A modeling study of light extinction due to cohesive sediments in a shallow coastal lagoon under well mixed conditions.

Cohesive sediments play an important ecological role in coastal systems, affecting light penetration in the water column and consequently the local biogeochemical processes. The main aim of this work is to study the water-column light extinction coefficient (k) dependence on cohesive sediment dynamics within Ria de Aveiro lagoon under sea level rise and extreme freshwater discharges events. A coupled estuarine hydrodynamic and transport model is explored, following previous modeling frameworks developed for the Ria de Aveiro. Three scenarios were simulated: a reference scenario (SC1), wherein present hydrodynamic, mean sea level and present freshwater discharges were used to simulate present hydrodynamic and cohesive sediments; a scenario considering local projections of sea level rise of 0.42 m (SC2); and a third one corresponding to maximum freshwater inflow for the lagoon's tributaries (SC3). Results show higher concentrations of cohesive sediments near the rivers mouths, gradually decreasing toward the lagoon's inlet. From SC2 scenario was found that the effect of sea level rise in sediment dynamics and in water-column light extinction coefficient is not significant. However, the tidal prism increase in this case induces a dilution of the sediments concentration from fluvial origin and consequently a slightly k decrease. Contrarily, in SC3 scenario, the extreme freshwater inflow enhances the cohesive sediment concentration in the entire lagoon, limiting the light penetration in the water column. In summary, extreme river discharge events have a highest impact on water-column light penetration in Ria the Aveiro than predicted sea level rise.