River discharge effects on the contribution of small-sized phytoplankton to the total biochemical composition of POM in the Gwangyang Bay, Korea

It is hypothesized that the proportion of small phytoplankton increases if the water temperature increases. However, limited information on the biochemical compositions of small phytoplankton and their ecological contributions is currently available for estuary and coastal environments. In the present study, our goal is to determine the roles of small phytoplankton under different environmental conditions, which influence higher trophic consumers. Water samples were obtained in the Gwangyang Bay, Korea from 2012 to 2014. The contribution of small phytoplankton to the total chlorophyll-a (chl-a) concentration increased from the first period (October 2012 April 2013) to the following period (August 2013 April 2014), which was mainly due to lower nutrient inputs available to phytoplankton because of decreasing river input. Based on biochemical compositions (carbohydrates, proteins, and lipids), the overall contributions of small phytoplankton to the total food material (FM) and caloric content were significantly higher than the chl-a contribution of small phytoplankton. This result indicates that the small phytoplankton assimilated more FM and energy per unit of chl-a concentration compared to those of the large phytoplankton. These findings reveal that the potential impacts of river inputs on phytoplankton cell size and the photosynthetic biochemical composition of this primary food source should be considered in coastal management plans for anthropogenic influences, such as dam constructions.