Feeding ecology of the copepod Lucicutia aff. L. grandis near the lower interface of
the Arabian Sea oxygen minimum zone: implications for carbon flux

Marcia M. Gowing¹ and Karen F. Wishner²

1: Institute of Marine Sciences, University of California, Santa Cruz, CA 95064
2: Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882

Abstract:

Feeding ecology of the calanoid copepod Lucicutia aff. L. grandis collected in
the Arabian Sea at one station during the spring Inter-monsoon and during the Southwest
Monsoon of 1995 was studied with transmission electron microscopy of gut contents.
Highest abundances of these animals occurred from ~400-1100 m, near the lower
interface of the oxygen minimum zone and at the inflection point where oxygen starts to
increase. We hypothesized that their gut contents would include particles and cells that
had sunk relatively undegraded from surface waters as well as those from within the
oxygen minimum zone, and that gut contents would differ between the spring Inter-
monsoon and the more productive SW Monsoon. Overall, in both seasons Lucicutia aff.
L. grandis was omnivorous, and consumed a variety of metazoans as well as detrital
particles, prokaryotic and eukaryotic autotrophs, gram-negative bacteria including metal-
precipitating bacteria, aggregates of probable gram-positive bacteria, microheterotrophs,
virus-like particles and large virus-like particles. Few significant differences in types of
food consumed occurred among life stages within or among various depth zones.
Amorphous and detrital material were significantly more abundant in guts during the
spring Inter-monsoon than during the late SW Monsoon, and recognizable cells made up
a significantly higher portion of gut contents during the late SW Monsoon. This is
consistent with the Inter-monsoon as a time when organic material is considerably re-
worked by the surface water microbial loop before leaving the euphotic zone. In both
seasons Lucicutia aff. L. grandis had consumed what appeared to be aggregates of
probable gram-positive bacteria, similar to those we had previously found in gut contents
of several species of zooplankton from the oxygen minimum zone in the eastern tropical
Pacific. By intercepting sinking material, populations of Lucicutia aff. L. grandis act as a
filter for carbon sinking to the sea floor. They also modify sinking carbon in several
ways: enhancing pelagic-abyssal coupling of carbon from cyanobacteria, eliminating part
of the deep-sea microbial loop by direct consumption of bacterial aggregates, and
redistributing particulate manganese and iron from association with suspended cells or
aggregates to containment in rapidly sinking fecal pellets.

Spatial patterns in phytoplankton growth and microzooplankton grazing
in the Arabian Sea during monsoon forcing

Michael R. Landry¹, Susan L. Brown¹, Lisa Campbell²,
John Constantinou¹, and Hongbin Liu¹

1: Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Rd.,
Honolulu, HI 96822, U.S.A.
2: Department of Oceanography, Texas A&M University, College Station, TX 77843,
U.S.A.

Abstract:

Spatial patterns in the rates of phytoplankton growth and microzooplankton
grazing were investigated in the Arabian Sea during the Southwest Monsoon (August-
September) and early Northeast Monsoon (December) seasons in 1995 using the seawater
dilution technique. Nutrient-enhanced growth rates (µn) averaged 1.2 d-1 in the upper
euphotic zone for both cruises and were similar between higher and lower nutrient stations,
the former (> 1.0 µM NO3) being characteristic of the upwelling-influenced western coastal
portion of the study region and the latter (< 0.5 µM NO3) being typical of the central basin.
Growth rates without added nutrients (µo) were also comparable between cruises but
strongly related to ambient nutrient conditions, averaging 1.1 d-l (91% of µn) at the higher
nutrient stations and 0.5 d-l (44% of µn) at the lower nutrient stations. The rate estimates
for phytoplankton losses to microzooplankton grazing (m) averaged 0.6 d-l for the upper
euphotic zone and did not vary systematically between low and high nutrient stations. As a
consequence, µo and m were largely in balance for the oligotrophic stations, while the
eutrophic stations showed a growth differential over grazing of about 0.6 d-l. These
experimental results are consistent with observed differences in community structure,
namely the dominance of picoplankton in oligotrophic offshore regions and the increased
importance of the large diatom - mesozooplankton grazing pathway in the richer coastal
areas. Overall, the spatial patterns, if not magnitudes, of the community responses to
Southwest and Northeast Monsoon forcing were remarkably similar in this study, allowing
for a relatively simple interpretation of the influence of enhanced nutrient supply on the
rates and fates of phytoplankton production.

Nano- and microplankton assemblages in the northern Arabian Sea during the
Southwest Monsoon, August-September, 1995: A US-JGOFS study.

D.L. Garrison, M.M. Gowing, and M.P. Hughes

Institute of Marine Sciences, University of California at Santa Cruz, Santa Cruz, CA.,
95064, U.S.A.

As part of the U.S. Joint Global Ocean Flux Studies (JGOFS) Arabian Sea
Program, we determined the abundance and biomass of autotrophic and heterotrophic
nano- and microplankton in the upper 100 m at 10 stations in the northern Indian Ocean
during the late Southwest Monsoon from 17 August through 15 September 1995.
Autotrophic nano- and microplankton biomass ranged from 0.2 to 68.0 ,µg C 1-1, with
most of the biomass in the upper 20-60 m. Phytoplankton assemblages varied markedly
in composition along a transect from onshore to about 1500 km offshore. Larger forms,
such as diatoms and colonies of the prymnesiophyte Phaeocystis, dominated stations
inshore of about 1000 m, whereas picoplankton dominated offshore. Heterotrophic nano-
and microplankton biomass varied from -1 to 12 µg C 1-1, and nanoflagellates,
dinoflagellates, and ciliates reached maximum biomass at different locations and depths.
Heterotrophs comprised 18 to 27% of the biomass over most of the transect. Biomass of
all groups of organisms was strongly negatively correlated with depth and positively
correlated with one another, suggesting a dynamic food web. Size structure of organisms
among stations suggested that larger consumers occurred where phytoplankton cells were
large. Sediment trap data indicate high organic carbon and biogenic silica flux at the time
of our study. Our findings of abundant diatoms over much of the study area and their
apparent transition from healthy-looking cells nearshore to senescent ones offshore
suggest that populations could have sunk as a bloom terminated, in addition to being
available for mesozooplankton grazers.

Upper ocean export of particulate organic carbon in the
Arabian Sea derived from Thorium-234

Ken Buesseler¹, Lary Ball¹, John Andrews¹, Claudia Benitez-Nelson¹, Rebecca Belastock¹, Fei Chai² and Yi Chao³

1: Department of Marine Chemistry and Geochemistry. Woods Hole Oceanographic Institution, Woods Hole MA 02543. U.S.A.
2: School of Marine Science. University of Maine, Orono ME 04469, U.S.A.
3: Jet Propulsion Laboratory. California Institute of Technology, Pasadena CA 91109, U.S.A.
As submitted to DSRII, Special Arabian Sea Issue- last updated Aug 27, 1997

Thorium-234 is used in the Arabian Sea as a tracer of sinking particulate fluxes.
Samples were collected from January to August 1995 on four cruises during the NE
Monsoon, the Spring Intermonsoon and the mid- and late-SW Monsoon periods. In this
study, 234Th activity distributions are used to quantify the 234Th flux on sinking particles
and the measured ratios of particulate organic carbon (POC) to particulate 234Th is used
to convert from 234Th to POC export at 100m. The calculated POC fluxes range from <I
to >25 mmols C m-2 d-1 and strong seasonal and spatial gradients are observed. The
single largest feature is the basin-wide export maximum associated with the late-SW
Monsoon cruise when POC export rates are 17-28% of the observed primary production
rates along the southern sampling line. During all other cruises, this export ratio is <2-
10%, with an increase near shore where POC fluxes are generally elevated. Also, during
the Spring Intermonsoon, a POC export maximum is observed along the northern
sampling line. Both this Spring export feature and late-SW Monsoon flux maximum
appear to be associated with a phytoplankton community structure that is dominated by
diatoms. The timing of the late-SW Monsoon flux peak agrees with the observed flux
maximum in the deep moored time-series sediment traps (Honjo et al., 1997). This
dramatic increase in export between the mid- and late-SW monsoon also corresponds to
measured decreases in the stocks of total organic C in the upper 150m (Hansell and
Peltzer, 1997) and a sharp decline in surface water Al and Fe (Measures and Vink, 1997).
These 100m flux results plus a series of POC flux profiles, allow for a more complete
understanding of the magnitude and timing of sub-euphotic zone export in the Arabian
Sea.

Abundance and biomass of nano- and microplankton
assemblages during the 1995 Northeast Monsoon
and Spring Intermonsoon in the Arabian Sea

Mark R. Dennett*
David A. Caron*
Sergey A. Murzov
Igor G. Polikarpov
Nelli A. Gavrilova
Ludmila V. Georgieva
Ludmila V. Kuzmenko

*Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA
02543 USA
Institute for the Biology of the Southern Seas, Sevastopol 335011, Crimea,
UKRAINE

Key words: Phytoplankton, zooplankton, protozoa, nanoplankton, microplankton,

Arabian Sea, microbial ecology.

