Links between surface sediment composition, morphometry and hydrodynamics in a large shallow coastal lagoon

Abstract

This study addresses the surface sediment composition and distribution in Mirim Lagoon, a large coastal shallow transboundary system located on the border of South Brazil and Uruguay, which is 3749 km2 and maximum depth is around 6 m, in relation to the hydrodynamic conditions evolved from predominant winds. Surface water currents in the marginal area displayed maximum speed values of 0.25 to 0.3 m s−1 flowing parallel to the coastline either under NE of SW wind predominance. The marginal zone, above the 6 m bathymetric contour comprise sandy sediments, indicating that under such a combination of bathymetric and hydrodynamic conditions, the resuspension of fine sediments is dominant. Lower surface current velocity observed in the central zone of the lagoon located below the 6 m isobath (i.e., 0.05 m s−1), together with an increase in the maximum width of the lagoon, promoted conditions for deposition of the clay sediment fraction. Therefore, the combined use of sedimentological and hydrodynamic data represents a useful tool to infer patterns of deposition and resuspension in coastal systems. Given the large size of the Mirim Lagoon and associated difficulties in sampling and monitoring the system, the information presented here is important for environmental management, and particularly for planning future paleolimnological research and set the coring stations on appropriate coordinates within the central region, where relatively calm conditions and dominance of clay sediments are observed.