Effects of droughts, sea level rise, and increase in outfall discharges on phytoplankton in a temperate estuary (Tagus Estuary, Portugal)

The effects of climate change on the estuarine environment are not fully understood. In temperate regions, droughts are expected to increase in frequency and severity, due to lower precipitation, and the mean sea level is expected to rise. This study aimed to assess how the estuarine environment will be affected by river flow’s reduction, mean sea level rise, and the increase in nutrients discharged from anthropogenic sources. Seven scenarios were simulated and analyzed in the Tagus estuary (Portugal), using the hydrodynamic and biogeochemical model SCHISM: (i) reference scenario, (ii) 10% increase of the wastewater treatment plant (WWTP) outfall’s discharge, (iii) 25% reduction of the river flow, (iv) 50% reduction of the river flow, (v) sea level rise of 0.5 m, (vi) sea level rise of 1 m, and (vii) the combination of 0.5 m of sea level rise and 25% reduction of the river flow. Both the reduction of the river flow and mean sea level rise led to higher salinities and lower nutrients and chlorophyll-a concentrations in the mid and upper areas of the estuary. The reduction in riverine nutrients in the estuary may increase the importance of nutrients from anthropogenic sources (e.g. WTTP discharges) in shaping the spatial variability of the phytoplankton communities in the future.