Abstract -- Phototrophic and heterotrophic nanoplankton (PNAN, HNAN; 2-20 µm
protists) and microplankton (PMIC, HMIC; 20-200 ,µm protists and micrometazoa)
are major components of the producer and consumer assemblages in oceanic
plankton communities. Abundances and biomasses of these microorganisms were
determined from samples collected along two transects during the Northeast
Monsoon and Spring Intermonsoon process cruises of the U.S. JGOFS Arabian Sea
Program in 1995. Vertical profiles of these assemblages were strongly affected by the
presence of a subsurface oxygen minimum layer. Abundances of all four
assemblages decreased dramatically below the top of this layer. Depth integrated (0-
160 m) abundances and biomasses of nanoplankton and microplankton were of
similar magnitude for most samples. Exceptions to this rule were primarily due to
PMIC (mostly diatom) species which dominated phytoplankton assemblages at a few
stations during each season. Depth integrated biomasses for the combined nano-
and microplankton averaged over all stations for each cruise were surprisingly
similar for the Northeast Monsoon and Spring Intermonsoon seasons in this
ecosystem (2.0 and 1.8 g C m-2 for the two seasons, respectively). Nano- and
microplankton biomass for these two time periods were a very signficant portion of
the total amount of the particulate organic carbon in the water column. Summed
over all stations, these assemblages constituted approximately 25-35% of the POC in
the top 160 m of the northern Arabian Sea.

Phytoplankton growth and mortality during the 1995 Northeast
Monsoon and Spring Intermonsoon in the Arabian Sea

David A. Caron
Mark R. Dennett

Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 USA

Key words: Herbivory, phytoplankton growth, phytoplankton mortality,
microzooplankton, Arabian Sea, trophic interactions, protozoa, microbial ecology.

Abstract -- Phytoplankton growth rates and mortality rates were experimentally
examined at 8 stations in the Arabian Sea along the U.S. JGOFS cruise track during the
1995 Northeast Monsoon (January) and Spring Intermonsoon (March-April).
Instantaneous growth rates averaged over an entire cruise were approximately twice as
high during the NE Monsoon than during the Spring Intermonsoon period (0.84 d-1 vs.
0.44 d-1). Average herbivore grazing (mortality) rates, however, were quite similar for
the two seasons (overall averages of 0.35 d-1 and 0.30 d-1 for the NE Monsoon and Spring
Intermonsoon, respectively). The absolute amounts of phytoplankton biomass
consumed during each season also were similar (29% and 25% of standing stock
consumed d-1 for the January and March-April cruises, respectively), as were the
geographical trends of this removal. These seasonal trends in growth and removal rates
resulted in net phytoplankton growth rates that were considerably higher during the
January cruise (0.48 d-1) than during the March-April cruise (0.14 d-1). That is,
phytoplankton production was more closely balanced during the Spring Intermonsoon
season (87% of daily primary production consumed) relative to the NE Monsoon season
(49% of daily primary production consumed). Station-to-station variability was high for
rate measurements during either cruise. Nevertheless, there was a clear onshore-
offshore trend in the absolute rate of removal of phytoplankton biomass (µg chlorophyll
consumed 1-1 d-1) during both cruises. Coastal stations had removal rates that were
typically 2-4 times higher than removal rates at oceanic stations.

The carbon dioxide system in the Arabian Sea

Frank J. Millero, Elizabeth A. Degler, Daniel W. O'Sullivan, Catherine Goyet* and Greg Eischeid*

Rosenstiel School of Marine and Atmospheric Science
University of Miami
4600 Rickenbacker Causeway
Miami, Fl.33149-1098
*Woods Hole Oceanographic Institution
MS 25, Woods Hole, MA 02543-1541

Abstract

In 1995 we participated on a number of research cruises in the Arabian Sea as part
of the Joint Global Ocean Flux Study (JGOFS) sponsored by the National Science
Foundation (NSF). This paper gives the results of our total inorganic carbon dioxide
(TCO2), total alkalinity (TA) and potentiometric pH measurements made on Arabian Sea
water samples during these cruises. Measurements made on Certified Reference Material
(CRM) indicate that the reproducibility of the measurements was ± 0.007 in pH, ± 3.2 µ
mol kg-1- in TA, and ± 1.2 µmol kg-1 in TCO2 (N = 180). The surface measurements of pH
and normalized TCO2 and TA were quite uniform throughout the year (pH = 8.0 ± 0.05,
NTCO2 = 2200 ± 20 µmol kg-1 and NTA = 2300 ± 8 µmol kg-1). The larger variations in
NTCO2 in the surface waters are related to changes due to primary production and the
upwelling in the coastal waters. The depth profiles of pH, pCO2, TA, and TCO2 were
similar to those in the Indian Ocean. The components of the carbonate system (CO2,
HCO3; CO32-) and the saturation state (½) for calcite and aragonite were determined from
the measurements of TA and TCO2. The waters below 600 m and 3400 m in the Arabian
Sea were undersaturated (½ < 1.0), respectively, for aragonite and calcite.
The CO2 measurements have been combined with the nutrient data to examine the
stoichiometric ratios of C/N, C/P, C/O2, and C/SiO2 of the waters. Marked differences
were found for the waters above and below the oxygen minimum zone. The surface waters
results have been used to develop the following equation for the production of
phytoplankton in the Arabian Sea
126 CO2 + 140 H2O + 14 HNO3 + H3PO4 + 13 SiO2 -
(CH2O)116(CH2)10(NH3)14(H3PO4)(SiO2)l3 + 159 O2
These results, together with the organic material data collected from the sediment traps,
should be useful in characterizing the formation and degradation of plant material in the
Arabian Sea.

Response of microbial community structure to environmental forcing
in the Arabian Sea

L. Campbell¹, M.R. Landry², J. Constantinou², H.A. Nolla²,

S.L. Brown², H. Liu², and D.A. Caron³

1: Department of Oceanography, Texas A&M University, College Station, TX 77843
2: Department of Oceanography, University of Hawaii, Honolulu, HI 96822
3: Woods Hole Oceanographic Institution, Woods Hole, MA 02453

corresponding author:

L. Campbell
Dept. Oceanography
Texas A&M University
College Station, TX 77843
409- 845-5706
lcampbell @ ocean.tamu.edu

Abstract-- The effect of environmental forcing on microbial community structure was
investigated in the Arabian Sea during four seasonal cruises: late Northeast Monsoon (January);
spring intermonsoon (March-April); late Southwest Monsoon (August-September); and early
Northeast Monsoon (December). The distributions of picoplankton populations -- heterotrophic
bacteria (HBac), Prochlorococcus(Pro),Synechococcus spp. (Syn) and < 3 µm picoeucaryotic
algae (Peuc)-- were determined by flow cytometric analysis. Seasonal variations in abundance
maxima, vertical profiles, integrated abundance (0-200 m), and estimated carbon biomass were
contrasted along two transects from the coast of Oman to 1500 km offshore. HBac were
numerically dominant in surface waters in all regions (1-3 x 106 cells ml-1), with higher
maximum abundances in coastal waters than at offshore stations. Conversely, Pro were most
abundant at the oligotrophic offshore stations, and 100-fold lower, or absent, at coastal stations,
except during the spring intermonsoon when abundances were extremely high along the entire
southern transect. Syn abundances were highly variable, with no consistent trend between
coastal and offshore stations. This variability may be explained by the prevalence of mesoscale
eddies, but could also be due to overlapping distributions of multiple Syn populations
distinguished by pigment type. Syn with a low-phycourobilin (PUB) to phycoerythrobilin (PEB)
ratio pigment type were more abundant at coastal stations, whereas Syn with a high PUB :PEB
ratio increased in abundance offshore. Average depth profiles for Pro, Syn, and HBac displayed
uniform abundance in the surface mixed layer, with a rapid decrease below the surface mixed
layer depth; however, during the intermonsoon most profiles had a peak at the base of the surface mixed layer. Distributions of Peuc typically displayed a subsurface maximum near the base of the surface mixed layer, except during the SW Monsoon when abundance peaked near the surface.
Microbial community structure in the Arabian Sea varied on both seasonal and spatial scales.
Overall, the eucaryotic component was more important at coastal stations, and the procaryotic
components were predominant at offshore stations. Pro abundance was restricted to warm
oligotrophic waters and was inversely related to surface nitrate concentrations; thus, an increase
in the % Pro as a fraction of total procaryote abundance was also indicative of oligotrophic
conditions. The effects of SW Monsoonal forcing on microbial community structure resulted in
an increase of Peuc, but this response was limited to coastal stations. Pro and Syn remained
dominant at offshore stations.

