Date: Wed, 11 Feb 1998 13:13:35 -0500

Final Report JGOFS/APFZ Survey II R/V Revelle January 8-February 8, 1998 Kenneth Coale, Chief Scientist In the report which follows, preliminary results from the APFZ, Survey II cruise are presented. As such, none of the data mentioned herein are final, but are offered here to give others a first glimpse of the APFZ and a transect to near the ice edge during January, a flavor of the hydrography and a taste of some of the more interesting findings during this cruise. The general purpose of this cruise was many-fold with these top three priorities 1) Obtain two detailed surveys of the APFZ using a towed undulating fish (SeaSoar) equipped with a variety of instrumentation with each survey separated by about 5 days, 2) Conduct a series of vertical profile stations spanning the range in hydrographic variation from Subantarctic to Antarctic waters and 3) Make a series of measurements and observations to provide continuity between the Process I and II cruises and a comparison to the Survey I results. For these reasons, the following cruise plan was executed. The ship steamed from Lyttelton directly towards the northerly part of our transect, a station at 57oS, 170o6' W (the N/S transect was offset by 6 min in order to avoid optical and sediment trap moorings along 170oW). In transit to 57oS, a test station at 53oS (Station 1) was performed to test equipment and rinse bottles. The SeaSoar was deployed at 57oS (Station 2) and towed at 7 knots until it was recovered at 67o 40' S, an estimated 60 miles from the ice edge in 0.5oC surface water. The SeaSoar provided a detailed two dimensional cross section of the upper 400 m of the water column for the following parameters: two wavelengths of fluorescence (Flashpacks), nine wavelengths of absorption and attenuation (AC9s) PAR, temperature, salinity and particle density (OPC). In addition, underway measurements of pCO2, fluorescence, pigments, nutrients, iron, aluminum, and all IMAT parameters allowed for the identification of a large chlorophyll maximum (pCO2 minimum) centered at 64o 50'S. From this initial survey together with the results from Survey I and Process I, we identified this chlorophyll maximum as a bloom feature which was increasing in magnitude and moving south, concomitant with the retreat of the ice edge. Since the surface water manifestation of the APF was weak with temperatures decreasing relatively continuously along the transect from 57oS to 67o 40'S, we decided to site our southerly vertical profiles to characterize pre-bloom waters (67o 40'S, Station 3), blooming waters (64o 50'S, Station 4) and post-bloom waters (62oS, Station 5). These comprised the first three of six detailed vertical profile stations. The first of the SeaSoar grids was then mapped out over the next 4 1/2 days. This grid was towed from 60oS to 61o30' S between 172o and 169o W, an area in which optical moorings were deployed during Survey I. In addition, four optical drifters and five velocity drifters were deployed in the core of the APF, in the western side of the grid, during this first SeaSoar survey. Based on the biological and hydrographic parameters mapped during this first SeaSoar survey, the next three stations were chosen. Station 6 was located at 60oS, 170o6' W, a location characteristic of the waters to the north of the APFZ with a very weak subsurface chlorophyll maximum and very low silicate in the surface waters. Station 7 was located at 61oS, 170o6' W, a location characterized by a developing subsurface chlorophyll maximum and a weak surface expression of enhanced fluorescence. We then steamed south at full speed, mapping all underway parameters in a transect through the bloom, turned around and held Station 8 again at the bloom maximum, now at 65o 10' S. A full station was completed again in this bloom feature and provided an opportunity to again map its N/S surface expression and deploy a fifth optical drifter buoy at this location prior to commencing the second SeaSoar survey. The second SeaSoar grid mimicked the first in location and duration. Another 4 1/2 days were spent in the same vicinity to allow for the description of the temporal development of features in this dynamic region. Following a make-up Mocness tow at midnight on February 3, the Revelle departed the APFZ for Lyttelton with all surface water analyses still underway. Throughout this cruise, I have been impressed by the fine attitude, high spirits and professionalism which has characterized both the scientists participating in, and the crew who made possible, this very productive cruise. We have accomplished all of our scientific objectives, lost no time to weather, lost no equipment, stayed