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Second Report from Chief Scientist

Date: Sun, 30 Jul 1995 15:09:41 +0000 (GMT)

Process Cruise #4 (P4) continues to be interesting, and a little different from P1 and P2. THOMPSON completed the northern U.S. JGOFS line and the jog to the east. As we approached the southernmost station of the "eastern jog" Capt. McClenahan and I realized we were at the closest point to Charlie Ericksen's profiling current meter mooring which had broken loose and was reporting its position via ARGOS. I elected to go after the mooring from that position to minimize the time we would spend on the mooring recovery. THOMPSON steamed right to the buoy, no search needed, and hauled it aboard with very little effort. The veteran UW skipper Bill Clampit came out of retirement to sail as Chief Mate on P4. With Capt. Clampit on the fantail and Capt. McClenahan on the bridge we are manned to carry out any kind of work. The sad news is that the profiling current meter with its precious data was not with the surface float. We didn't get the data, but Charlie can rest a little easier now knowing that his current meter rests with King Neptune. After the float recovery we set a schedule with the METEOR for an intercalibration station at 14deg27'N and 65deg00'E at the eastern end of the southern U.S. JGOFS line. Running the JGOFS box clockwise this station is #17, but counterclockwise it is #11, and on P4 it is #13. As veterans of P1 and P2 know, the U.S. JGOFS stations have no permanent names or numbers. On each process cruise, each station is formally assigned a consecutive number, and since the actual stations occupied on the U.S. JGOFS effort have varied on P1, P2 and P4, station locations towards the end of the cruise have different numbers. Lou Codispoti says to refer to latitude and longitude. So, at 14deg27'N and 65deg00'E we carried out an intercalibration with METEOR. The Chief Scientist, Bernt Zeitschel, arranged for labeled leucine and thymidine to be passed to us, so Helen Quinby and Gary Schultz were able to begin their work on bacterial production. For the intercalibration, we each made a cast to 2000 m and exchanged water from 10, 80, and 2000 m. Intercalibration was done on salinity, oxygen (fixed), nutrients, total inorganic carbon, alkalinity, chlorophyll, HPLC pigments, bacterial counts, trace metals, aerosols, calcite, DOC and POC. Thorium comparisons will be made, but no water was exchanged. In situ primary productivity measurements were done by the U.S. and German teams for comparisons. THOMPSON deployed the prim. prod. in situ array at 0400 with the lights of METEOR blazing in the darkness (at 3 miles distance); METEOR picked up her array at 0600. The in situ incubations are a day apart, but will still be an interesting comparison. In addition, the nutrient labs exchanged standards and oxygen reagents and the CO2 labs exchanged Dickson standards. The transfer of standards, reagents, water, and a few gifts was carried out using the METEOR's inflatable. Bernt Zeitschel, Chief Sci. of METEOR cruise M32/5 and Klaus von Broekel, deputy Chief Sci., came over to THOMPSON with the samples. They displayed both courage and agility getting in and out of the plunging inflatable without mishap. They had a tour of the THOMPSON and lunch and met most of the scientific party. The intercalibration methods and comparison of data will be of great value to JGOFS. Bernt Zeitschel was actually visiting THOMPSON in a second capacity. Bernt chairs the international JGOFS Executive Council, so he was also making a field inspection of his JGOFS troops. His visit was both fun and profitable for the THOMPSON'S scientific party. THOMPSON is now at the southernmost station at 10degN snf 65degE. We have seen three oceanic regimes. Within sight of the Omani coast we encountered cool (23.3 deg) water, high nutrients, high productivity, and high chlorophyll. Oxygen and carbon net productivity values were the same at about 4.0 gC/m2/day. The rest of the northern U.S. JGOFS line graded to more or less typical oligotrophic waters with 27 deg water and nutrient depletion. On the eastern jog out to 15deg23'N, 68deg45'E, the water was warm and the mixed layer got deeper and deeper as we went east. The mixed layer contained a lot of particulate matter, but Dickson and Orchardo found close to zero net productivity using oxygen. Their interpretation is that we were observing a post-bloom crash. Helen Quinby and Gary Schultz carried out incubation experiments to obtain some growth rate data using AODC and DOC analyses done by Ed Peltzer and Nancy Hayward. The bacterial abundance was about 1x10^6 per ml in the mixed layer and declined by 90% in the strong O2 minimum layer. On the jog to the east the O2 minimum layer had a secondary fluorescence maximum that Ralf Goericke and Amy Shankle found to be an almost pure population of Prochlorococcus. As in P1 and P2, the population was in a 10-m thick layer just below the depth where the oxygen electrode shows zero oxygen. On the eastern jog a bottle was tripped in the layer at several stations providing samples for Rob Olson and Alexi Shalapyonok as well as the pigment team. The eastern jog stations have proved to be interesting; they do not have a typical tropical structure, yet nutrients are depleted. The southern jog along 65E from 14deg23'N to 10degN had more typical tropical ocean conditions, similar to the northern JGOFS line. We were surprised by the persistent and widespread oligotrophy during steady SW monsoon winds. Nutrients in the mixed layer were higher over the northern and eastern segments of the JGOFS box in P1 during the NE monsoon than they are now during the SW. We expect that there will be more action on the southern line in regard to mixing and upwelling. We are repeatedly impressed by the high zooplankton abundance. Ken Buessler has seen lots on his in-line filters and the on-deck incubator lines have clogged coontinually on big crustacea. The sea state is moderate, official weather observers on METEOR said the waves were 4 to 6 ft during our transfers. That doesn't seem very impressive, but it does mean that Bernt Zeitschel was jumping into a boat that was moving up and down about 5 ft. The small boat handling by the METEOR and THOMPSON crews was excellent during our transfers. As we start the southern line, all hands (at least all hands in the scientific party) hope we'll see some upwelling. Two weeks of moderate winds and seas have wetted our appetites for some real SW monsoon conditions. Regards, Dick Barber Richard T. Barber is