Spatial distribution of the summer chlorophyll a and nutrients in the Pearl River Estuary

Estuaries, critical land-sea interfaces, mediate terrestrial inputs and sustain coastal ecosystems, with high ecological and economic significance. The Pearl River Estuary (PRE), the largest estuary in southern China and a core of the high-density Guangdong-Hong Kong-Macau Greater Bay Area, connects the Pearl River to the South China Sea, experiencing complex runoff, intense anthropogenic activities, and sensitive ecosystems. Eutrophication has emerged with rapid regional development. Based on field surveys conducted in the PRE and adjacent waters during summer 2023, this study employed a three-end-member mixing model to analyze the distribution of chlorophyll-a (Chl-a) and nutrients, as well as their regulatory mechanisms controlled by environmental factors. Results showed that high Chl-a zones were concentrated in the salinity transition zone, whose formation was synergistically regulated by factors such as salinity, light availability, and nutrient inputs. Although nutrient concentrations were high within the estuary, Chl-a levels remained relatively low due to light limitation caused by suspended particulate matter. The study area exhibited a nitrogen-rich and phosphorus-limited pattern. Notably, in the estuary and near-mouth regions, where significant changes in Chl-a concentration occurred, a moderate negative correlation (r = -0.456, p < 0.05) was identified between Chl-a and PO₄-P. The three-end-member mixing model confirmed that salinity-driven mixing served as the primary driver of nutrient distribution in the PRE. Model results showed that nutrient processes within the estuary were dominated by additive mechanisms, while consumptive processes prevailed outside the estuary mouth. These findings provide key insights into biogeochemical dynamics and eutrophication management in estuaries.