Influence of hydrographic conditions on picoplankton distribution in the East China Sea

An investigation was made on picoplankton distributions in relation to physical and chemical conditions in the East China Sea, a marginal sea of the Northwest Pacific, in July 1998. Synechococcus, pico-eukaryotes and heterotrophic bacteria were ubiquitous, with average abundance at the order of 10(4), 10(2) and 10(5) cells ml(-1), respectively. Prochlorococcus was present at most locations beyond the 50 m isobath at around 10(4) Cells ml(-1). Responses of these picoplankters to the hydrographic conditions were evident in both vertical and horizontal distributions. Prochlorococcus were basically associated with oceanic warm currents, and sudden changes in cell abundance often occurred within a relatively short distance of the currents. In the surface mixed layer, Prochlorococcus were usually present only when temperature was >26degreesC, salinity >30 psu, total inorganic nitrogen < 3 muM and phosphate <0.4 muM in the study period. Vertically, however, Prochlorococcus were distributed down where temperature was as low as 16degreesC and nutrient levels were also higher. No pronounced subsurface peaks in Prochlorococcus abundance were recorded in the oceanic warm currents although Prochlorococcus outnumbered Synechococcus by at least an order of magnitude. Synechococcus were most abundant in the coastal area associated with high nutrient levels. Pico-eukaryotes usually developed very well in the front areas on the continental shelf. Along offshore directions, pico-eukaryotes often centered farther from the shore and deeper in the water column than did Synechococcus. Heterotrophic bacteria showed the least variation in abundance among the 4 picoplankters, but still decreased distinctly in offshore directions, following a similar trend in the total biomass of pico-eukaryotes and Synechococcus. In the Yangtze River plume area, light availability was also important in regulating picoplankton distribution patterns. The relationship between Prochlorococcus and bacteria biomass was negative along gradients in the marginal sea.