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This is a message from NOAA Ship Malcolm Baldrige
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Date: May, 1995
Subj: Ortner, Smith & Baars
Arabian Sea Report from RV 'Malcolm Baldrige' (NOAA)
Baldrige departed Colombo, Sri Lanka, on April 27 on schedule.
During our westward transect to Africa, along the southern border
of the Arabian Sea at about 6-7 N, we observed typical tropical
ocean conditions, with very low chlorophyll concentrations in the
mixed layer (about 0.05 mg/m3) and a conspicuous deep chlorophyll
maximum at 70-90 metres.
Off Somalia, we occupied 48 hour stations at 5 N and at 10 N, and
although the Somali Current seemed to have established itself
ready, the SW monsoon winds were still irregular in speed and
direction. The beginning of upwelling was observed at 10 N (Ras
Hafun), where on 7 May we encountered isotherms upsloping towards
the coast, a drop in sea-surface temperature of ca. 1 degree
Celcius, and chlorophyll concentrations of ca. 1 mg/m3.
Measurements of pCO2 (by David Ho for the OACES program ) in the
upper water layer during the transect from Sri Lanka to Somalia
and up the Somali coast revealed relatively constant
concentrations of 390-405 uatmosphere, but at the 10 N station
pCO2 decreased to 345-360 locally in phytoplankton blooms. This
suggests that at low upwelling rates, the uptake of CO2 by
photosynthesis exceeds the vertical flux of CO2-rich deeper water
to the surface. OACES is collecting pCO2, fluorescence and pH
data continuously throughout the GLOBEC cruise. Additional
monitoring of the surface waters includes 13C/12C, 18O/16O,
foraminiferans and >75um phytoplankton (S.Broerse, Free
University/ Amsterdam) and copepod species composition (S.
Smith, RSMAS).
Regular CTD casts have been made on route for salinity, oxygen,
chlorophyll and macro-nutrient concentrations. These same casts
obtain water for a carbon dynamics study (C.Wiebinga,
NIOZ/Texel). Measurements are made of phytoplankton composition
(HPLC, flow cytometry), bacterial enumeration and production (3H-
thymidine and 3H-leucine), dissolved and total organic carbon,
particulate organic carbon and nitrogen, 13C and 18O, and delta
13C, 14C and 15N of particulate organic matter. Additional
physical measurements include the deployment of ARGOS-tracked
lagrangian drifters (M. Bushnell, AOML) and continuous collection
of ADCP data (D. Wilson, AOML) as well as logging the standard
meteorological data.
This being a GLOBEC cruise, the principal activities are
zooplankton and nekton sampling. A variety of gear are regularly
deployed: neuston nets, live Reeve-type nets and NIOZ vertical
nets in the surface and near surface and MOCNESS 1 (1 m2 opening,
150 um mesh size, 8 strata) and MOCNESS 10 (10 m2 opening, 3 mm
meshsize, 4 strata) where possible to 2000 metres (well below the
OMZ). The catches at depth have been particularly worthwhile,
with a diverse composition near Somalia and a number of rare
species. Calanoides carinatus were sampled at depths greater than
previously reported (S. Smith, RSMAS). By the time we reached
the ONR mooring site, diversity of fishes and invertebrates had
declined significantly, and biomass in the very low oxygen waters
was low. During night MOCNESS operations simultaneous tows are
made with neuston nets. Samples contain considerable numbers of
myctophids and other fish, as well as Halobates and masses of
forams and acantharians (K. Hartel, MCZ and M. Baars, NIOZ). At
Somalia 10 N, patches of crab zoea occurred, whereas underway to
the ONR mooring site massive catches of crab megalopa were made.
Most probably these are Charybdis smithii whose annual
reproduction cycle is regulated by the monsoons.
Zooplankton displacement volumes from 50 um vertical net tows (M.
Baars, NIOZ) in the upper 150 m up to and including the Somali
station at 5 N were among the smallest ever recorded in the
northwestern Indian Ocean. The mean displacement volume of the >
300 um fraction was only 5 ml/m2, more than four times lower than
during the NE monsoon. The 50-300 um fraction added only 2 ml/m2
to this value. At 10N the upwelling copepods Calanoides carinatus
and Eucalanus monachus were present in considerable numbers. All
crew members are also enjoying the creatures brought aboard alive
by our divers Larry Madin and Pat Kremer. Some of their
collections may represent unreported species. Diving
observations have revealed a fauna generally similar to that of
the oligotrophic Atlantic, with several species of salps that may
contribute to fecal carbon flux.
Animal distributions at depths to 1500m are being sampled and
digitally recorded continuously on route and on station with hull
mounted acoustic transducers (12 and 100kHz) (P. Ortner and V.
Holliday) and with the ship's ADCP array (150kHz) (S. Smith and
C. Flagg). The migration of the DSL has been evident throughout.
On MOCNESS stations fine-scale distributions (both vertical and
horizontal) are being determined by a towed vehicle providing
realtime display of acoustic (250kHz to 3.0mhz), optical
(backscatter, particle size distribution and fluorescence) and
physical data (P. Ortner). The same system is regularly used for
documenting mesoscale structure by long-term tows at 8kts between
stations. Remarkably acute vertical discontinuities are being
seen at the top of the anoxic layer when it shoals to <150m.
Until the last few days seas have been calm which has enabled our
whale watchers to record over 150 sightings, the most remarkable
being groups of Bottlenosed Whales. This mammal has never been
recorded north of 30 S before (L. Ballance and R. Pittman, SWFC).
Seabird observations total 29 species to date, helping to clarify
the ranges of many species whose dispersal in the Indian Ocean is
poorly known. The most unexpected record was a Streaked
Shearwater, associated with a school of Spinner Dolphins, the
first known sighting of this seabird from the Arabian Sea. The
sightings of Jouanin's Petrels delineate the at sea distribution
of this Indian Ocean endemic, the nesting grounds of which are
still to be discovered (M. Force).
At present we are near the central mooring arrray, where we will
spend several days. The MOCNESS-1 was instrumented with realtime
optical and acoustic sensors and for comparison purposes samples
were obtained at four hour intervals throughout the diel cycle as
close as possible to the similarly instrumented central ONR (P.
Ortner and V. Holliday). Between deployments we moved 10 kms
further from the mooring and used the MOCNESS-10 to sample
greater depths. The shallow MOCNESS tows document a vertical
migration by mesopelagic fish larvae that are distributed
throughout the surface layers during the day but concentrated
just above the anoxic layer at night (E. Clarke, RSMAS).
We will shortly be moving to the JGOFS 'Time Series Station'
16 N, 62 E where we will obtain both night and day MOCNESS and
other net tows and deep and shallow CTDs. Before our departure
a shallow CTD cast was made to obtain oxygen measurements (P.
Kremer) data for comparison with mooring oxygen probes (C.
Langdon). If time and schedules permit, intercomparison CTD
casts might be made with both the RV Meteor, now doing its first
JGOFS leg just north of our position and the RV Thomas G.
Thompson which is also in the area. This may be impossible since
we are committed to surveying the JGOFS Line, the coast of Oman
and to sampling the myctophid populations in the Gulf of Oman.
By an odd coincidence our GLOBEC cruise with its focus on faunal
change might be the first to sample both coastal and open ocean
upwelling during this field season. Upon arrival in Muscat on
May 24, we should have a big reunion-party with the JGOFS crew of
the Thomas G. Thompson arriving a day earlier!
On behalf of all participants of GLOBEC Arabian Sea Leg One
chief-scientist Peter B. Ortner