Intra-annual variability of phytoplankton biomass and nutrients in a tropical estuary during a severe drought

This study describes the infra-annual variability of environmental characteristics, hydrologic regime and their potential consequences for phytoplankton biomass in a low-inflow estuary on the Brazilian semi-arid coast. The estuary was subjected to two conditions based on the water and salt balance: a short hyposaline and a long hypersaline period. They were marked by considerable differences in precipitation, evaporation and water flushing time that resulted in noticeable variations in the salinity gradients, nutrient concentrations and total phytoplankton biomass (TPB). TPB peaked at the middle and end of the dry season, when salinity reached maximum levels (up to 63) and the water residence time was higher. Higher TPB was also related to a decline in inorganic nitrogen due to phytoplankton uptake and a decline in water transparency due to phytoplankton shading. Rainfall rather than drought constituted a disturbance for the phytoplankton as TPB declined in the rainy period despite available nutrients. Given the prevalent climate change, increases are predicted in the frequency and intensity of droughts and salinity in estuaries. Although the phytoplankton community seemed adapted, the limits of its adaptability and the effects of hypersalinity should be further evaluated.