Fluorescence-based characterization OF Synechoccus
community structure in the Arabian Sea during the Northeast
and Southwest Monsoon (1994-95)

A. Michelle Wood¹,² Michael Lipsen¹, and Paula Coble³

lDept. of Biology, University of Oregon, Eugene, Oregon 97405*
2Naval Research Laboaratory, Code 7330, Stennis Space Center, Mississippi 39529
3Dept. of Marine Science, University of South Florida, St. Petersburg, FLA 33701

ABSTRACT

Scanning fluorescence spectroscopy was used to investigate the spatial and
temporal variability in the community structure of phycoerythrin-containing organisms in
the Arabian Sea during the early Northeast and early Southwest Monsoon (1994-95).
During both periods, fluorescence microscopy confirmed that chroococcoid cyanobacteria
were responsible for essentially all the phycoerythrin signal measured in our samples; the
filamentous nitrogen-fixing cyanobacterium Trichodesmium, however, was present in net
hauls obtained at some stations and the fluorescence excitation and emission spectrum for
isolated trichomes are also presented in this paper. Peak phycoerythrin (PE) emission
was relatively invariant among all the samples collected on either cruise, and ranged from
563-572 nm; PE excitation spectra always showed either a strong shoulder or a peak at
wavelengths absorbed maximally by phycourobilin chromophores (PUB). Thus, the
Arabian Sea appears to be different from the Black Sea in that PUB-lacking forms of PE
rarely, if ever, dominate the PE signal. Fluorescence excitation spectra revealed three
basic types of phycoerythrins dominating the samples. These were categorized based on
the relative excitation of phycoerythrin emission at wavelengths absorbed by
phycourobilin chromophores (~495 nm, ExPUB) and wavelengths absorbed by
phycoerythrobilin chromophores (-550 nm, ExPEB): those with very low (~0.6), very high
(~1.8), and intermediate ExPUB:ExPEB ratios. The distribution of these different types was
investigated intensively during the early Southwest Monsoon and communities dominated
by the low ExPUB:ExPEB ratios were closely associated with cooler, fresher water
influenced by coastal upwelling. Persian Gulf Influenced Water (PGIW), which is very
warm and salty, is also dominated by communities with low ExPUB:ExPEB ratios and we
found a similar low ExPUB:ExPEB ratio in water of high temperature and high salinity in
the south central Arabian Sea at the beginning of the SW Monsoon; we discuss evidence
from models of general circulation which support the hypothesis that this water was
transported to the South Central Arabian Sea from the Gulf of Oman.

Temporal and spatial variability of dissolved iodine speciation in the Arabian Sea

Anna M. Farrenkopf*, George W. Luther

College of Marine Studies University of
Delaware; Lewes DE 19958 and Piers Chapman Texas A&M University; College Station
TX 77840

(*corresponding author: afarren@ccalmr.ogi.edu Presently at Oregon Graduate Institute of
Science and Technology; P.O. Box 91000, Portland OR 97291-1000)

Abstract

Data were collected in the Arabian Sea as part of the U.S. Joint Global Ocean Flux
Study (JGOFS) northeast intermonsoon cruise (March-April, 1995) and as part of the World
Ocean Circulation Experiment (WOCE) southwest monsoon cruise (July-August, 1995).
We report seasonal and hydrographic variability in the distribution of dissolved iodide and
total iodine. Primary productivity and denitrification were reflected in the iodine species
measured. Iodide concentrations increased at intermediate depths within the oxygen
minimum zone across the basin from west to east. The eastern Arabian Sea was most
denitrified (NO2- < 6.5 mM) and contained the highest concentrations of reduced iodine
(0.25 < I- < 0.9 mM). No free sulfide was detected at any of the stations sampled along the
northern transect during (March and April 1995). The vertical profiles from the Arabian
Basin during (March - April 1995) are contrasted with southwest intermonsoon samples
collected (September-November 1992) (Farrenkopf, 1993; Farrenkopf et al., 1997a).
Temporal and spatial differences in iodine speciation indicated a rapid cycling between the
reduced and oxidized forms of iodine. Since oxygen concentrations were typically less than
5 uM throughout the OMZ, observations of seasonal redox cycling point toward evidence of
biologically enhanced reduction of iodate.

Spatial and temporal variations of total organic carbon in the Arabian Sea

Dennis A. Hansell¹ and Edward T. Peltzer²

1: Bermuda Biological Station for Research, Inc
St. Georges, GE-01, BERMUDA
2: Department of Chemistry
Woods Hole Oceanographic Institute
Woods Hole, MA 02543 USA
2: Current address:
Monterey Bay Aquarium Research Institute
PO Box 628
Moss Landing, CA 95039 USA

Abstract

Concentrations of total organic carbon (TOC) were determined on samples collected during
6 cruises in the northern Arabian Sea during the 1995 US JGOFS Arabian Sea Process Study.
TOC concentrations and integrated stocks in the upper ocean varied strongly, both spatially and seasonally. Highest TOC concentrations (80-100 µM C) were observed in the mixed layer near the coast when upwelling was not active, while upwelling tended to reduce local concentrations. In the open ocean, highest TOC concentrations (80-95 µM C) developed in winter (period of the NE Monsoon) and remained through mid summer (early to mid periods of the SW Monsoon). Lowest open ocean concentrations (65-75 µM C) occurred late in the surrlmer (late SW Monsoon) and during the Fall Intermonsoon period. The changes in TOC concentrations resulted in seasonal changes in mean TOC stocks of 1.5-2 moles C m-2, with lowest stocks found late in the summer, toward the end of the SW monsoon. The transition between high and low TOC stocks in the upper layer over the course of the SW monsoon was correlated with other biogeochemical changes in the water column, likely reflecting a change in food web composition during this period. Total annual accumulation of TOC north of 15°N was 31-41 x 10l2 g C, mostly taking place over the period of the NE Monsoon, and equivalent to 6-8% of annual primary production estimated for that region in the rnid-1970's. TOC concentrations varied in vertical profiles as a function of water mass layering, with the Persian Gulf Water showing TOC concentrations elevated relative to deeper waters. Variability in TOC concentrations did not correlate with indicators of denitrification (such as nitrite maxima or nitrate minima), suggesting that the horizontal introduction of organic matter was not the primary source of substrate to the denitrifiers. Deep water (>2000 m) TOC concentrations were uniform across the basin and over the period of the cruises, averaging 42.3±1.4 µM C.

A One-year record of atmospheric forcing from the Arabian Sea

R. A. Weller¹, M. F. Baumgartner¹, S. A. Josey², A. S. Fischer¹, and J. C. Kindle³

1: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
2: Southampton Oceanography Center, Southampton, U. K. SO14 3ZH
3: Naval Research Laboratory, Stennis Space Center, Mississippi 39529

Abstract - Accurate, year-long time series of winds, incoming shortwave and longwave
radiation, air and sea temperatures, relative humidity, barometric pressure, and precipitation have been collected from a surface mooring deployed off the coast of Oman along the climatological axis of the Findlater Jet. Wind stress, heat flux, and freshwater flux were computed using bulk formulae. The Northeast Monsoon was characterized by steady but moderate winds, clear skies, relatively dry air, and two months, December and January, in which the ocean, on average, lost 45 W m-2 to the atmosphere. The Southwest Monsoon had strong winds, cloudy skies, and moist air. Because of reduced latent and longwave heat loss, it was accompanied by sustained oceanic heat gain, with the strongest monthly mean warrning, 147 W m-2, in August. Large differences are found between the observations and older climatologies. Recent climatologies agree better with the observations; the Southampton Oceanography Center data set (SOC, Josey et al., 1996 and 1997a) means for 1980-1995 are close to the buoy monthly means. The SOC data set shows that 1994-1995 was a typical year, with surface meteorology and air-sea fluxes closer to the long term means than one standard deviation. Concurrent data from the NCEP, ECMWF, and FNMOC show that the models provide realistic surface winds, but fail to replicate other observed surface meteorology and to produce realistic heat fluxes. Annual and monsoonal mean net heat fluxes from the models differed from those of the buoy by 50 to 80 Wm2. Because of these differences, some care is indicated in selecting and using air-sea flux fields in studies of the Arabian Sea.

A comparison of phytoplankton populations of the Arabian Sea
during the Spring Intermonsoon and Southwest Monsoon of 1995 as described by HPLC-analyzed pigments

MIKEL LATASA¹ and ROBERT R. BIDIGARE²

1: Instituto de Ciencias del Mar, Passeig Joan de Borbo s/n 08039, Barcelona, Spain.
2: Department of Oceanography, University of Hawaii, Honolulu, HI 96822, U.S.A. e-mail:

Abstract--We used diagnostic pigments to estimate the relative abundances of different algal
groups in the Arabian Sea during the Spring Intermonsoon (cruise TN045) and late Southwest
Monsoon (cruise TN050) of 1995. Two track lines were followed during each cruise and
designated as the northern and southern transects. These transects started near the coast of the
Arabian Peninsula at 22.38°N and 18.50°N, respectively, and extended >1200 Km offshore. The
pigment concentrations at the offshore stations (>1000 Km from the coast) were low during both
cruises, and the composition of the phytoplankton resembled that found in oligotrophic, open-
ocean waters. A well developed deep chlorophyll maximum (DCM) was always observed for
these regions. The pigment compositions at the inshore stations (<1000 Km from the coast) were
markedly different during cruises TN045 and TN050.
During TN045, the inshore waters of the northern transect were dominated by a mixture of
diatoms and haptophytes. Large particles (>18µm) accounted for up to 23% of the total
chlorophyll a-related pigments (TCHLA, chlorophyllide a + monovinyl chlorophyll a + divinyl
chlorophyll a). Pigment biomass (~500 ng TCHLA L-l) was homogeneously distributed in the
upper water column at these stations. By contrast, pigment biomass at the inshore stations of the southern transect was concentrated in a pronounced DCM. The phytoplankton were clearly
partitioned in the water column, with photosynthetic prokaryotes and eukaryotes dominating in the upper mixed layer and below, respectively. High levels of divinyl chlorophyll a were observed at these stations (up to 300 ng L-1), and represent the highest concentrations ever recorded for this prochlorophyte marker. The size-fractioned pigment analyses revealed that 85% of the TCHLA passed through 2 µm filters, indicating dominance by the picophytoplankton.
During TN050, the inshore portion of the northern transect displayed a clear example of a
diatom to Phaeocystis successional event. Diatoms dominated at coastal upwelling stations and
were progressively replaced by Phaeocystis at offshore stations. At the inshore stations of the
southern transect, diatoms dominated the phytoplankton community and a large fraction of the
TCHLA (21-48%) was retained by 18 µm filters. Circulation patterns during the Southwest
Monsoon period were complex and characterized by numerous mesoscale features. A filament
with high diatom biomass was observed 700 Km offshore, documenting that horizontal advection
is an important mechanism for removing phytoplankton from coastal waters in this region.
However, lower than expected TCHLA concentrations were measured during this period relative to that observed in the CZCS imagery. Several hypotheses are discussed to explain this enigma,
including sampling biases, losses by grazing and sinking, phytoplankton dilution via upwelling,
and Si limitation of diatom growth.

