Suspended particulate matter and dissolved inorganic nutrients transport in a small urban shallow subtropical estuary

Abstract

The Biguaçu River estuary is a small system that traverses a highly urbanized area and flows into a larger estuarine system, the Baía Norte, in Santa Catarina, Brazil, thus being subject to various anthropogenic stresses. In the present study, we investigated the hydrodynamic regime of this system, assessing sedimentary and biogeochemical dynamics to evaluate the role of this system as a source or sink of materials for the adjacent coastal zone. A sampling campaign was conducted to record hydrodynamic data (water level and currents) and water properties (salinity, temperature, suspended particulate matter (SPM), dissolved oxygen, dissolved inorganic nutrients (nitrite, nitrate, ammoniacal nitrogen, and phosphate), chlorophyll-a, and phycocyanin during two complete cycles of semi-diurnal tides (25 h) under spring tide and average river discharge conditions. The results allowed us to classify the estuary as partially stratified. Based on the Richardson number diagnosis, periods of increased mixing occurred during flood tides, while stable stratification was observed during ebb tides. The variability in SPM concentration was associated with resuspension processes during flood periods, while during ebb tides, the variability was associated with advection. The residual transport of SPM was directed into the estuary. The low levels of chlorophyll-a and phycocyanin found indicate low primary productivity within the estuary, likely due to light limitation or inadequate relative nutrient ratios. The hydrodynamics of this small-scale estuary tend not to favor the establishment of planktonic communities. Additionally, the apparent high organic load related to the poor wastewater treatment in the watershed tends to favor a saprobic rather than a trophic state, limiting autotrophic activity. Nutrient variation showed that nitrate is associated with riverine origin through the estuary, but for ammoniacal nitrogen, nitrite and phosphate, processes occurring in the bay also influenced the estuary.