Regime Shift in a Coastal Pelagic Ecosystem with Increasing Human-Induced Nutrient Inputs Over Decades

Abstract

Human-induced nutrient inputs to global coastal waters are leading to increasing nutrients and escalating eutrophication. However, how aquatic ecosystem functioning responds to these changes remains insufficiently studied. Here we report the long-term changes in the nutrient regime and pelagic ecosystem functioning in the Daya Bay, a typical subtropical semi-enclosed bay experiencing rapid economic and social development for several decades. Time-series (from 1991 to 2018) data with a mostly quarterly resolution were collected to depict long-term changes in dissolved inorganic nutrients and plankton abundances, based on which we constructed simplified abundance size spectra (SASS) and plankton abundance ratios to describe the functioning of the pelagic ecosystem. The results revealed a long-term increase in system productivity but a decrease in integrated energy transfer efficiency of the pelagic ecosystem, with rising concentrations of dissolved inorganic nitrogen (DIN). Shifts in the nutrient regime and pelagic ecosystem functioning were detected at a tipping point around 2006–2007. The shifts were characterized by abrupt changes in the trends of nutrient (phosphate, ammonia, nitrite) concentrations, nutrient ratios (DIN/phosphate, silicate/phosphate), plankton abundance, total plankton biomass, system productivity, and integrated energy transfer efficiency. Compared to the nutrient regime, the pelagic ecosystem functioning shifted several years later. Overall, this study indicates that the nutrient regime and pelagic ecosystem functioning can shift significantly in response to long-term increasing input of human-induced nutrients in coastal waters such as the Daya Bay. We discussed the implication of the regime shifts to fishery production, nitrous oxide emission, and ecosystem management in the Daya Bay. Furthermore, our study demonstrates that SASS and plankton abundance ratios can serve as tools for describing long-term changes in pelagic ecosystem functioning.

Keywords: nutrient loading, ecosystem functioning, simplified abundance size spectrum, tipping point, energy transfer efficiency, Regime shift