Distribution and optical properties of CDOM in the Arabian Sea during the 1995 Summer Monsoon

Paula G. Coble*, Carlos Del Castillo, and Bernard Avril

Department of Marine Science
University of South Florida
St. Petersburg, FL 33701

ABSTRACT

Spatial and temporal variability in concentration and optical properties of colored
dissolved organic matter (CDOM) were examined in the Arabian Sea during the 1995 Summer
Monsoon NRL/Seasoar Cruise, June 21 - July 12, 1995. Classical upwelling patterns were
observed near the coast, with strong fronts characterized by cold water, high chlorophyll, and
high nutrients. CDOM concentration showed a negative correlation with temperature in the
surface waters throughout the study region, reflecting upwelling as the major source of CDOM to
surface waters.
Three water mass types were identified and found to have distinctive excitation-emission
matrix (EEM) fingerprints. Persian Gulf Water had two fluorescence maxima due to UV and
visible humic substance fluorescence. Maximum CDOM concentrations were found in the core of
this water mass. Upwelling water had five or six fluorescence peaks, due to new and old CDOM
and to biomolecules including proteins and chlorophyll. Elevated CDOM concentrations at the
depth of the chlorophyll maximum indicated production of new CDOM associated with the
phytoplankton bloom. Oligotrophic water was highly bleached and only UV-absorbing and
fluorescing compounds remained. This is consistent with photobleaching as the major sink of
CDOM in surface waters.
Two new EEM fingerprints are presented, one for oligotrophic "bluewaters" showing
photoresistant DOM, and one for upwelling water which may be of biological origin.

Patterns of co-variability between physical and biological parameters in the Arabian Sea

Danielle M. Bartolacci*+ and Mark E. Luther*

Abstract- The relationship between physical forcing and biological response observed in
the Arabian Sea for the years 1978 through 1986 were examined. Spatial and temporal
patterns of variability in a climatological time series of three suspected physical forcing
parameters and CZCS-derived phytoplankton pigment concentration during the annual
cycle were quantified using single and joint Empirical Orthogonal Function (EOF), and
Singular Value Decomposition (SVD) analyses. Monthly composites of the NASA
regional pigment data were interpolated to fill data voids, and binned corresponding to
the physical flux data. Nearly all the spatio-temporal analyses consistently partitioned a
large portion of the variability using only 1 or 2 dominant modes and indicated a lag in
the timing of the peak pigment concentration behind the maxima in physical forcing. In
all cases major modes of variability resembled the Southwest Monsoon pattern, with the
Northeast Monsoon contributing very little to the total variance and covariance. The
Joint EOF and SVD analyses incorporated subtle features surrounding the peak
Southwest Monsoon phenomena. Correlation maps of the joint EOF analysis depicted
differences in spatial variability of pigment concentration associated with stress and curl,
showing areas of curl-driven upwelling distinct from coastal upwelling, with possible off-
shore advection of the curl-induced high pigment waters.

*Department of Marine Science, University of South Florida, 140 Seventh Avenue South St. Petersburg, FL 33701
+Present address: Wave Hill, Inc., 675 West 252nd Street, Bronx, NY 10471

Particulate organic carbon fluxes: Results from the U.S. JGOFS Arabian Sea Process Study by the Arabian Sea Carbon Flux Group

C. Lee¹, D. W. Murray², R. T. Barber³, K. O. Buesseler⁴, J. Dymond⁵, J. I. Hedges⁶, S. Honjo⁴, S. J. Manganini⁴, J. Marra⁷, C. Mosers⁵, M. L. Peterson⁶, W. L. Prell² and S. G. Wakeham⁸

1: State University of New York, Stony Brook NY 11794-5000; 2: Brown University, Providence RI 02912-1846; 3: Duke University, Beaufort NC 28516-9721; 4: Woods Hole Oceanographic Institution,Woods Hole MA 02543; 5: Oregon State University, Corvallis OR 96331; 6: University of Washington, Seattle WA 98195-7940; 7: Lamont-Doherty Earth Observatory, Palisades NY 10964; 8: Skidaway Institute of Oceanography, Savannah GA 31411

Abstract

Organic carbon fluxes in the Arabian Sea were measured as a function of
depth, season and distance from the coast of Oman. We summarize here
measurements of primary production, water-column export flux and sediment
accumulation of organic carbon over a full annual monsoon cycle on a 1500 Km
transect from the coast of Oman toward the central Arabian Sea. This represents an
integration of measurements spanning one day (primary production) to 1000 years
(sediments) and gives a broad overview of organic carbon removal and
remineralization in the highly productive, seasonally varying region of the
northern Indian Ocean.
Organic carbon fluxes decreased from the surface to the sediments by a factor
of 500 to 10,000, with the largest changes in the upper ocean and at the sediment-
seawater interface. Organic carbon fluxes generally decreased with distance offshore,
with the largest gradient between surface and seafloor being at the offshore station.
Sediment accumulation rates of organic carbon changed by a factor of 40 between
nearshore and offshore, while primary productivity varied only by a factor of 2. The
decrease in carbon flux with depth that occurs between the deepest traps and the
sediment becomes a greater proportion of the total loss with increasing distance
from shore. Thus, the influence of processes at the sediment-water interface on the
proportion of primary productivity preserved in the sediment increases offshore
relative to upper water column processes. Carbon fluxes changed greatly with
season, with highest fluxes during the southwest monsoon. Export fluxes varied
more with season than primary productivity or mid-water fluxes.

Seasonal variations in the distribution of Fe and Al in the surface waters of
the Arabian Sea
C.I. Measures and S. Vink

Department of Oceanography, University of Hawaii, Honolulu, HI 96822

Abstract

Surface water mixed layer concentrations of Al and Fe are presented
for five cruises during the 1995 Arabian Sea JGOFS expedition.
Concentrations of both Al and Fe were relatively uniform between Jan.
and April, the NE Monsoon and the spring intermonsoon period, ranging
from 2 to 11 nM Al (mean 5.3 nM) and 0.5 to 2.4 nM (mean 1.0 nM) In
July/Aug., after the onset of the SW Monsoon, surface water Al and Fe
concentrations increased significantly (mean Al 10 nM, mean Fe 1.3 nM),
particularly in the NE part of the Arabian Sea, as the result of the input
and partial dissolution of eolian dust. Using the enrichment of Al in
surface waters, we estimate this is the equivalent of 2.2 - 7.4 g m-2 dust,
which is comparable to values previously estimated for this region.
Approximately one month later (Aug./Sept.), surface water concentrations
of both Al and Fe were found to have decreased significantly (mean Al 7.4
nM mean Fe 0.90 nM) particularly in the same NE region, as the result of
export of particulate material from the euphotic zone. Fe supply to the
surface waters is also affected by upwelling of sub-surface waters in the
coastal region of the Arabian Sea during the SW Monsoon. Despite the
proximity of high concentrations of Fe in the shallow sub-oxic layer,
freshly upwelled water contains insufficient Fe to support complete NO3
removal. Continued deposition of eolian Fe into this water as it advects
offshore provides the Fe required for the full utilisation of the NO3.

Upper ocean currents in the northern Arabian Sea from
ADCP measurements during the 1994-1996 JGOFS program

Charles N. Flagg and Hyun-Sook Kim
Oceanographic and Atmospheric Sciences Division
Brookhaven National Laboratory
Upton, NY I 1973

Abstract

A large upper ocean velocity data set was obtained using a shipboard acoustic Doppler current profiler (ADCP) on seventeen RV T.G. Thompson cruises during the JGOFS and ONR expedition to the northern Arabian Sea from September 1994 through January 1996. Seven of the cruises followed a large area survey track centered over the Arabian Basin, four cruises conducted SeaSoar surveys on either side of the Findlater jet axis, and six cruises were for the deployment and maintenance of moored instrumentation, together providing some 380 cruise-days and 96,000 track kilometers of coverage. The ADCP data extended over the upper 250 to 400 meters of the water column depending upon the temporal/spatial distributions of acoustic scatterers. The velocity data revealed several items that differed significantly from the historical perspective. Maximum current magnitudes in this area varied from more than I m/sec along the Arabian coast to 10 to 20 cm/sec well offshore. Perhaps the most important result was the complete dominance of the velocity field by eddies which had offshore correlation length scales of roughly 100 km, a spectral peak at around 300 km, and kinetic energies that ranged from 70 to more than 90 percent of the total kinetic energy. The total kinetic energy was highest within about 300 km of the shore and decreased significantly in magnitude and vertical extent offshore. Within the coastal region, the temporal variability was such that currents of 50 cm/sec or more could completely reverse within a two week period. Mean and seasonal velocities also differed from historical results. There was a large anti-cyclonic feature located for most of the year south of Ras ash Shabatat which intensified during the southwest monsoon. There was a strong jet-like current off Ras al Hadd which also intensified during the southwest monsoon, at which time it flowed southwestward against the wind. In contrast to the historical ship drift data which indicated that the surface currents followed the monsoonal winds, the ADCP data over the upper 200 to 400 meters was highly variable with overall seasonal means that were often directed against the wind.