on schedule and have some very unique results to show for it. As Chief Scientist, it has been a privilege to serve such a talented and dedicated group. The following are summaries submitted by the participating groups. S-242 Sea-Soar Grids and Transects Chris Wingard, Andrew Dale, Kipp Shearman, Robert O'Malley, Marc Willis, Linda Fayler Oregon State University Sea-Soar measurements began over the period 12-16 Jan with a transect some 1200 km in length along 170W from 57S to near 68S. Evolution of the density field since Survey I (Oct 25-26 '97) was most marked in the establishment of a warm, less saline surface mixed layer. North of the Antarctic Polar Front (APF) this layer was around 50m thick, decreasing to 30m south of the front and shoaling further to around 20m at the southern end of the transect. Sea surface temperatures were between 3 and 5 degrees warmer than during Survey I, and the surface manifestation of the APF was less clear than previously. Current measurements from the ship's ADCP showed a weaker and less localized frontal jet with largely barotropic eastward currents of up to around 25cm/s spread over approximately 1 degree of latitude between 60S and 61S. Jets of similar magnitude were observed elsewhere in the transect, so the APF jet was not a clearly dominant feature of the circulation. The two flashpak fluorometers and AC9 spectrophotometer showed high chlorophyll in the surface waters between 64S and 65S as well as within the thermocline to the north and south, extending north as far as the APF and decreasing to the south. Two five day grid surveys (20-25 Jan and 29 Jan-3 Feb) examined the mesoscale structure of the APF between latitudes of 60S and 61.5S by means of north-south sections spaced by approximately 20km. Again, the frontal jet was found to be a broad eastward flow, with no distinct core. Whilst there was variation between adjacent north-south grid lines, there was no clear large scale meandering, although the second grid showed a weak meander to the north. Throughout both surveys there was evidence of the subduction of parcels of cold water along isopycnals from the cold tongue at around 100m depth south of the front. The fluorometers and AC9 showed chlorophyll levels that were low compared to those further south, with greatest values occurring in the thermocline, especially where it overlay cold water south of the front. S-214 Carbon Dioxide Stephany Rubin, Rebecca Esmay LDEO of Columbia University The objectives of the carbon dioxide group on the JGOFS APFZ Survey II cruise fall into two categories: 1)underway /surface water sampling and 2) discrete water sampling while on station. To accomplish these goals, four carbon dioxide systems were set up to measure - 1) underway pCO2, 2) underway TCO2, 3) discrete pCO2 at 4 deg C, and 4) discrete TCO2. The underway systems were activated upon leaving the harbor of Lyttleton. The discrete systems were activated shortly thereafter. Throughout the cruise, discrete surface samples for both pCO2 and TCO2 were taken from the continuous flow-through system while underway. These results and those for the underway TCO2 system will not be discussed at this time. 1) Continuous underway pCO2: a) North - South Transits: On the first transit south along 170.1 deg S, the surface pCO2's of ~352 uatm observed at 57 deg S on 12Jan98 (local) were close to the atmospheric value of ~350.8 uatm (= 360.9 ppm * 985 mbar /1013.25 mbar). As we headed south, the surface pCO2's slowly dropped off to ~315 uatm at 62 deg S on 14 Jan98. This was followed by a precipitous drop to ~240 uatm within in the next 1.5 degrees. A pCO2 minimum was observed on 15Jan98 between 64.8-64.85 with values around 228 uatm. This was coincident with the observed chlorophyll maximum and considered to be the bloom "max." The pCO2 quickly rose back up to 270-280 uatm at 65.5 deg S and stayed there for the next 1.25 degrees. At ~66.75 deg S the pCO2's rapidly rose to ~340 uatm by 67 deg S on 16 Jan 98. The return transit began on 17 Jan 98 after occupying a 24 hour station at 67.784 deg S. The surface pCO2 values were unchanged from values observed on the transit south until ~65.3 deg S reached late on 17 Jan 98. Between ~65.3 deg S and ~63.2 deg S, the surface pCO2 values on average were ~25 uatm higher than those observed only 2-4 days earlier despite the temperature being ~1 deg C colder. Between ~63.2 and ~62.2 deg S the pCO2 values for the northern transect were consistent with those observed earlier on the run south. When the track was reoccupied approximately one week later, the surface pCO2 values, between 66 deg S and ~62.25 deg S, were on average another 20 uatm greater than observed on