High contribution of picophytoplankton to phytoplankton biomass in a shallow, eutrophic coastal sea

We explored picophytoplankton in the surface (0 m) and bottom (2.3-8.7 m) layers of a shallow (<10 m) eutrophic coastal system (Isahaya Bay, Japan). We found that picophytoplankton (principally Synechococcus) constituted the major phytoplankton in spring and summer. The chlorophyll a (chl.a) concentration in the 0.7-2.0-mu m picophytoplankton fraction (hereinafter 'pico-sized chl. a') and picophytoplankton abundance in Isahaya Bay were higher than those in other eutrophic coastal waters. The pico-sized chl. a concentration and the picophytoplankton abundance counted with an epifluorescence microscope was up to 49.31 mu g L-1 and 1.9 x 106 cells mL-1, respectively. Higher contributions of pico-sized chl. a to the total chl. a were evident in summer (up to 63.5%), relative to spring (up to 32.1%), at both depths. Picophytoplankton abundance and the pico-sized chl. a concentration was positively correlated with water temperature and dissolved inorganic phosphorus (DIP) concentrations. Thus, both temperature and DIP may be major controllers of picophytoplankton in Isahaya Bay. The pico-sized chl. a concentration and picophytoplankton cell number at the bottom layer were positively correlated with those in the surface layer, suggesting that picophytoplankton in bottom layers may have sunk from the surface layers. The results imply that the picophytoplankton affects the biogeochemical processes in the bottom of Isahaya Bay more than previously thought. This may be true not only for this estuary but also for other eutrophic coastal seas.

Automated summary

We investigated picophytoplankton in the surface and bottom layers of a shallow eutrophic coastal system in Isahaya Bay, Japan. Picophytoplankton, particularly Synechococcus, dominated in spring and summer. The concentrations and abundance of picophytoplankton in Isahaya Bay were higher compared to other eutrophic coastal waters. Water temperature and dissolved inorganic phosphorus were identified as major controlling factors for picophytoplankton in Isahaya Bay. The presence and distribution of picophytoplankton in the bottom layer suggest a sinking from the surface layer. This study highlights the significant influence of picophytoplankton on biogeochemical processes in Isahaya Bay and potentially other eutrophic coastal seas.