Mesozooplankton biomass in the upper 1000 m in the Arabian
Sea: Overall seasonal and geographic patterns, and relationship
to oxygen gradients

Karen F. Wishner¹, Marcia M. Gowing², and Celia Gelfman¹

1: Graduate School of Oceanography, University of Rhode Island, Narragansett RI 02882 USA,
2: University of California, Santa Cruz, CA 95064 USA

Abstract - Mesozooplankton biomass distributions in the upper 1000 m of the Arabian
Sea were studied as part of the U.S. JGOFS project. Samples were collected in vertically-
stratified MOCNESS tows during 4 seasons in 1995 at 6 - 8 stations spanning the Arabian
Sea. This paper describes results from the size-fractionated dry weight and wet weight
biomass profiles and their relationship to the seasonal monsoon cycle and the pronounced
oxygen minimum zone (OMZ). The total mesozooplankton biomass and most size
fractions exhibited a significant onshore/offshore biomass gradient during most seasons
and at most depths. The gradient was strongest in August during the Southwest Monsoon
and weakest in March during the Spring Intermonsoon, when maxima of some size
fractions occurred offshore. Seasonal changes in zooplankton biomass were small. The
Southwest Monsoon was the time of higher biomasses for near shore stations, while the
Spring Intermonsoon was the time of higher biomasses for offshore stations. Most of the
zooplankton biomass present in the upper 1000 m of the water column occurred in the
upper 200 m, but there was substantial diel vertical migration, especially of the large size
class, down to 300 - 400 m, well within the suboxic water of the OMZ. A subsurface
biomass increase occurred near the depth of the oxygen inflection point (about 500 - 600
m) in the OMZ. The enhanced zooplankton biomass, higher potential food levels, and
possible short food chains suggested active biological modification of the sinking flux in
this depth zone. Biomass depth profiles showed close relationships to oxygen profiles.
The shapes of the zooplankton biomass and oxygen profiles varied with geographic
location across the Arabian Sea but were remarkably consistent over the year at each
station. Geographic differences in the shapes of the profiles were dependent on the vertical
extent of the suboxic zone and may correspond to broad regional differences in the
processing of the export flux in midwater. However, the consistency of the shapes over
time implies longterm stability in the structure and function of the midwater ecosystem at each location.

The hydrographic milieu of the U.S. JGOFS Arabian Sea Process Experiment

J. Morrison¹, L.A. Codispoti², S. Gaurin², B. Jones3, V. Manghnani¹, and Z. Zheng3
l: Department of Marine, Earth & Atmospheric Sciences, Box 8208, 1125 Jordan Hall,
North Carolina State University, Raleigh, NC 27695-8208, U.S.A.
2: Center for Coastal Physical Oceanography, Crittenton Hall, Old Dominion University,
Norfolk, VA 23529, U.S.A.
3: Department of Biological Sciences, Allen Hancock Foundation, University of
Southern California, Los Angeles, CA 90089, U.S.A.

Abstract

Between September 1994 and December 1995, the U.S. JGOFS Arabian Sea Process
Experiment collected arealy extensive, high quality hydrographic data (temperature, salinity,
dissolved oxygen and nutrients) during all seasons in the northern Arabian Sea. An analysis
of this unique suite of data suggests many features that are in general accord with canonical
descriptions of the region, but these new data provided the following insights.
1) Although the seasonal evolution of mixed layer depths was in general agreement with
previous descriptions, the deepest mixed layer depths in our data occurred during the late NE
Monsoon instead of the SW Monsoon.
2) The region exhibits considerable mesoscale variability resulting in extremely variable
temperature-salinity (TS) distributions in the upper 1000 db. This mesoscale variability is
readily observed in satellite imaging, in the high resolution data taken by a companion ONR
funded project, and in underway ADCP data.
3) The densest water reaching the sea surface during coastal upwelling appeared to have
maximum offshore depths of ~150 m and o~'s close to the core value (24.8) for Arabian Sea
Water (ASW). The densest water found at the sea surface during late NE Monsoon
conditions also had ~'s >24.8 and relatively high salinities suggesting that they are a source
for the salinity maxima used to define ASW.
4) There was little evidence of an influence of high-salinity Red Sea Water (RSW) in the
JGOFS study region.
5) Persian Gulf outflow mixes rapidly with ambient waters and has only a minor impact on
nutrient distributions.
6) Although a minor component, the Persian Gulf outflow produces a salinity maximum at a
~ of about 26.6. This signal is associated with the suboxic portions of the Arabian Sea's
oxygen minimum zone, and "outbreaks" of suboxic waters containing secondary nitrite
maxima may be associated with this salinity maximum. The influence of nitrate reduction
and denitrification on relationships between nitrate and phosphate and nitrate and oxygen are
obvious on this surface.
7) Inorganic nitrogen to phosphate ratios were lower (frequently much lower) than the
standard Redfield ratio of 15/1-16/1 (by atoms) at all times and all depths suggesting that
inorganic nitrogen was more important than phosphate as a limiting nutrient for phytoplankton
growth, and that the effects of denitrification dominated the effects of nitrogen fixation.
8) The waters upwelling during the SW Monsoon came from depths of ~150 m where
inorganic nitrogen/silicate ratios are higher (2:1) than the ~1:1 ratio often assumed as the ratio
of uptake during diatom growth.
9) The temporal evolution of inorganic nitrogen/silicate ratios suggests major alteration by
diatom uptake only during the late SW Monsoon cruise in August-September 1995.
10) Widespread moderate surface layer nutrient concentrations occurred during the late NE
Monsoon. This study and recent data from Indian expeditions suggest that the nutrient
enrichment of the surface layers that occurs during the NE Monsoon may not have been
emphasized sufficiently in most previous studies.
11) A zone of high offshore nutrient concentrations was encountered during the SW
Monsoon, but instead of being associated with offshore upwelling it might represent offshore
advection from the coastal upwelling zone, or the influence of an eddy, or both.
12) Although the suboxic portion of the oxygen minimum zone may have been weakened
during the SW Monsoon, we did not find as strong a tendency for re-oxygenation or for the
secondary nitrite maximum to be associated with dissolved oxygen concentrations above 5 µM as has been suggested by others.

Aerosols over the Arabian Sea: Atmospheric
transport pathways and concentrations of dust and sea salt.

N.W. Tindale & P.P. Pease

Neil W. Tindale
Depts. of Meteorology and Oceanography
Texas A&M University
College Station, TX 77843-3150
Patrick P. Pease
Dept. of Geography
TAMU, College Station, TX 77843-3147.

ABSTRACT

Many studies have indicated that the input of nutrients and
micronutrients from the atmosphere can impact biogeochemical cycles and
biological productivity in coastal and open ocean surface waters and the flux
of material from the surface to the deeper ocean. One of the marine regions
that is most likely to be affected by atmospheric inputs is the Arabian Sea.
Strong seasonal monsoon winds drive ocean current reversals, increase
surface mixing and upwelling, and significantly affect productivity. The
transport and input of dust from surrounding deserts is also heavily
influenced by the seasonal forcing. The United States Joint Global Ocean
Flux Study (U.S. JGOFS) Arabian Sea Process Study was an excellent
opportunity to study the atmospheric flux to the ocean. Specific questions
included: what was the influence of aeolian dust on primary productivity;
and, how did this input vary during the different monsoon seasons?
This paper provides an overview of dust transport pathways and
concentrations over the Arabian Sea during the 1995 study. Results indicate
that the transport and input of dust to the region is complex, being affected
by both temporally and spatially important processes. Dust levels vary
considerably, by the day, week, month, and seasons. Highest values of dust
were found off the Omani coast and in the entrance to the Gulf of Oman.
Surprisingly, dust levels were generally lower in summer than the other
seasons, although even in summer they were relatively high compared to
other oceanic regions. The Findlater jet, rather than acting as a source of
dust from Africa, appears to block the transport of dust to the open Arabian
Sea from northern desert dust source regions. Dust transport aloft, rather
than at the surface, seems to be important during certain periods. In an
opposite pattern to dust, sea salt levels were exceedingly high during the
summer monsoon, presumably due to the sustained strong surface winds.
The high sea salt aerosols during the summer months may be impacting on
the strong aerosol reflectance and absorbance signals over the Arabian Sea that are detected by satellite each year.

