Estuarine algal responses to increasing nitrate concentrations during closed mouth conditions of oligotrophic systems: a laboratory microcosm experiment

Abstract

Abstract The increasing incidence of eutrophication has potentially detrimental socio-economic and ecological consequences. This study aimed to elucidate the temporal dynamics of algal communities in response to increasing initial concentrations of inorganic nitrogen (particularly nitrate) – central components of eutrophication. A contained microcosm experiment was designed to mimic the conditions of shallow oligotrophic estuaries with high water residence times. Phytoplankton, microphytobenthos and filamentous algal community dynamics were observed over a 28-day experimental period under different nitrate regimes. Key observations included (1) accelerated filamentous algal growth, (2) rapid loss of phytoplankton biomass and abundance, and (3) reduced benthic diatom species diversity and richness in the “1.0 μm Nitrate Addition” treatment. Additionally, model results highlighted the positive relationship between filamentous algal growth and increased water temperature. From a global change perspective, the decline in microalgal abundance and diversity at the onset of filamentous algal growth in warm, N-enriched environments suggests a potential uncoupling of trophic pathways. However, the “Control” and “0.5 μm Nitrate Addition” treatments were similar in their algal responses, highlighting the ability of ecosystems to absorb small disturbances. Thus, it is critical that estuarine resilience is preserved to ensure continued provision of invaluable ecosystem services.