Check Impact of anthropogenic forcing on the environmental controls of phytoplankton dynamics between 1974 and 2017 in the Pearl River estuary, China

How phytoplankton responded spatiotemporally to the co-occurrence of multiple variables is of central importance for ecosystem management. In this study, extensive investigations in the Pearl River estuary (PRE), southern China, between 1974 and 2017 revealed a strong anthropogenic effect on the environmental drivers of phytoplankton dynamics (in terms of chlorophyll a). The empirical cause-and-effect chain summarized as human activity -> hydrology -> nutrient -> chlorophyll may constitute fundamental quantitative tools for predicting effects of coastal eutrophication. Change in subaqueous topography induced by human stress resulted in shoal-trough instability, which unexpectedly shifted nutrient dynamics across the entire regions. Over time, dissolved inorganic nitrogen (DIN) and phosphorus (PO4) increased, as did the molar N/P ratio (DIN/PO4), indicating that phytoplankton growth was potentially phosphate-limited in most PRE. The adsorption of phosphorus by sediment particles was stronger than that of nitrogen. The quantification of phytoplankton drivers demonstrated the importance of river-tide dynamics, which incorporated the interplay between environmental variables. Particularly, the influence of riverine input varied over time and site, affecting hydrological control variables and nutrient limitations. Our structural equation modelling results (p < 0.05) further suggested that nutrients and water temperature directly affected phytoplankton chlorophyll, but salinity and suspended sediment relatively contributed more on nutrients. Characterizing the relationships among phytoplankton chlorophyll, nutrients, and hydrological factors over the last 40 years enables us to develop effective long-term ecosystem management strategies, and to design studies more focused on ecological health and function.