Effects of monsoons on the seasonal and spatial
distributions of POC and chlorophyll
in the Arabian Sea

Jan S. Gundersen
Wilford D. Gardner
Mary Jo Richardson
All at
Department of Oceanography
Texas A&M University
College Station, Texas
and
Ian D. Walsh
College of Oceanic and Atmospheric Science
Oregon State University
Corvallis, Oregon

ABSTRACT

The Arabian Sea is of special interest because of reversals in the extreme
atmospheric forcing that lead to great seasonal variability. An intensive series of cruises
were conducted in the Northern Arabian Sea as part of the 1995 U.S. Joint Global Ocean
Flux Study. Temporal and spatial variations of POC and chlorophyll were determined
via transmissometers and fluorometers during a monsoonal cycle. Seasonal variations of
the standing stock of POC and chlorophyll were on the same order of magnitude as
spatial variations. The abundance and distribution of POC and chlorophyll varied
throughout the monsoonal cycle but the differences between the northeast and southwest
monsoons were much smaller than expected. Variations in the POC and chlorophyll
were directly related to the biology and were greatly affected by nutrient concentrations
and mixed layer depths. Concentrations throughout the year were high near the coast
and decreased offshore. Horizontal gradients were patchy and were influenced strongly
by the frequent presence of mesoscale features. During the spring intermonsoon, a
subsurface layer of POC and chlorophyll was ubiquitous. This subsurface maximum
would likely be missed by satellite measurements which would, therefore, underestimate
standing stocks during the intermonsoon.

The role of diel variations in mixed-layer depth on the
distribution, variation, and export of carbon and chlorophyll in the Arabian Sea.

Gardner, W. D., J. S. Gundersen, M. J Richardson,

Department of Oceanography
Texas A&M University
College Station, TX

I. D. Walsh.
College of Oceanic and Atmospheric Sciences
Oregon State University
Corvallis, OR

ABSTRACT

The Arabian Sea experiences annual reversals of the wind field which
dramatically alter the biogeochemical cycles of the upper ocean. A major physical factor
in the biogeochemical changes is the variation in the depth of the mixed layer both
spatially and temporally. Variations in the depth of the mixed layer result not only from
local wind mixing and convective cooling, but from regional variations in the wind
stress curl that cause open ocean upwelling and downwelling. During the southwest
monsoon mixed layers start at 10-20 m nearshore and deepen to ~100 m mid basin.
Mixing is active throughout the diel cycle, so particulate organic carbon (POC) and
chlorophyll distributions are also quite uniform. During the spring intermonsoon the
mixed layer is shallow in the southern sector and its diel variation isn't large. Nutrients
become depleted in the surface, but there is a strong subsurface maximum in POC and
chlorophyll. During the northeast monsoon the mixed layer tends to be somewhat
deeper than during the southwest monsoon, especially near the margins. However, there
is usually a clear diel oscillation in the depth of active mixing as solar heating stratifies
the surface water during the day, creating a thin mixing layer; wind stress and convective
cooling create deeper mixing at night. POC and chlorophyll produced in the shallow
mixed layer during the day are mixed downward at night. As a shallow, stratified mixed
layer forms the following day, the biogenic material mixed downward remains at depth
and, being isolated from the active mixing that increases particle residence time in the
surface, settles unimpeded by mixing. Although this "mixed layer pumping" may move
carbon downward during the northeast monsoon, it appears unimportant during the
southwest monsoon when the major flux of carbon occurs. Nevertheless, regular
oscillations in the depth of mixing can be an important forcing function for vertical
exchange in surface waters of any biological or chemical species that has a strong
gradient across the zone of mixing.

Seasonal response of mesozooplankton to monsoonal reversals in the Arabian Sea

Sharon Smith¹, Michael Roman², Karen Wishner³, Marcia Gowing⁴,
Louis Codispoti⁵, Richard Barber⁶, John Marra⁷, Irina Prusova⁸,
Charles Flagg9

1: University of Miami, Rosenstiel School of Marine and Atmospheric
Science, 4600 Rickenbacker Causeway, Miami, Florida, 33149
2: University of Maryland, Horn Point Environmental Laboratory,
Cambridge, Maryland, 21613
3: University of Rhode Island, Graduate School of Oceanography,
Narragansett, Rhode Island, 02882
4: University of California, Institute of Marine Science, Santa Cruz,
California, 95064
5: Old Dominion University, Center for Coastal Physical Oceanography,
Norfolk, Virginia, 23529
6: Duke University, Marine Laboratory, 111 Pivers Island Road,
Beaufort, North Carolina, 28516
7: Columbia University, Lamont Doherty Earth Observatory, Palisades,
New York, 10964
8: Institute of Biology of the Southern Seas, Sevastopol, Ukraine
9: Brookhaven National Laboratory, Oceanographic and Atmospheric
Sciences Division, Upton, New York, 11973

Abstract - The absolute maximum in biomass of epipelagic
mesozooplankton in the entire study was observed during the
Southwest Monsoon season inshore of the Findlater Jet in the area of
upwelling. The greatest contrast between high and low biomass in
the study area was also observed in the Southwest Monsoon season,
as was the strongest onshore-offshore gradient in biomass. Lowest
biomass throughout the study was observed at the most offshore
station, positioned outside the direct influence of the monsoon
forcing. Seasonal peaks in biomass varied depending upon the
subarea of the study region: in the upwelling area (S2, S4) and most
offshore area (S15), the peak was in the Southwest Monsoon season;
offshore of the Findlater Jet (S11) and in the most intensely suboxic
area (N7), the peak was in the Intermonsoon season. Virtually no
diel vertical migration took place in any season at the station with
strong subsurface suboxic conditions (N7), suggesting that these
conditions suppress migration. The greatest day/night contrasts in
biomass were observed nearshore in all seasons, with night-time
biomass exceeding daytime in the Northeast Monsoon season, but
daytime exceeding night-time in the Southwest Monsoon season. The
diel vertical migration patterns in general reversed between the
Monsoons at all stations on the southern line. Based on the
distribution of biomass, we hypothesize that inshore of the
Findlater Jet mesozooplankton grazing on phytoplankton is the
dominant pathway of carbon transformation during both Monsoon
seasons, whereas offshore the mesozooplankton feed primarily on
microplankton or are carnivorous, conditions which result in reduced
carbon flux mediated by the mesozooplankton. In the area of strong
subsurface suboxic conditions, the food web operates like the
offshore area during the Northeast Monsoon, but in the Southwest
Monsoon there is potential for cell sinking to be an important factor
in carbon flux because mesozooplankton biomass remains relatively
low. Predation by mesopelagic fish, primarily myctophids, on
mesozooplankton may equal daily growth of mesozooplankton inshore
of the Findlater Jet during all seasons. This suggests that the food
web inshore of the Findlater Jet is well integrated, may have
evolved during past periods of intensified upwelling, and has a distinctly annual cycle.

Picophytoplankton dynamics and production in the Arabian
Sea during the 1995 Southwest Monsoon

Susan L. Brown¹, Michael R. Landry¹, Richard T. Barber², and Lisa Campbell³

1: Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Rd.,
Honolulu, HI 96822, USA
2: NSOE, Duke University, Beaufort, NC 28516, USA
3: Department of Oceanography, Texas A&M University, College Station, TX 77843, USA

Abstract

Community growth and grazing rates, the dynamics of picophytoplankton
populations, and their contributions to total primary productivity were investigated in in situ
dilution experiments, concurrent with primary productivity experiments, at six stations in
the Arabian Sea during the Southwest Monsoon, 1995 (Cruise TN050; August-
September). Depth-integrated estimates of community growth rates from dilution
experiments ranged from 0.49 d-l offshore to an average of 1.12 d-l at coastal stations.
Growth and grazing rates showed nearly identical trends with depth; however, mortality
due to grazing only accounted for 54% of community growth at high-nutrient coastal
stations as compared with 81% at the oligotrophic stations. Comparisons of growth
estimates from dilution and 14C-uptake experiments yielded different results, with dilution
estimates 2 to 3 times greater at highnutrient stations and at depth. Prochlorococcus was
present at two oligotrophic stations, and its maximum growth approached two doublings
per day. Synechococcus growth rates were highest offshore, exceeding two doublings per
day, and decreasing towards the coast. Depth-integrated growth rates of picoeukaryotic
algae reached 1.29 d-l offshore and decreased to 0.78 d-l at the most coastal station.
Mortality due to grazing averaged 91, 71, and 98% of growth for Prochlorococcus,
Synechococcus, and picoeukaryotes, respectively. Net changes in picoplankton
populations during the 24-h incubations accounted for a significant amount of l4C-uptake,
usually 20-40% of measured rates of primary production. Picoplankton production based
on specific growth rates, however, point to an even greater importance of the picoplankton,
approaching or exceeding 100% of the measured community production. The present
study demonstrates that the picoplankton were a dynamic and important component of the
phytoplankton community in the Arabian Sea during the 1995 Southwest Monsoon;
however, both the importance of component populations and the picoplankton as a whole
varies among stations.

Variability in primary production as observed from moored observations in the
central Arabian Sea in 1995.

