Studies on the small invertebrate plankton of the Sargasso Sea

During the German Eel Expedition in Spring 1979, the horizontal and vertical distribution of the invertebrate plankton was studied in the epipelagic zone of the western central Sargasso Sea, based on 55 µm and 100 µm mesh net samples. In the isothermal waters north of the thermal front, plankton biomass was on average 2–3 times higher than in the warmer stratified waters south of the front. With regard to the fraction of small invertebrates (nauplii and microcopepods) the differences in numerical abundance between the two areas were similar to those reported in the literature for other size ranges of organisms. No divergency was obvious in the plankton composition in terms of major taxonomic groups and size classes. In both parts of the area, organisms smaller than 400 µm, which form a fraction not quantitatively sampled by the conventional 200 µm or 300 µm mesh nets, accounted for 71–92 % of the total number of organisms in the 55 µm net samples and for more than 50 % in the 100 µm net samples. Average concentrations of the potential food supply for early larval fish stages in the upper 100 m appear to be comparable with values reported in the literature for areas well known for larval fish development, such as the California Current.