Impact of the Patos Lagoon jetties on inner-shelf circulation and water properties

Abstract

The jetties on the Patos Lagoon mouth are 4 km long and form a conspicuous feature in a monotonous coast line. Under steady northeasterly winds-the prevailing local wind-an eddy is formed in the leeward of the jetties. This study investigated the circulation and water properties in the inner shelf affected by this eddy. We conducted a field experiment to collect hydrodynamic and water property data in the lee of the jetties during a prolonged event of strong northeasterly winds. These conditions promote lagoon outflow and eddy formation. Current velocity and direction were measured using four acoustic doppler current profilers (ADPC) deployed parallel to the coast line. Vertical profiles of water properties (salinity, temperature, turbidity, chlorophyll and dissolved oxygen) were measured using a CTD along transects. To complement the field experiment, high-resolution Sentinel-2 satellite imagery acquired under similar conditions was analyzed, along with wind reanalysis data from ERA5. Results revealed that eddy formation occurs under both NE and W/NW winds, the spatial extent and core position of which are modulated by wind intensity and relative angle to the jetties. Brackish water outflow through the jetties induces strong stratification, leading to a decoupling of the upper and bottom layers. This produces a vertically sheared flow, with the surface layer moving seaward and the bottom layer moving landward, toward the jetties. However, the observed salinity and temperature distributions suggest that the bottom layer also exhibits a cross-shelf (onshore) component indicating upwelling, which may contribute to the transport of fine sediments shoreward. An identified region of low dissolved oxygen saturation suggests hypoxic conditions likely associated with the intense stratification.