J. Marra ¹
T.D. Dickey,²
C. Ho¹
C.S. Kinkade¹
D.E. Sigurdson,²
R. Weller³
R. T. Barber⁴
1: Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964
2: ICESS/Department of Geography, UC, Santa Barbara, CA 93106-3060
3: Woods Hole Oceanographic Institution, Woods Hole, MA 02543
4: Marine Laboratory, Duke University, Beaufort, NC

Abstract- Carbon assimilation was calculated using surface irradiance and fluorescence data
collected from moored sensors with an assumed quantum efficiency, and an independently
estimated phytoplankton absorption coefficient. Fluorescence was calibrated to chlorophyll a.
Estimated primary production (C assimilation) varied seasonally, and was roughly correlated
with chlorophyll biomass. Variations in chlorophyll-specific carbon assimilation occurred as a
result of the magnitude and distribution of pigment in the water column, which affected the
variability in the penetration of irradiance in the euphotic zone. The variation in integral primaryproduction estimated from the moored observations as a function of integral chlorophyll is interpreted in terms of the variations in mixed layer depth and possible losses of chlorophyll
biomass. Deep mixed layers suggest lower chlorophyll-specific production, and variations in
chlorophyll may indicate grazing losses.

Prochlorococcus and Synechococcus growth rates and contributions to production in the Arabian Sea during
the 1995 Southwest and Northeast Monsoons

H. Liu¹, L. Campbell², M. R. Landry¹, H. A. Nolla¹, S. L. Brown¹ and J. Constantinou¹

1: Department of Oceanography, School of Ocean and Earth Science and Technology,
University of Hawaii, Honolulu, HI 96822
2: Department of Oceanography, Texas A&M University, College Station, TX 77843

corresponding author: H. Liu
Department of Oceanography
University of Hawaii
1000 Pope Road, Honolulu, HI 96822

After December 1: Marine Science Institute, The University of Texas at Austin, Port Aransas, TX 78373

Abstract

Growth rates of Prochlorococcus and Synechococcus spp. and their relative
contributions to carbon production were investigated at five stations in the Arabian Sea
during the late Southwest and early Northeast Monsoon seasons in 1995. Estimates of
Prochlorococcus growth rates were based on diel cell cycle analysis. For Synechococcus,
populations with highly variable cell cycle patterns (such as imperfect phasing, multiple
DNA-replication peaks, and dark-arrested division) prevented accurate determination of
the duration of the cell cycle terminal event, e.g., ts+G2. Consequently, growth and
mortality rates of Synechococcus were estimated from diel variations in population
abundance. The assumptions of this approach were validated by observations that
Synechococcus cell division occurred only during the daytime and the good agreement
between the growth rates of Prochlorococcus estimated from this approach and the cell
cycle analyses.
Prochlorococcus growth rates typically less than 1 doubling per day, although growth
rate in excess of one doubling per day occurred in the surface waters at an offshore site
(Stn.N7) during the SW Monsoon and at a coastal station (S2) during the NE Monsoon.
Synechococcus spp. grew much faster than Prochlorococcus in the upper water column at
almost every station during both seasons, but the depth range of its maximum growth rate
was shallower and its growth and abundance decreased sharply in deeper waters.
Maximum growth rates > 2 d-1 were observed at onshore stations during both seasons.
Synechococcus growth rate increased with nutrient availability whereas Prochlorococcus
growth rates did not vary dramatically with nutrient conditions. Although there was no
significant difference in Synechococcus growth rates between the late SW and early NE
Monsoon seasons, the estimated carbon production and relative contribution to primary
production was greater during the early NE Monsoon owing to the larger biomass of
Synechococcus and lower total primary production. Maximum Prochlorococcus
production was found only in the most oligotrophic regions and Prochlorococcus was not
a major contributor of primary production for the most part of the Arabian Sea during the
SW and NE Monsoons. Maximum Synechococcus production occurred in mesotrophic
(nitrate concentration 0.1-3 µM) areas during both SW and NE Monsoon, but decreased
at offshore and coastal stations. Overall, the importance of Prochlorococcus and Synechococcus to primary production was inversely related.

Seasonal variability of bio-optical and physical properties
in the Arabian Sea: October 1994 - October 1995

T. Dickey¹, J. Marra², D.E. Sigurdson¹, R.A. Weller³, C.S. Kinkade²,
S.E. Zedler¹, J.D. Wiggert⁴, and C. Langdon²
1: ICESS/Department of Geography, University of California at Santa
Barbara, Santa Barbara, CA 93106-3060
2: Lamont-Doherty Earth Observatory of Columbia University,
Palisades, NY 10964
3: Woods Hole Oceanographic Institution, Woods Hole, MA 02543
4: NASA Goddard Space Flight Center, Code 971, Greenbelt, MD 20771

Abstract

A mooring instrumented with optical and physical sensors within the
upper 300 m was deployed for two consecutive 6-month periods
(October 15, 1994 through October 20, 1995; sampling intervals of a
few minutes) in the central Arabian Sea (15° 30'N, 61° 30'E). Both
the northeast (NE; November 1, 1994 - February 15, 1995) and
southwest (SW; June 1 - September 15, 1995) monsoons were
observed. During the NE monsoon, wind speeds averaged 6 m s-1
and reached up to 15 m s-1 during the SW monsoon. Intermonsoon
periods (spring, SI: February 16 - May 31, 1995 and fall, FI:
September 16 - October 15, 1995) were characterized by weak and
variable winds. Shortwave radiation and photosynthetically
available radiation (PAR) displayed biannual cycles, peaking during
the intermonsoon periods. Two mixed layer depth definitions have
been used to describe our results. The first is based on a
temperature difference of 0.1°C of the surface temperature, MLD0.1°C
and the second is based on a difference of 1.0°C, MLD1.0°C The
maximum winter mixed layer depth (MLD1.0°Cc~110 m) was deeper
than the summer mixed layer (MLD1.0°C~80 m), primarily because of
surface cooling and convection. A biannual cycle in chlorophyll was
evident with greater values occurring during each monsoon and into
the intermonsoon periods. High chlorophyll values associated with
cool mesoscale features were also apparent during each monsoon.
These mesoscale features and others have been documented using
remotely sensed sea-surface height anomaly maps.
Time series of the 1% light level depth, h1%, tracked the depth-
integrated chlorophyll. In general, h1% was deeper than MLD1.0°C
during the latter half of the spring intermonsoon (low chlorophyll
periods) and shallower than h1% during the latter portions of the
monsoons (high chlorophyll periods). During the SI, the penetrative
components of net solar radiation at the base of the mixed layers,
En(MLD1.0°C) and En(MLD10.1°C), reached values of ~60 and 120 W m-2,
respectively, when the net surface heat flux was 120 W m-2. The
highest mixed layer radiant heating rates occurred during the
intermonsoon periods with peak values greater than 0.25 and 0.15 °C
d-1 for MLD0.1°C and MLD1.0°C, respectively. These values are
consistent with those previously suggested for the central Arabian
Sea. Our results indicate that biological variability is important for
the seasonal variability of the upper ocean heat budget of the central
Arabian Sea.

Advection of upwelled waters in the form of plumes off Oman during the Southwest Monsoon

Vijayakumar Manghnani, John M. Morrison, Thomas S. Hopkins and Emanuele Bohm

Dept. of Marine, Earth and Atmospheric Sciences
North Carolina State University, Raleigh, NC 27695-8208, USA.

Advanced Very High Resolution Radiometer (AVHRR) imagery, Topex/Poseidon (T/P) altimetry, and modeled surface wind stress fields have been used in conjunction with
other ancillary data to describe the influence of the 1995 southwest monsoon on the
distribution of upwelled waters off the coast of Oman. The region offshore of the Oman
coast is characterized by cold plumes extending from the coast into the deep ocean with
apparent disregard for rapid depth changes. The most prominent of these plumes is found
off Ras Madraka. A mechanism behind the development of such an offshore entrainment of
upwelled water is suggested, and validated by observational data. It is proposed that the
location of the plume is primarily governed by the sea level structure away from the coast
and that coastally upwelled water is passively advected offshore through regions of low sea
level. Analysis of the surface wind stress fields show significant spatial variability
associated with the predominantly cyclonic mean curl, with relatively weak curl observed
off the region south of Ras Madraka and north of Ras Merbat. Decomposition of the
surface wind stress fields through Principal Component Analysis shows, at certain periods,
the development of strong along-shore winds and cyclonic curl to the north of Ras
Madraka. Using this information along with concurrent observations from T/P altimetry,
AVHRR derived sea surface temperatures (SST), and surface current measurements, it is
concluded that the combined effect of a strong along-shore wind field and the positive wind
stress curl force a depression in the sea level off the region north of Ras Madraka. The sea
level gradient due to the presence of a sustained high sea level structure to the south of Ras
Madraka, causes geostrophic advection of coastally upwelled waters away from the shelf.
Acoustic Doppler Current Profilers velocity measurements along with SST maps further
prove that the upwelled water was geostrophically advected offshore as opposed to being a
deflection of a wind driven coastal current. Comparison of interannual Sea Level Anomaly
features suggests that the plumes off the Oman coast may not be directly linked to the
coastal topography or bathymetry but are a result of interaction between mesoscale
variations, both in the wind field and the underlying ocean. The strong along-shore winds
and cyclonic curl to the north of Ras Madraka are found to be enhanced when the Findlater
Jet moves closer to the Oman coast than its mean position.

