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Final Report from Chief Scientist

Date: Mon, 21 Aug 1995 15:19:07 +0000 (GMT)

Process Cruise #4 (P4) is over. THOMPSON arrived in Muscat on August 15 after steaming towards Muscat in relatively calm seas. About August 9 there was a break in the SW monsoon giving the final days of P4 a relatively smooth ride. Overall the SW monsoon was weak during P4 with winds that averaged closer to 25 knots than 40 knots; there were two breaks when winds decreased to around 15 knots. wind direction stayed steady from the SW. On the southern line in to the Omani coast we found oceanographic conditions that were the same as those seen by Ken Brink's SeaSoar cruise in July and the NASA aircraft in early July. There was slightly cooler water in the vicinity of the moorings. Inshore, between the moorings and the coast, the water was warmer; just offshore of the shelf break the water was cooled and nutrients were abundant. Like the METEOR we found 20 deg water with 20 micromolar nitrate at the innermost station on the shelf in 80 m of water. We put in a final in situ primary productivity array at the 80 m station and did final incubations for N15, O18, C14 and pigment labeling. The in situ array was a solid string of bottles, but Joe Orchardo gets bottles on the array line right where they belong, so the deployment went smoothly even when all those bottles were ganged up in a 30 m euphotic zone. Productivity at the shelf station was about 2.0 gC/m2/d. METEOR found about 4.0 gC/m2/d, but they were there during a period of stronger winds. The shelf station was dominated by green flagellates during THOMPSON's visit and by the diatom Chaetoceros curvisetus during METEOR's. As always in active coastal upwelling sites, the time-space variability of dominant phytoplankton taxa was dramatic. Overall all P4 was extremely productive for all hands and went smoothly; except for losing the trace metal rosette it went more or less as planned. There were, however, lots of scientific surprises and things we noted with special interest. The SW monsoon was impressive, but really didn't affect operations. The scientific party learned to move carefully and use handholds. The northern Arabian Sea is wonderfully complex in a hydrographic sense. Adjacent profiles are often different with regard to the secondary nitrite maximum, or stepping of the pycnocline or fluorescence in the oxygen minimum. Watching the traces develop on the CTD cast gave one more surprises than watching the soaps. After the predictable patterns that unfold for station after station in the vast Pacific, the Arabian Sea is quite exciting. Other interesting aspects: delta pCO2 was positive throughout the cruise track (i.e., water higher than air) and at the inshore stations ocean pCO2 was twice that of the atmosphere. Surface nitrate was depleted to detection levels on the entire cruise track except for two regions (the inshore stations on the northern line, and the region from the moorings to the inshore end of the southern line). Similarly, surface temperatures were over 27 deg except for the two regions cited above. Despite the warm, low nutrient condition, observed productivity on P4 was higher than that on P1. The following table compares P1 and P4 productivity. Sta# P1(NE) P4(SW) 7 1.1 0.9 13(17) 0.7 1.6 17(13) 1.0 1.6 21(22) 1.2 1.6 24(27) 1.0 1.5 26(29) 1.4 1.6 Data are gC/m2/d for in situ 24-hr incubations. The station numbers are those used in P1 when the JGOFS track was run counterclockwise. Numbers in parentheses are those assigned to the same locations during P4 when we diverted to chase a loose mooring and inserted stations. Other issues: pigments and the flow cytometer showed that there was diversity in the dominant taxa. An interesting Prochlorococcus maximum was present in the anoxic layer at some stations. Oxygen productivity showed a wide range of variation in the ratio of net to gross photosynthesis. Coccolith productivity was a high proportion of total productivity at some stations. Thorium depletion showed that export was very high. The trace metal analyses indicated that there was around a nanomole of dissolved iron in the euphotic zone. DOC was extremely constant in the deep vertical profiles, but surface values showed an upwelling pattern, i.e., low DOC, low temperature, high pCO2, and high nitrate. Bacterial production was very high and showed a strong vertical gradient. About the only property for which we don't have preliminary data is atmospheric deposition. It will be interesting to see that data given that we sampled a weak and fluctuating monsoon. The scorecard for P4 was: 137 CTD casts (including 26 to 2000 m and 6 to the bottom); 50 thorium slurper casts; 19 thorium surface pumps; 13 deployments of the underway fish for trace metals; 12 TM rosette casts and 9 deployments of the in situ primary productivity array. The Omani observers, Ali Al Riyami and Salim Al Suqri, provided valuable help and good company. The marine technicians, Mike Grogan, Tony Burke, Mike Realander and Jim Postel, kept things working and ensured that P4 went smoothly. Capt. Al and the entire crew gave us excellent support. Now the scientific party looks ahead to working on the P4 data. It's a rich resource that will keep us busy for some time. Regards, Dick Barber, Chief Scientist, Process Cruise #4 Richard T. Barber is