Assessing Nutrient Limitation in Yeongsan River Estuary Using Bioassay Experiments

Abstract

The Yeongsan River estuary is experiencing increased concentrations of nutrient and organic matter due to its estuary bank (sea dike). The opening of floodgates at the estuary bank leads to a substantial inflow of freshwater into the saltwater zone, thereby resulting in water quality changes. Our study evaluated spatiotemporal variations in nutrient limitation in the freshwater and saltwater zones in the Yeongsan River estuary, which is expected to fluctuate with the changing seasons. We utilized the N:P ratio to evaluate the potential nutrient limitation and conducted bioassay experiments to directly assess actual nutrient limitation. The N:P ratio showed that P was the limiting nutrient in both the long-term (2004–2008) and during our field investigation. However, the bioassay experiment revealed that in the freshwater zone, P was limited in spring and winter (p < 0.05), while no nutrient was limited in summer and fall. In the saltwater zone, we observed P limitation in spring and winter (p < 0.05) and N limitation in fall (p < 0.05), whereas no nutrient limitation was observed in the summer. These results demonstrate that actual nutrient limitation, which directly influences phytoplankton growth, varies spatiotemporally in response to freshwater discharge in the Yeongsan River estuary.