Response of phytoplankton assemblages to nitrogen reduction in the Laizhou Bay, China

Abstract The response of phytoplankton assemblages to decreases in nitrogen loading is an essential index to assess the recovery of aquatic ecosystems from eutrophic status. However, a positive signal is hard to observe from short-term operations considering ecosystem complexity. Here, we used decadal data from the Laizhou Bay, China to track the seasonal and annual variations in phytoplankton assemblages after ammonia reduction in the Xiaoqing and Yellow Rivers. Annual trends show reduced phytoplankton abundance and a decline in harmful algal blooms, indicating the positive recovery of phytoplankton assemblages. Phytoplankton assemblage in the sea region adjacent to the Xiaoqing River shows a lower H ′ index, higher cell abundance, and higher seasonal variability than the sea region neighbouring the Yellow River. The spatial variability might result from the differences of nitrogen species, runoff and sediment contents between the two rivers; this finding indicates a demand for more aggressive decreases in nitrogen loads.