Diel bio-optical variability in the Arabian Sea as observed from moored sensors

C.S. Kinkade¹,
J. Marra¹,
T.D. Dickey ²,
C. Langdon ¹
D.E. Sigurdson ²
R. Weller³
1: Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, 10964
2: ICESS, UC Santa Barbara, CA, 93106
3: Woods Hole Oceanographic Institution, Woods Hole, MA, 02543

Photosynthetically active radiation (PAR), particulate beam attenuation (Cp),
stimulated fluorescence (FLU), and dissolved oxygen (DO2), were collected from
moored sensors in the Arabian Sea from October 1994 to October, 1995, as part of the
Forced Upper Ocean Dynamics Program. Diel bio-optical signals were recorded during
both intermonsoon periods at 10, 35, and 65 meters. Spectral analysis showed significant
diel cycles of Cp, FLU, and DO2 but the strength of these cycles was not constant over
time. Daily periodicity was lowest for all bio-optical signals just after a strong storm
during the Fall Intermonsoon period. During a phytoplankton bloom associated with a
cool advective feature, the FLU and DO2 diel signals were most pronounced. Although
these signals are biological responses to the daily cycle of irradiance, they are mediated
by hydrographic conditions; strongest when phytoplankton are caught within the mixed
layer or stratified water that exposes them to light intensities long enough to display these
responses to PAR.
Because of fluorescence quenching at 10 meters, the ratio of fluorescence to
particulate attenuation showed a diel signal at 10 m, but not at 35m, with the dark period
needed for complete recovery. Diel changes in Cp when scaled to particulate organic
carbon, suggest a net production of 28 and 18 mmol C m-3 d-1 at 10 and 35 meters.

Remotely-sensed sea surface temperature and flow
features in the JGOFS Arabian Sea Process Study
W. Shi, J. M. Morrison, E. Bohm and V. Manghnani

Department of Marine, Earth and Atmospheric Science, North Carolina State University, Raleigh, NC 27695-8208.

Abstract

Sea surface features in the Arabian Sea are described using 1995-96 satellite-derived sea surface temperature data (SST), 1993-1996 TOPEX/Poseidon altimeter data (T/P), Navy Operational Global Atmosphere Prediction System (NOGAPS) model wind data and hydrographic data collected during the JGOFS Arabian Sea Process Study (ASPS) cruises. The SST undergoes a semi-annual oscillation. For the entire basin, the highest SST (31° C) is found in early May. The lowest SST appears during SW monsoon (late summer - early fall) in the upwelling region off the coast of Oman, but further offshore the lowest SST is observed during the NE monsoon (winter).

The wind stress distribution over the basin and SST are highly correlated. During SW monsoon, the SST drop is caused by the coastal upwelling along the Arabian coast where the SST decrease can reach 10° C. In the central basin, the SST drop is due to the evaporation and convection caused by the high monsoonal winds. Even though the wind may is stronger in the central basin than in the upwelling region, the SST decrease is only a few degrees. In the upwelling area near the coast, the SST drop is in phase with the onset of strong SW winds signifying the beginning of SW monsoon, but the increase in SST in the upwelling region generally lags the collapse of the winds at the end of the monsoon. The cold upwelling waters can persist for nearly a month after the SW monsoon is over. "Empirical orthogonal function" (EOF) analysis was caTried out on the SST variation at the location of the ASPS standard station locations. The first mode, which has a 6-month period that reflects the summer cooling in the Arabian Sea, explains 66.7% of the observed variance. The amplitudes of the first mode at each station show a decreasing tendency with increasing offshore distance. Using T/P sea level anomaly data, clear circulation patterns are observed in the surface geostrophic velocities in the Arabian Sea during NE and SW Monsoons.

During the SW monsoon, surface waters circulate clockwise (anticyclonic) with an observed mean northward geostrophic velocity of approximately 5 cm s-1. The longitude where the flow direction changes from northward to southward along the 15.75° N parallel is at the intersection (63° E) between the axis of Findlater Jet and the 15.75° N parallel. During the NE monsoon, the circulation becomes anticlockwise (cyclonic), but the flow is weak and unstable. In addition, surface water motion was studied in the area encompassed by the ASPS standard cruise track as well as in the Omani upwelling region. In summer, the mean flow is out of the upwelling region. For the ASPS region, the mean flow shows a periodical change; it is dominated by the flow across the southern transect. During the summer, the net surface flow is into the region.

The oxygen minimum zone in the Arabian Sea during 1995

John M. Morrison¹, Louis A. Codispoti², Karen Wishner³, Charles Flagg⁴, Wilford D. Gardner⁵, Steve Gaurin², S.W. A. Naqvi⁶, Vijayayakumar Manghnani¹, Linda Prosperie² AND Jan S. Gundersen⁵

1: Depart. of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695-8208, John_Morrison@ncsu.edu
2: Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, VA 23529, lou@ccpo.odu.edu
3: Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, 02882, kwishner@geosunl .gso.uri.edu
4: Oceanographic and Atmospheric Science Division, Brookhaven National Laboratory, Upton, NY 11973, flagg@bnlarm.bnl.gov
5: Depart. of Oceanography, Texas A&M University, College Station, TX 77843-3146, wgardner@ocean.tamu.edu
6: National Institute of Oceanography, Dona-Paula, Goa 403 004, India

Abstract

The paper focuses on the characteristics of the oxygen minimum zone (OMZ) as observed in the Arabian Sea over the complete monsoon cycle of 1995. Dissolved oxygen, nitrite, nitrate and density values are used to delineate the OMZ, as well as identify regions where denitrification is observed. The suboxic conditions within the northern Arabian Sea are documented, as well as biological and chemical consequences of this phenomenon. Overall, the conditions found in the suboxic portion of the water column in the Arabian sea were not greatly different from what has been reported in the literature with respect to oxygen, nitrate and nitrite distributions. Within the main thermocline, portions of the OMZ were found that were suboxic (oxygen less than 4.5 µM) and that contained secondary nitrite maxima with concentrations that sometimes exceeded 4.5 µM, suggesting active nitrate reduction and denitrification. Although there may have been a reduction in the degree of suboxia during the Southwest Monsoon, a dramatic seasonality was not observed as has been suggested by some previous work. In particular, there was not much evidence for the occurrence of secondary nitrite maxima in waters with oxygen concentrations greater than 4.5 µM. Waters in the northern Arabian Sea appear to accumulate larger nitrate deficits due to longer residence times even though the denitrification rate might be lower as evident by the observed lower-nitrite concentrations in the northern part of the basin.

Organism distributions showed strong relationships to the oxygen profiles, especially in locations where the OMZ was pronounced, but the biological responses to the OMZ varied with type of organism. The regional extent of intermediate nepheloid layers in our data correspond well with the region of the secondary nitrite maximum. This is a region of denitrification and the presence and activities of bacteria are assumed to cause the increase in particles. ADCP acoustic backscatter measurements show diel vertical migration of plankton and movement into the OMZ. Daytime acoustic returns from depth were strong, and the dawn sinking and dusk rise of the fauna were obvious. However at night, the remaining biomass in the suboxic zone was so low that no ADCP signal was detectable at these depths. There are at least two groups of zooplankton (used loosely), one that stays in the upper mixed layer and another that makes the daily excursions.

CZCS-derived phytoplankton pigment for 1978 - 1986 and correlations with physical surface flux fields in the Arabian Sea

Karl Banse¹ Danielle M. Bartolacci,², ³David C. English¹, ²
and Mark E. Luther²

1: School of Oceanography, Box 357940, University of Washington, Seattle, WA 98195-7940, U.S.A.
2: Department of Marine Sciences, University of South Florida, 140 Seventh Avenue South, St. Petersburg, FL 33701-5016, U.S.A.
3: 120 Main Street, Apt. 1, Nyack, NY 10960, U.S.A..

Abstract-The pigment observations by NASA's Coastal Zone Color Scanner (CZCS) between late 1978 and mid-1986 over the Arabian Sea north of 10°N, including the outer Gulf of Oman, are depicted as daily means, often only five days apart, for 13 subregions beyond the continental shelves. The data were reprocessed with a more restricted cloud screen than used for NASA's Global Data Set to eliminate bias from electronic overshoot. To elucidate physical mechanisms underlying the large-scale spatial and temporal variability of pigment distribution, monthly pigment means for 11 subregions south of 23°N are correlated with climatological monthly means, as well as those for individual years, of wind pseudostress, wind pseudostress curl, and total heat flux in subregions essentially of the same shape.

The pattern, derived from older in-situ observations, of one bloom during the Southwest (summer) Monsoon almost everywhere and one additional late-winter bloom in the north, is confirmed. The correlation analysis, handicapped by the incomplete or lacking pigment coverage during three months of the summer monsoon, suggests that pigment increases follow the months of physical forcing by a lag of one or possibly two months. The most conspicuous lack of correlation between the physical processes and pigment concentrations is the absence of pigment enhancement in the central Arabian Sea during winter when a deep, cool mixed layer is regularly present. It is unclear whether the bloom induced by the Southwest Monsoon in the open sea lasts for two or four months. The biological reasons for the genesis of blooms are discussed. In this context, the utility of satellite-derived pigment concentrations (as opposed to temporal changes of such pigment) for testing models with more than one phytoplankton compartment is questioned.