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Date: Thu, 7 Dec 1995 18:55:47 +0000 (GMT)

12/7/95 Mid-Cruise Report from R/V THOMPSON Despite a late air shipment, the R/V Thompson sailed on time for the last Arabian Sea Process Cruise. Clear skies, calm seas, and an escort of dolphins got us off to a good start. A school of tuna also put on a show. We're running the stations clockwise and have just completed station 10 ahead of schedule. Results below are mostly limited to stations 1-7. Mike Grogan and Tony Burke have all systems up and running and are working diligently to keep operations on track. Thanks to the planning and experience from previous process cruises, sampling operations are going like clockwork. Everyone is ready when their operation is on deck. There is water for everyone despite having lots of water catchers. Several satellite AVHRR images come in every day from Mark Luther and give us an important perspective of our sampling regime. The chemical and physical hydrographic program (Lou Codispoti, Rob Masserini, Doug Masten, Maureen Noonan, Gene Pillard, Jim Postel, Erik Quiroz, Logan Respess) has gone well so far with no significant equipment problems. The data fit with what we know of the seasonal cycle in the northern Arabian Sea. Mixed layers are fairly deep (50-60 m), and will deepen as winter cooling continues. Nearshore surface nitrate exceeded 3 micromolar at station 2 and ammonium and nitrite were about 0.7 and 0.2 micromolar in the surface layer. Dissolved oxygen was undersaturated at the sea surface suggesting the effect of upwards mixing of high nutrient- and low oxygen-waters. Nutrient concentrations fell and surface oxygen values rose as we proceeded seawards. At station 7, for example, the sum of nitrate, nitrite and ammonium in the surface layer was about 0.2 micromolar. Phosphate and silicate remained well above limiting levels at all stations. At station 7 surface phosphate was ~0.35 micromolar and silicate was about 3 micromolar. On past cruises, the suboxic portion of the main oxygen minimum intensified as we approached the interior of the Arabian Sea with maximum nitrite levels in excess of 5 micromolar. Sporadic occurrences of shallow suboxic zones that occur outside of the main oxygen minimum zone also seem to occur and have been noted on past cruises as well. Complex TS structure is evident in the upper 500 m which is related at least in part, to the outflow of high salinity water from the Persian Gulf and to production of relatively dense waters over the shelves of the northern Arabian Sea. Overall, hydrographic conditions during this leg are very similar to those found on Process Leg 1, when adjusted for the fact that this cruise occurs earlier in the winter. We have hopes, therefore, of piecing together a nice annual cycle when all the JGOFS Arabian Sea data are assimilated. Dan O'Sullivan and Beth Degler have been sampling for total inorganic carbon (TCO2), total alkalinity (TA), pH, and partial pressure of CO2 (pCO2) from the start of the cruise and have taken nearly 200 samples up to station 8. The surface water TCO2 decreased from 2065 +/- 5 umole/kg in the nearshore stations 1&2 to 2037 +/-5 umole/kg. Continuous measurements of pCO2 in surface waters show a steady decrease from 410 ppm to 370 ppm as we move offshore, similar to what was seen on Process 2. In contrast to the gradual decline in surface pCO2 with distance from shore, we encountered a 2-hr stretch where the pCO2 decreased by 45-50 ppm steaming away from station 2. Surface concentrations of DOC (Tye Waterhouse & Liz Caporelli) ranged from 85-90 uM C at stations 1, 2 &4 and 75-80 uM C at station 3, 5-7. Below the mixed layer DOC decreased steadily, reaching a minimum of 40 uM C at approximately 800 m and remained constant to 2000 m, their deepest sampling depth. Shevaun Fennell is running trace metal samples almost continuously, and Susan Gilbertz is sucking in all the aerosols she can take. On the particulates front, one of our Omani observers, Ali Al Harrasi, is doing intensive sampling for POC on the same hydrocasts on which the TAMU group (Jan Gundersen, Rick Morton, Wilf Gardner) is filtering for total particulate matter concentration - PMC. This will provide calibration or the transmissometers, light sccattering sensors (LSS) and ac3 instruments on the CTDs. One ac3 meter, which measures beam attenuation and chlorophyll concentration, is interfaced with a CTD independent of the main rosette and is providing good data for intercomparisons of this new instrument with transmissometers, fluorometers, and chlorophyll. It has also been succesfully tested on the main rosette to 3200 m. A second ac3 is hooked in-line with the ship's continuous-flow seawater in addition to a fluorometer and pCO2 analyzers. A third ac3 is integrated into the TAMU Large Aggregate Profiling System (LAPS), which has made several good profiles. There are far fewer aggregates during this inter-monsoon period that during the SW monsoon. The same is true for small particles and chlorophyll, suggesting they have been grazed, remineralized or settled out of surface waters. Particles and chlorophyll (fluorescence) had maximum values at the surface nearshore, but as we moved across the northern transect the surface values decreased and the maximum was subsurface. During the SW monsoon, subsurface