LAUSR

Abstract We studied phytoplankton community structure at 25 stations in the Busan coastal region (South Korea), including the Nakdong River estuary, during four seasons in 2013. Together with the observational study, we conducted an experimental study to assess the effects of nutrient enrichment on phytoplankton community structure over seasonal succession. Over the study period, abiotic environmental factors, such as water temperature, salinity, and nutrient concentrations, in the Busan coastal region were associated with the amount of discharge from the Nakdong River; salinity and nutrient levels varied depending on the amount of river discharge, and the water temperature in winter near the Nakdong River estuary was significantly lower due to the increase in river discharge compared to other regions. However, the water temperature in offshore stations was relatively high owing to the introduction of the Tsushima Warm Current (TWC). During winter and summer, diatoms were the predominant taxa; the primary dominant species were Stephanodiscus hantzschii (winter), Leptocylindrus danicus (spring), and small diatoms (summer). In summer, blooms of the dinoflagellate Cochlodinium polykrikoides occurred at several stations near the Nakdong River mouth and several offshore stations, and small diatoms were abundant at the sampling stations. In autumn, Cryptomonas spp. were dominant (>60%) at most sample stations. To investigate the response of phytoplankton communities to nutrient loading, bioassay experiments with phytoplankton samples collected from five sampling stations during four seasons were conducted through the addition of inorganic nutrients: nitrate (+N; 20 µM), phosphate (+P; 2 µM phosphate), and nitrate and phosphate (+NP; at the same concentrations). The +NP treatment exhibited the highest growth enhancement of phytoplankton in most samples. In addition, depending on the NP ratio and phytoplankton community structure, there was a distinct response of phytoplankton to nutrient loading. These results imply that phytoplankton can grow under conditions of pulsed nutrient loading, irrespective of season, but there may be seasonal difference in response of phytoplankton communities in the Busan coastal region.