Spatial and temporal variations of sediments in a subtropical microtidal estuary

Abstract

Sediment dynamics are determined by the synergy between the hydrodynamic condition and sediment properties. This study analyzed, between the years 2015-2017, the spatial and temporal changes in sediment texture of a shallow embayment in the Patos Lagoon estuary (Southern Brazil), and the influence of environmental parameters in these variations. Two areas were selected: an area more exposed to the estuary main channel dynamics and NE winds, and the other protected from them by islands. Each area was divided in shallow (mean depth <2m) and deeper zones (mean depth ≥2 m). The organic matter content presented a spatial and temporal variation, with maximum values higher than previously reported. Due to elevated rainfall rates during the winter and spring of 2015, higher values of fine sediments and organic matter were recorded, mainly at deeper zones. The protected area presented smaller grain sizes and their variability was correlated with wind speed and direction. While in the exposed area the water depth has a higher influence in the granulometric variations, decreasing grain sizes with depth increases. The higher seasonality in the granulometric variation was observed in the deeper zones, which presented the smaller grain sizes and lower values of asymmetry, decreasing these values in winter and spring. In conclusion, few centimeters in water depth may cause large changes in bottom types (or in the relative amount of sand, silt and clay) of shallow estuarine areas. From the environmental variables measured, wind, rainfall and bathymetry were the most influential in the temporal variation of the sediments, while hydrodynamic exposure on the spatial differences.