Increasing nutrient inputs shift the regime of marine ecosystem over decades in Daya Bay

Human-induced nutrient input into the world's coastal waters is leading to an increase in nutrients and increasing eutrophication. However, how the functioning of aquatic ecosystems responds to these changes is still poorly understood. In this study we report on the long-term changes in nutrient regime and planktonic ecosystem function in the Daya Bay, a typical subtropical semi-enclosed bay that has experienced rapid economic and social development for several decades. Time-series data (from 1991 to 2018) were collected at a mostly quarterly resolution to visualize the long-term changes in dissolved inorganic nutrients and plankton abundances, based on which we constructed a simplified abundance size spectra (SASS) and plankton abundance ratio to describe the functioning of the planktonic ecosystem. The results showed a long-term increase in the system productivity but a decrease in integrated energy transfer efficiency of the planktonic ecosystem with increasing dissolved inorganic nitrogen (DIN) contents. Changes in the nutrient regime and functioning of the planktonic ecosystem were detected at a tipping point or threshold around 2006–2007, the shifts of which were characterized by abrupt changes in the trends of nutrient contents (phosphate, ammonia and nitrite), nutrient ratios (DIN/phosphate and silicate/phosphate), plankton abundance and total plankton biomass. Compared to the nutrient regime, the functioning of the planktonic ecosystem shifted a few years later. Overall, our findings suggest that the pelagic ecosystem in coastal waters such as Daya Bay may change significantly in response to long-term increases in human nutrient inputs. These shifts may have profound implications for fisheries production and ecosystem management in the bay.