Between 17 April and 31 May 1990, 81 stations were sampled at or near the transect of the JGOFS North Atlantic Plot Study from 33°N to 60°N at the 20°W meridian with RV Tyro. In the mainly pelagic upper ocean programme 177 CTD/Rosette casts and 93 net tows were completed. In addition several water samples were collected from an inflatable for trace metals, and from the Tyro with single Niskin bottles for algae and bacteria and a large volume sampler for microzooplankton. In the national framework of JGOFS two sediment trap moorings were recovered and redeployed. Two piston cores were collected for the university of Utrecht. An American team from Woods Hole employed in situ pumps for filtration of isotopes. As with leg I in 1989 the 16 scientists on board took charge of most of the JGOFS Level 1 measurements and activities. Additionally there were special studies of coccolithophorid algae, dimethyl sulfite, and slides were prepared by microphotography of conspicuous zooplankton species. The CTD data were recorded in the CTD data system and the SHIPMAN database. There was an intercalibration of corresponding methods applied at the British Charles Darwin and the Tyro on 16 May, but the British data have not been received yet. Preliminary results show also in the spring situation a sharp discontinuity at about 40 °N, to the north of which nutrient concentrations in the upper mixed layer steeply rise. In the south the depth of the mixed layer (to 200 m at 33°N) was remarkably large. Here the primary production per m³ was low, at most 20 mgC near the surface. But because production stretched to about 90 m, per m² it still was about 700 mgC. Only small flagellates were dominant. The thermocline rose going north to about 25 m at 47°N and 20 m at 60°N, hence here the mixed layer was much shallower. The surface production was > 100 mgC per m³ here, and total production about 1500 mgC per m². Bacteria were associated with the chlorophyl peaks, which tended to be at about 50 m in the south and just above the thermocline in the north. Coccolithophorids everywhere were often dominant in number at all latitudes, although in the north the larger diatom chains comprised most of the algal biomass. Zooplankton biomass in the south seemed higher than in 1989. Egg production was on a higher level than in 1989. By the frequent sampling and also continuous recording of surface parameters the horizontal variation in hydrography, productivity and community composition along the south-north transect can eventually be mapped with a high resolution. |
