Hydrodynamic drivers of nutrient transport and water quality in a coastal estuary–bay system: A temporal evolution

Estuaries are characterized by high nutrient concentrations and are strongly influenced by anthropogenic activities, which may enhance ecosystem productivity or degrade water quality in estuarine and adjacent coastal environments. This study investigated hydrodynamic processes in the Camboriú River estuary and their effects on nutrient dynamics and water quality in the adjacent bay. Data were collected between March 31 and April 1, 2022, during a spring tide over a 25-hour period, with hourly sampling in the estuary and two discrete sampling events in the bay (high tide and low tide). Measurements included flow, sea level, salinity, temperature, turbidity, and dissolved oxygen. Surface and bottom water samples were collected to quantify nutrient concentrations, chlorophyll-a, particulate organic carbon (POC), suspended particulate matter (SPM), and coliforms. Temporal changes in nutrient concentrations were evaluated through comparison with datasets from 1999, 2020, and 2022. The estuary exhibited elevated concentrations of nutrients and pollutants, particularly NH 4 ⁺ (maximum of 298.30 µM), POC (maximum of 203.08 µM), and thermotolerant coliforms (155,300 MPN 100 mL -1 ) at the surface during low tide, indicating inputs of untreated or partially treated effluents. Episodes of severe hypoxia and anoxia were observed, with dissolved oxygen reaching a minimum of 0.02 mg L -1 , revealing signs of eutrophication and direct impacts on water quality. Sampling sites in the bay near the estuary mouth showed increased concentrations during low tide, emphasizing the estuary’s influence. Overall, the results indicate poor water quality and increasing nutrient concentrations from 1999 to 2022, highlighting progressive environmental degradation over the past two decades. • The estuarine plume strongly influences bay water quality. • Elevated nutrients and fecal coliforms indicate untreated effluent inputs. • Estuarine anoxia indicates advanced eutrophic conditions. • Water quality has progressively declined over the past two decades.