the northbound leg. b) SeaSoar Grids: On average, the pCO2's at SST are 10 uatm higher during the second tow. In both cases, there is a fairly strong gradient in the pCO2 at SST just south of 60 deg S. This is coincident with the location of the Polar Front as indicated by the SST. The pCO2's calculated at 4 deg C for both tows show a local maximum between 60.5 and 61 deg S and again around 61.5 deg S, in agreement with observed local minima in the chlorophyll data.2) Discrete pCO2 and TCO2 at stations: Discrete pCO2 at 4 deg C and TCO2 measurements were made on samples collected from the CTD casts at all stations. The three stations south of 63 deg S have similar profiles from 60m downwards for both pCO2 and TCO2. The three stations north of 63 deg S have similar pCO2 and TCO2 profiles down to 60m. In terms of pre-bloom, bloom and post-bloom stations, station 3 at 67.784 deg S (pre-bloom) has the highest TCO2 concentrations and pCO2 values in the upper 40m of the water column. The two bloom stations, #4 at 64.833 deg S and #8 at 65.167 deg S, show the lowest TCO2 concentrations and pCO2 values in the upper 20m of the water column. The three post-bloom stations, including station 6 north of the Polar Front, all show similar TCO2 and pCO2 concentrations down to 60m, with the values being closest to those of station 8. The mixed layer in terms of CO2 is deepest at these stations, being down to 60m, whereas at the bloom station it was only 20m and at the pre-bloom station it was even shallower. Since the water masses of the stations south of 63 deg S are different from those to the north, based on the T-S diagrams, no interpretation of the profiles below the mixed layer will be made at this time. S-213Total Organic CarbonGreg Eicheid Woods Hole Oceanographic Institution TOC underway samples were drawn at the beginning of the first transect south at the rate of one sample every 2 hours. This continued during SeaSoar grids. A total of ~250 underway samples were taken and analyzed with the last 70 samples being collected, frozen, and prepared for analysis at home. Samples were also collected from all depths at each regular CTD cast, a total of 106 CTD samples were analyzed during the cruise. Surface TOC values post bloom were in the 40-50 umC range, bloom values were highly variable with highs in the 50-60 umC range, pre-bloom values were similar to post-bloom values. All other surface values were 40-50 umC, except for close in to the NZ coast where highs of 75 umC were observed. Not many deep samples were analyzed, but those that were available showed a typically low value 30-40 umC. S-212 Optical Properties/FRR Fluorometry Claudia Mengelt and Ricardo Letelier Oregon State University During the SURVEY-II leg the group S-212 had four main tasks: 1) to deploy optical and velocity drifters to monitor sea surface water mass displacements and optical properties along the Antarctic Polar front, 2) to deploy a Tethered Spectroradiometer buoy whenever possible to monitor ocean color, reflectance, and natural fluorescence, 3) to obtain photosynthetic parameters of the phytoplankton communities through the use of fast repetition rate (FRR) fluorometer, and 4) to collect pigment samples in support of the optical mooring array data. All drifters but one were deployed on January 21 and 22 at approximately 61 degrees South and 171 Degrees West. Before loosing e-mail contact there track indicate displacement toward East-North East. These tracks seem to follow the contours of areas of high subsurface chlorophyll, as mapped by the SeaSoar fluorometers. The last optical drifter was deployed on January 29 at approximately 65 degrees South and 170 degrees West to track the evolution of the bloom. Although we know that the drifter is transmitting we have not yet received the data from Oregon State University. The Tethered Spectroradiometer Buoy was deployed at every Station and the results will be analyzed back at Oregon State University. Because of the many problems that plagued the Fast repetition rate fluorometer's performance, the record obtained with this instrument is less complete than originally expected. However, we were able to obtain a complete transect record across the Bloom area, as well as three depth profiles, one in the bloom area. Preliminary analyses of the data suggest that in none of the areas surveyed phytoplankton assemblages are able to reach maximum photosynthetic efficiencies (as derived from the estimation of the maximum quantum yield efficiency of photosynthesis). During the transect the maximum quantum yield reached its minimum value (< 0.1) in the bloom area. The values increased both toward the South and the North of the bloom