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Diatom densities in the surface water and dinoflagellate cysts in bottom sediments of Gwangyang Bay were studied to determine changes in the phytoplankton community structure in response to anthropogenic eutrophication and to assess the use of dinoflagellate cysts as indicators of coastal eutrophication. Our results show that, in nutrient-enriched environments, diatoms are particularly benefited from the nutrients supplied and, as a consequence, heterotrophic dinoflagellates that feed on the diatoms can be more abundant than autotrophic dinoflagellates. In short-core sediment records, a marked shift in autotrophic-heterotrophic dinoflagellate cyst compositions occurred at a depth of approximately 9-10 cm corresponding to the timing of the 1970s industrialization around Gwangyang Bay. This tentatively indicates that diatom and dinoflagellate communities here have undergone a considerable change mainly due to increased nutrient loadings from both domestic sewage effluent and industrial pollution. Our study suggests a possible potential use of dinoflagellate cysts in providing retrospective information on the long-term effects of coastal eutrophication.

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We examined the dynamics of dinoflagellate cyst and phytoplankton assemblages in eutrophic coastal waters of Korea, adjacent to fish and shellfish farms. Water temperature showed seasonality, whereas salinity and pH remained relatively consistent. Dissolved inorganic nutrient levels were higher in September and at the inner stations, where aquaculture fish farms are located than those in May and at the outer stations. Canonical correspondence analysis and artificial neural network analysis revealed multiple environmental factors that affect the distribution of phytoplankton and dinoflagellate cysts. Diatoms dominated in the phytoplankton assemblages, while the protoperidinioid group dominated in the dinoflagellate cyst assemblages. Cyst abundance was higher at the outer stations than at the inner stations due to transport by fast currents, and phytoplankton abundance was positively correlated with cyst abundance. An increase in diatom abundance led to an increase in heterotrophic/mixotrophic cyst abundance, indicating that excessive uneaten food and urinary waste from the fish farms caused eutrophication in the study region and fast growth of diatoms, thereby contributing to the growth of heterotrophic/mixotrophic dinoflagellates and consequently, high abundance of heterotrophic/mixotrophic dinoflagellate cysts.