Multiple stressors influencing the general eutrophication status of transitional waters of the Brazilian tropical coast: An approach utilizing the pressure, state, and response (PSR) framework

Modeling approaches are useful tools for assessing the general state of eutrophication and understanding the effects of multiple stressors on coastal ecosystems. Thus, we hypothesized that anthropogenic pressures and multiple stressors would increase the trophic state of macrotidal estuaries. Datasets from long-term (2012−2020) environmental and biological monitoring in the Itapecuru River estuary (IRE) were analyzed using the pressure-state-response (PSR) approach and nonlinear methods. Our results indicated a low dilution of nutrients and a moderate flushing potential of urban effluent (2150.94 ton year−1) in the estuary. The estuary was consequently classified as being in a high trophic state, being susceptible to and suffering high estuarine pressure to develop eutrophic symptoms. The Assessment of estuarine trophic status (ASSETS) model indicates that eutrophication is seasonal and depends on climatic variation. La Niña events (2019–2020) contributed to chlorophyll-a (>40 μg L−1) and orthophosphate (0.04 mg L−1) concentrations, principally during periods of low river discharge. According to the GAM's model, brackish waters (salinity>10) with high temperatures (> 30 °C) and high dissolved oxygen (>4 mg L−1) have more intense trophic conditions, especially in the mixing zone. The low dissolved oxygen (DO) levels (DO<3 mg L−1) and high concentrations of chlorophyll-a in the seawater zone indicate that the lower portion of the estuary was the most susceptible. In addition, the random forest model selected salinity, DIP and Chl-a as the principal stressors that intensified eutrophication in the macrotidal systems. According to the ASSETS final ranking (worsen-high) for the next decade (2021−2031), the primary planned strategies should be to reduce anthropogenic contributions and improve trophic conditions in the IRE. From these results, the interactions and predictions of eco-hydrological effects could facilitate the characterization of future risks and the management of macrotidal estuarine systems.