area. With respect to the photosystem II (PSII) absorption cross section, our preliminary results do not indicate a consistent spatial pattern. However, the analyses of iron enrichment experiments performed during the cruise (K. Coale's group) suggest that the absorption cross section increases significantly upon additions of this micronutrient. The photosynthetic apparatus response to iron enrichment (changes in photosynthesis quantum yield and absorption cross section) is detectable after 3 hours of incubation in the dark, and appears to reach its maximum expression after 15 hours. Samples for HPLC pigment analyses were routinely collected during the grid surveys and the bloom transects. Furthermore, 0-200 profile samples were also collected at every station. These samples will be analyzed at Oregon State University during the first half of 1998. S-261, S-262 Optical profiling, Transmissommetry, fluorometry John Wieland, Berzas Bichnevicius Scripps Institution of Oceanography The optical data accumulated on our deployments requires intensive review using complex mathematical algorithms and queries. Because of the failure of e-mail communications, results from the optical MER data sent back to Scripps have not been available. The real time data received during our casts, from our CTD, transmissometers, fluorometer, did however correspond with the data received from the CTD deployments directly pre-ceding or following our casts. Most of our lab work/filtration i.e. AP, AS, POC, HPLC, also require further analysis back at the lab. Extrapolation of our particle counts revealed the following:Particle size: No strong differentiation in particle size between our sampling areas was observed but overall, pre-bloom and bloom waters saw the largest particle sizes. We also observed distinct speciation occurring in the 2.0-3.0 um size range in these pre-bloom and bloom samples above the euphotic zone. Further taxonomic and HPLC analysis will determine the species present here. Particle Number: We saw a higher particle number in pre-bloom and bloom waters. A significant difference when compared to post-bloom samples which showed much lower counts. The highest particle counts were recorded during our first occupation of the bloom at station 004. Conversely, we saw a significant decrease in particle number during our re-occupation of the bloom during station 008. Counts seen in 008 samples were similar to those observed during our post-bloom measurements. We did not see a significant change in particle size between the two bloom stations, however, the absence of distinct speciation in the samples accumulated during our bloom re-occupation was noticed. S-255 P vs E, chlorophyll, pigments, POC, PON, species composition Sarah Goldthwait Bermuda Biological Station for Research Chlorophyll profiles were established at each of the 8 stations. In addition discrete chlorophyll samples were collected at varying intervals to calibrate the underway system. Chlorophyll levels peaked at the bloom site (65 S) reaching 2.7 ug/l on our first visit (St. 4) and 1.7 ug/l on our second visit (St. 8) with fairly uniform distribution from the surface down to about 30 m. It appeared that the bloom may have moved south during station 8 as values dropped from 1.7 ug/l to 0.8 ug/l within about 7 hours. South of the bloom (St. 3) chlorophyll was about 0.9 ug/l in the surface waters reaching a maximum of 1.5 ug/l at 20 m. The northern post-bloom stations showed a subduction of the bloom to 80 m with peak values of only 0.4 ug/l. Underway data and calibration samples showed little variation during both Sea-Soar grids with most samples falling between 0.2-0.35 ug/l on the first grid and 0.1-0.2 ug/l on the second grid. Phytoplankton slides/preserved samples, HPLC, AP, and POC/PON samples were also collected. P vs. E incubations conducted at each station. S-249 Microbial Communities Mike Landry, Susan Brown, Scott Nunnery University of Hawaii The microplankton team of Mike Landry, Sue Brown and Scott Nunnery investigated plankton community structure, phytoplankton growth, and microzooplankton grazing using a combination of descriptive (shipboard flow cytometry and video image analysis microscopy) and experimental methods (dilution and FLB uptake). The community in the area of the physical front was dominated by small picophytoplankton (mostly Prymesiophytes) and was relatively featureless in hourly mixed-layer sampling collected during the SeaSoar grids. The bloom region south of the front was rich in large centric diatoms, with many of the slides showing an intense network of mucus strands or threads suggestive of aggregation processes. Further south of the bloom site, but before