Phytoplankton community dynamics in response to seawater intrusion in a tropical macrotidal river-estuary continuum

Coastal environments are at the frontline of sea-level rise effects, and seawater intrusion constitutes one of the most important causes of salinization, changing the ecological patterns. Hence, we hypothesized that seawater intrusion would alter environmental gradients, causing either abrupt or gradual changes in the phytoplankton of Itapecuru River estuary. Physical and chemical and biological variables were collected bimonthly at six sites between 2019 and 2020. Seventy-six phytoplankton indicators were selected based on their specific functional traits and indicator value. Polymyxus coronalis was a good indicator of the limit of seawater intrusion. Multivariate analyses revealed high species dispersion among the estuarine sectors governed by variations in salinity, suspended particulate matter, cell size, and silicate. The distribution of freshwater species in the upper sector was correlated with low nutrient values and salinity. The marine species were transported between the middle and lower sectors under the opposite conditions. The seawater intrusion negatively affected the community, primarily in the dry season when the displacement of the turbidity maximum zone estuarine altered the structure, reducing its density, diversity, and biomass. The present study generated important information about seawater intrusion effects on the spatiotemporal variation in the phytoplankton community and provided a tool for the sustainable management of tropical estuaries.