Dynamics of marine intrusions in a microtidal coastal lagoon and their potential influence on meroplankton inflow

It is essential for the functioning of intermittently closed/open lagoons and lakes (ICOLLs) to maintain a connection with the adjacent sea. This connectivity allows larval stages, like planktonic larvae, of marine estuarine-related species to move between environments by relying on water flows when the sandbar is open. This study aims to research the dynamics of marine intrusions into a microtidal coastal lagoon - Laguna de Rocha, Uruguay as a case study - during the open state of the sandbar. A 2-D hydrodynamic model was constructed, and model results were calibrated and validated with in situ data. A seawater tracer was used to determine the frequency, duration and intensity of marine intrusion under different wind, sea level and tributary discharge scenarios. Results showed that under calm conditions (light wind and basal discharge) at least one marine intrusion per day occurred in the vicinity of the inlet channel due to high tide. The sea level was essentially the main determinant of the duration of marine intrusions. Sea level rise has led to an increase in the duration (up to 22 hours per day) and spatial extent of marine intrusions. Conversely, tributary discharge primarily determined the intensity of marine intrusions attenuating them under high discharge scenarios. Strong SE (onshore) winds increased the duration of marine intrusions in all scenarios and the intensity of intrusions in the low sea level scenarios. Results about the importance of sea level for marine intrusions in Laguna de Rocha broaden the possible mechanism by which meroplankton can enter these microtidal systems. For instance, it could involve a combination of forces that overlap to raise the sea level above the lagoon water level (e.g. onshore winds, spring tides). Results suggest that the main force triggering seawater and larval entry may vary over time in the same lagoon. Therefore, in the microtidal Laguna de Rocha, we propose a mechanism for selecting incoming currents for the entry of estuarine-related larvae that, unlike other proposed mechanisms, may not be solely due to the effect of flood tide.