the ice edge, were areas of greater importance of pennate diatoms and the motile form of Phaeocystis. Phaeocystis colonies were not observed in any numbers anywhere during this cruise. Phytoplankton growth rates varied from about 0.2 to 0.7 day^-1, averaging 0.3 day^-1. Microzooplankton grazing rates varied from 0.1 to 0.6 day^-1, averaging 0.2 day^-1. There were rate differences among stations, but no clear north-south pattern. Specific growth and grazing rates in the vicinity of the bloom were about comparable to cruise averages. S-225 Dissolved Iron and Aluminum Sue Vink, Kendra McDonough University of Hawaii Fe and Al concentrations determined in surface waters on station and underway from the fish during Survey 2 showed similar trends over the study area. Overall Fe concentrations varied between 0.1nM in the region of the bloom (63-65o S), increasing to 0.3 nM at the southernmost station. A small Fe concentration maximum (0.19nM) was also observed in the region of 62oS. Within the region of the polar front Fe concentrations averaged 0.15nM. Al concentrations varied between 0.3-0.6nM with minimum concentrations in the region of the bloom and maximum concentrations to the South. Concentrations of both elements determined during Survey 2 were similar to those determined during Process 1. S-219 Trace Elements in the APFZ Kenneth Coale, Steve Fitzwater, Mike Gordon, John Haskins Moss Landing Marine Laboratories The goal of our research is to determine the concentrations and distributions of a wide suite of the transition elements both in the dissolved and particulate phases. We seek to understand the processes which give rise to both the concentrations and distributions of these elements and to identify the extent to which some elements may control phytoplankton growth and biomass in these HNLC waters. Because of our recent findings both in the equatorial Pacific and the Ross Sea, our current studies in the APFZ have focused on iron. Using 30 liter Go Flo bottles deployed on kevlar hydrowire, the MLML group collected both dissolved and particulate samples from vertical profiles at each station (surface to 500m) and from a horizontal surface water transect through the bloom. To understand the roll or iron in the pre-bloom and post-bloom waters and determine the extent of iron limitation in these regions, two iron enrichment experiments were conducted. These involved filling 8, 20 liter polycarbonate carboys with water from the mixed layer at station 3 (pre-bloom) and again at station 5 (post-bloom). From each series, two carboys were used as controls (nothing added) and the others were treated with 0.2 nM Fe, 0.5 nM Fe, 1 nM Fe, 2.5 nM Fe, 2.5 nM Fe + 5 nM Zn and 5 nM Zn. Subsamples from each carboys were drawn every other day with aliquots analyzed for nutrients, species composition, POC, DOC, PON, chlorophyll, and silicate uptake rates. In addition, initial samples for trace metal analysis were drawn from every carboy. Since the samples collected for dissolved and particulate trace elements will be analyzed ashore, no data from these samples is yet available. Initial chlorophyll and nutrient concentrations from the enrichment experiments, however, do indicate iron limitation in these waters. The enrichments initiated south of the bloom (pre-bloom) showed the most dramatic effects with final chlorophyll in the iron treatments reaching 13 micrograms per liter relative to 2 micrograms per liter in the controls. As with the equatorial Pacific and the Ross Sea, this response seems to saturate at about 1 nM Fe. In contrast to the previous Ross Sea results, no enhancement in growth was seen in samples receiving Zn. In the enrichment experiment initiated north of the bloom (post bloom) only 2 micrograms per liter chlorophyll was reached in the iron treatments whereas controls only yielded 0.5. Although there is a much enhanced effect for iron enrichment in waters containing high silicate, both regions showed evidence of iron limitation. Another major difference between the pre-bloom and post-bloom waters were the high numbers of copepod nauplii and pteropods in the post-bloom waters. We believe that grazing pressure in these samples may have consumed some of the phytoplankton produced via iron enrichment, thus the apparent effect was suppressed. Nutrients were consumed at different ratios depending upon whether iron was added or not. In the controls and in the treatments receiving only Zn, the ratio of N:Si drawn down was about 1:2. In samples receiving iron, however, this ratio was 1:1 indicating that rates of nitrate uptake are greater when iron is present. Iron limitation may, therefore, account for some of the low draw down ratios of