The influences of nitrogen form on phytoplankton community and stoichiometry in estuarine and coastal waters

Abstract

Agricultural runoff transports significant amounts of nitrates (NO₃^-) to estuaries and coastal areas. Wastewater discharge leads to increased ammonium (NH₄⁺) concentrations in estuaries. However, the impact of nitrogen (N) forms on phytoplankton community composition and stoichiometry has received relatively little attention. Nutrient enrichment bioassay experiments were conducted in representative areas of the Pearl River Estuary and adjacent coastal areas in summer. Our results demonstrated that diatoms and dinoflagellates prefer NO₃^- and NH₄⁺, respectively. N:P uptake ratios were higher in the NO₃^--added treatment than those in the NH₄⁺-added treatment. Hence, nitrogen forms not only modulate the phytoplankton community composition, but also phytoplankton stoichiometry. The input of NO₃^--rich river discharge favored diatom growth in the Pearl River Estuary, increasing phosphorus consumption per unit of nitrogen and thus exacerbating phosphorus limitation. Our findings provided new insights into the potential mechanism for phosphorus limitation in the river-impacted coastal areas.