Process Cruise 5 Update

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Sharon Smith, Chief Scientist
Date: Mon, 28 Aug 95 04:42:43 +0000

We are approaching the southernmost station of the sampling plan (10N, 65E). On the second U.S. JGOFS station, one day out of Muscat, we had a rendezvous with the R/V Knorr (WOCE) where we did companion 2,000m and 3,000m casts and exchanged nutrient standards. Conditions at the offshore end of the northern line were oligotrophic, with the sum of nitrate, nitrite and ammonium in the surface layers being between 0 and 0.15 micromolar. This is in contrast with nearshore stations where surface concentrations were greater than 18 micromolar. The transition was monotonic over the first four stations (Codispoti and the HydroTeam). The phytoplankton assemblages nearshore were dominated by colony-forming diatoms (Chaetoceros cruvisetum, Thalassionema nitzschoides, Rhizosolenia spp.), with large dinoflagellates, Phaeocystis, and tintinnids also conspicuous. In the region of nutrient transition, the phytoplankton became dominated by 1-5um flagellates and small, naked dinoflagellates which were most abundant in a subsurface maximum. Among larger sarcodines, acantharians were most abundant. The conspicuous change in phytoplankton assemblages suggests that on the northern line we sampled a coastal upwelling regime and an oligotrophic oceanic regime (Garrison, Gowing). Primary productivity showed a similar gradient over the four nearshore stations, declining from 9 gC/m2/day to 2.3 gC/m2/day (yes, nine, one of the highest rates ever measured). After the transition to oligotrophic, oceanic conditions, primary productivity was reduced to 0.7 gC/m2/day (Barber, Chai). Growth rates measured by dilution were high nearshore (three divisions per day), half of which was grazed. Nutrient addition did not alter the growth rate. Offshore, however, the growth rate was one division per day, all of which was grazed, and nutrient addition boosted the growth rate (Landry). Concentrations of iron in surface waters are similar to previous measurements, with highest concentrations observed near the coast. Aluminum concentrations are similar to those measured on Process Cruise 2 (Vink). Dust particulates seem to have dropped precipitously compared to previous cruises (Process Cruises 1 and 2), with practically nothing collected during the first 36 hours of continuous sampling (Rosinbaum). The oxygen minimum zone is well developed. Nitrite concentrations in excess of 5 micromolar have been measured within suboxic portions of the oxygen minimum zone where oxygen concentrations are typically less than 2 micromolar (Codispoti and the HydroTeam). Profiles of aggregates of marine snow size (greater than 0.5 mm) show that they are most abundant in the upper water column (10's per liter), and present to 400 m in concentrations greater than 1 per liter. Preliminary analysis of the video records show a local maximum in the oxygen minimum zone around 250 m at some stations. A bottom cast recorded few aggregates below 700 m until a nepheloid layer 150 above the bottom. The optical particle sensors on the CTD have recorded low particle concentrations in the surface mixed layer and a peak on the pycnocline coincident with the fluorescence peak (Walsh). The first day/night pair of MOCNESS tows show that, qualitatively, the deeper zooplankton appear similar in species composition to the collections from previous seasons. There is a clear increase in abundance and some apparently opportunistic species that occur at the base of the oxygen minimum zone. Diel migration of some fish and euphausiids to 300-400 m during the day (well within the oxygen minimum zone) is clearly evident. Upper layer zooplankton nearshore appeared to contain species typical of upwelling areas in other parts of the Arabian Sea, while offshore size and abundance declined (Wishner, Gowing, Smith, Roman). The acoustic Doppler current profiler shows that vertical migration is completed in 1 to 2 hours at sunrise and sunset. In the day, most of the biomass is between 250 and 350 m; between 150 and 250 m in daytime, no currents are being measured because of the lack of scattering plankton and nekton. At night, the ADCP shows that all the biomass is concentrated in the upper 100m (at 100 m, the oxygen concentration is 0.2 ml/l), and biomass is so sparse below 150 m at night that no currents are measured (Smith, Flagg). The winds remain 20 knots gusting to 30 in rain squalls.