N:Si seen in the bloom and the high N:Si ratios remaining in post-bloom waters. Subsamples from the enrichment experiments were distributed to other investigators looking at isotopic fractionation (M. Altabet); silicate uptake and dissolution (V. Franck), Fast Repetition Rate Fluorometry (C. Mengelt and R. Letelier) and Cadmium:Phosphate ratios in particles. S-236Stable Isotopes (N-15) Mark AltabetUniversity of Massachusetts The goal of this project is to characterize and understand the processes controlling nitrogen and carbon isotopic ratio in the Southern Ocean. Considering that these measurements need to be done at a shore-based laboratory, the principal at-sea activity is to collect samples for transfer to the isotope lab at U. Mass. Dartmouth. Samples were collected underway approximately every 0.5 deg. of latitude. Particulate samples were collected in size fractions of >100, 100 to 10, >3, and 3 to 0.7 microns. There were two modes of collection; small volume in which 10 to 40 l were filtered and large volume in which 500 to 3000 l were filtered. Filtrate samples for isotopic analysis of nitrate and DOM were also collected. At each station, hydrocasts were made to 300 m and a similar sample suite (small volume filtrations only) was also collected at 12 depths. At some stations samples for isotopic analysis of dissolved ammonium were collected. Visually, the bloom stations were obvious in having high amounts of >100 micron material of phytoplankton origin. Carboy incubations were carried out at several stations to measure the isotopic fractionation effect occurring with the drawdown of nitrate. There was generally rapid utilization of nutrients and accumulation of large sized phytoplankton biomass. This occurred whether Fe was added or not. However, I suspect our controls had incidental Fe addition. Station 5 incubations did not respond, though, until silicate was added to stimulate the growth of diatoms. This apparent post-bloom station had very low ambient silicate concentrations. S-234/S-235 Biogeochemical Silicon Cycling Bill Golden, Valerie Franck University of California, Santa Barbara The purpose of our involvement in JGOFS Survey II was to assess the biogeochemical cycling of silicon in the Antarctic Polar Front during the summer bloom; our measurements included dissolved silicon (silicic acid) concentrations, the kinetics of silicic acid uptake, the production of biogenic silica and its dissolution, and natural variations in the ratio of stable silicon isotopes in both silicic acid and biogenic silica. (The ratio of 30Si:28Si increases in dissolved silicon as diatoms preferentially take up the lighter isotope.) During the initial north to south transect along 170 W, starting at 57 S and for every half degree latitude thereafter we collected surface water samples to measure dissolved Si concentrations, particulate Si (biogenic and lithogenic), Si uptake rates at ambient and enhanced Si (using the radiotracer 32Si), and variations in the natural abundance of 28Si and 30Si (mass spectrometry). Our station work involved measuring rates of dissolved Si uptake and particulate Si dissolution throughout the water column (8 light levels). In addition to measuring Si uptake rates at ambient Si concentrations, we conducted similar experiments to measure Si uptake rates after adding 0.5-50uM Si to water from several different light levels, including a full kinetic curve at 50% surface light. In a separate set of experiments, Valerie Franck, working together with the Moss Landing Marine Laboratory group lead by Dr. Kenneth Coale, set out to investigate the effect of iron and zinc on silicon uptake rates over time. After the initial Fe/Zn enrichments were set up at Stations 3 and 5, samples were drawn at discrete time points for several weeks from all 8 treatments. Samples were taken daily to measure dissolved silicon, and, less frequently, to measure particulate silicon (both biogenic and lithogenic) and silicon uptake rates both at ambient and enhanced Si concentrations (+50uM). In addition, near the end of each experiment, samples were also taken to measure 15N uptake in order to assess changes in initial and final Si:N uptake ratios with enhanced Fe and Zn. S-249 Thorium Scavenging/Proxy for Carbon Export David Hirschberg, Huan Feng State University of New York Measurements of dissolved and particulate 234Th were made in surface water along the 170W transit from 58S to 66S, and at the six major hydrographic stations. Preliminary data indicate substantial 234Th removal at all stations with the greatest removal around thearea of the plankton bloom, about 170W 65S, where the 234Th was reduced to about 25% of its equilibrium value.