Phosphorus availability shapes size structure, co-occurrence patterns and network stability of surface microeukaryotic plankton communities in an urbanized estuarine ecosystem

The size structure of microeukaryotic plankton communities exerts a fundamental control on marine ecosystem functions, such as food web dynamics and element flows. The global expansion of nitrogen (N) and phosphorus (P) nutrient pollutants into estuaries and coastal waters is causing public health and environmental concerns associated with microeukaryotic plankton activities. In the context of eutrophication, understanding how size structure and biotic activities of microeukaryotic plankton communities responds to anthropogenic nutrient inputs is vital to evaluate the future fate of coastal ecosystems. Here, we conducted onboard incubation experiments to provide the first investigation of how size structure and co-occurrence network of microeukaryotic plankton communities change under the dynamic nutrient conditions in the Pearl River Estuary. Our study revealed (1) a transition from P limitation to N limitation or potential NP co-limitation for the growth of pigmented protists along the Pearl River-South China Sea continuum, and isohaline ∼ 31.5 being the geographic boundary; (2) P availability drove the microeukaryotic plankton community size structure that the increased P inputs favored the dominance of nano-sized (3–20 μm) protists due to the increased relative abundance of Chlorophyta; (3) increased P inputs enhanced the species competition among phototrophic protists and improved the network stability of microeukaryotic plankton communities. In the future, if there is an increase in anthropogenic input of P in the Pearl River Estuary during summer, which would alleviate the pressure of P-deficiency, it is expected to increase the biomass of microeukaryotic plankton, with a dominance of nano-sized Chlorophyta. Altogether, this study advances our understanding of the geographic nutrient limitation pattern for pigmented protist plankton in the Pearl River Estuary, an urbanized estuarine ecosystem. Moreover, it enhances our knowledge regarding the response of marine microeukaryotic plankton communities to future environmental changes.