Phytoplankton communities, with emphasis on picoplankton and nanoplankton, were investigated in Gamak Bay, South Korea, where freshwater input and coastal water intrusion shape ecosystem functions. Shellfish farms and fish farms are located in the inner bay and outer bay, respectively, and tides translocate uneaten food and urine production from aquaculture farms toward the inner bay. Water masses were distinctly different based on a significantly different density between the surface and bottom layer and among three water masses, including the inner bay, outer bay, and Yeosu Harbor. Phytoplankton communities were quantified using flow cytometry and size-fractionated chlorophyll-a (chl-a) was measured. Salinity was a principal variable separating phytoplankton communities between the surface and bottom layer, whereas Si(OH)4 controlled the communities in the inner bay, and NH4 + and PO4 3- governed the outer bay communities. While phycocyanin-containing (PC) cyanobacteria dominated in the outer bay, phycoerythrin-containing (PE) cyanobacteria dominance occurred with cryptophyte dominance, indicating that nutrients affected the distribution of pico- and nanoplankton and that cryptophytes potentially relied on a mixotrophic mode by feeding on PE cyanobacteria. Interestingly, picoeukaryotes and eukaryotes larger than 10 μm were mostly responsible for the ecological niche in the western region of the bay. Given that chl-a levels have historically declined, our study highlights the potential importance of increased small phytoplankton in Gamak Bay. Particularly, we urge an examination of the ecological role of small phytoplankton in the food supply of cultivated marine organisms.

The seasonal variations of picoplankton including Prochlorococcus, Synechococcus and Picoeukayotes around Ulneung Island were investigated by flow cytometry in spring, summer and autumn in 2006. All groups of picoplankton showed clear seasonal patterns in population abundance. Among the group, Synechococcus showed the most prominent seasonal variation during the study period. The maximal abundance of Synechococcus occurred in summer and the lowest in autumn. The seasonal distribution of Prochlorococcus displayed the reverse tendency with that of Synechococcus. The abundance of Prochlorococcus ranged from 2.9×103 cells/ml in summer to 311×103 cells/ml in autumn. However, the seasonal distribution of Picoeukaryotes was shown to be relatively constant, and the maximal abundance was 81.5×103 cells/ml in summer. The highest abundance of Picoeukaryotes occurred in summer and the lowest in autumn and the seasonal distribution in abundance of Picoeukaryotes showed a similar trend with that of Synechococcus. The estimated total carbon biomass of picoplankton were ranged from 74.7 mgC/m2 to 1,055.9 mgC/m2. The highest total carbon biomass occurred in summer, but lowest occurred in autumn. The pattern of the contribution of three picoplankton to total autotrophic picoplankton carbon is different. The contribution of Synechococcus to total autotrophic picoplankton carbon is increased to 75%, but the contribution of Prochlorococcus dropped to 12% in summer. The contribution of Picoeukaryotes is ranged from 24% in summer to 72.5% in spring.