The major source of nutrients in both the Gamagyang Bay and the Yeoja Bay is mixing. However, water discharges also seem to contribute significant quantities which may influence local phytoplankton growth. Data collected inn the study area shows that natural fluctuations in the environmental conditions of the entire area play a dominant role in determining the phytoplankton populations of the two Bays. The predominating phytoplankton forms are neritic diatom species including some dinoflagellate species in summer. Neritic groups are relatively more abundant in inner areas of the Bays. The oceanic groups and species are seasonal, and associated with advective effects. One neritic species is of overwhelming numerical importance and occurs when a specific seasonal conditions is formed. Species cycles in the two Bays may be regarded as largely the result of successional changes or cycles of autochthonous species introduced by advective processes.