maxima were the norm along this transect. A particle and chlorophyll minimum occurs regularly at the top of the OMZ with a small secondary peak beneath the minimum. This layer was investigated during process 2 and was discussed in John Marra's email report of 3/28/95. Along the nearshore to offshore transect (Stations 2,4,6) David Smith and Grieg Steward found that bacterial abundance in the mixed layer decreased steadily from around 0.8 to 0.5 x 10^9/1 while bacterial production decreased more dramatically from 6-8 down to around 1.5 ugC/l per day. Below the mixed layer, bacterial abundance and production decreased with depth, but varied little between stations. These results are similar to those obtained during the Process 1 cruise in January 1995. The Primary Production Team (Zachary Johnson - Duke, Carol Knudson - LDEO, and Khalfan Al Rashdi - Oman), show integrated production from several deck incubations to be 3-5 gC/m2/d. these values show a remarkable increase over values from just a month ago, and are consistent with measurements last January. Quantum yield has remained relatively constant since Process 6 last month, about 0.015 mol C/mol Ein, but the vertical structure of biomass, as measured by chlorophyll, has become more well-mixed up to station 7. Consequently, nutrients have been entrained and phytoplankton are provided with a better overall light regime. Thus, we have come full cycle in production measurements as started almost a year ago. The primary production measurements are complemented by frequent optics profiles (Dan Sullivan). The microplankton and HPLC chlorophyll group (Lisa Campbell, Sue Brown, Hongbin Liu, Hector Nolla) have measured community growth rates at nearshore stations of approximately 2 divisions /day, which fall between the results from dilution experiments made during the SW monsoon (3d/day) and last winter (1d/day). Nutrient additions did not alter growth rates nearshore. Flow cytometry enumeration at the first 8 stations show cyanobacteria consistently at 10^4 - 10^5/ml; small algae at 10^3 -10^4/ml. Larger (>3 um) algae decreased from 1000/ml to 100/ml as we moved offshore. Marsha Gowing has completed slide preparations of upper water column profiles and reverse flow (60 liters) concentrations at one depth at three intermediate stations for later analysis. She has taken mixed layer phytoplankton tows at all stations. Quick scans of aliquots showed that Rhizosolenia spp. occur along the entire transect, and that Phaeocystis sp. occurred at station 4 and Trichodesmium at station 5. Acantharians dominated the larger protozooplankton along the entire transect. Sharon Smith and Peter Lane report that the plankton of the upper 300 meters at stations 1-4 included the copepods associated with upwelling during the southwest monsoon (Calanoides carinatus, Eucalanus monachus) and the swimming crab Charybdis smithii. These species were found from Somalia to Ras al Hadd during the southwest monsoon season in areas influenced by coastal upwelling. During the intermonsoon season (May) larvae of C. smithii were collected in large numbers by neuston nets from the RV Baldrige; those larvae are now adult crabs of approximately 5 cm carapace width. Paired day/night tows of a 0.25 m^2 MOCNESS suggest much of the mesozooplanktonic biomass is contained in the upper 150 meters day and night. The acoustic Doppler current profiler continues to show a regular diel vertical migration taking place between 300-400m (day depth) and 0-100m (night depth). This pattern has been sustained throughout all cruises (September/October 1994 to present). Tows of the double 1 m^2 MOCNESS suggest that euphausiids and myctophid-like fish may be prominent taxa of the migrating layer. Grazing experiments are conducted at night when the upper 50 m contain adult Eucalanus and Undinula and also migrating copepods such as the genus Pleuromamma. At station 7, in the area of more intense oxygen depletion, the large-bodied copepods near the surface at night were primarily the carnivorous genus, Euchaeta. Finally, on our 4th process cruise, we obtained a successful day/night Big Moc tow series to 1100 m at Sta. 7 (Station with the most intense O2 minimum), report Karen Wishner, Marsha Gowing and Cindy Venn. Some deep samples were obtained there on previous cruises, but never a complete day/night series due to equipment trouble shooting. The faunistic change at the lower oxygen minimum zone (where the very sparse OMZ community is replaced by an abundant and colorful fauna of red shrimp and copepods and black fish) occurred at about 1000-1100 m. There were suggestions of particle layers (numerous spikes in the MOCNESS transmissometer records) just below the depth where the oxygen values start to increase at the lower OMZ interface (just below 1000 m at Sta. 7). Clumps (>20 um) of large bacteria (4 um long) were collected by David Smith and Grieg Steward at the base of the oxygen minimum where the transmissometer showed spikey records, but preliminary viewing of the LAPS videos showed no increase in marine snow-sized particles (>500 um) at that depth. As usual, Captain Glenn Gomes and his merry crew make life at sea enjoyable. They initiated a Christmas tree ornament contest and we'll trim our tree in about ten days. Spirits are high and seas are low. Happy holidays from all of us! Wilford D. Gardner is