Observations of Bubble Plumes at Convergent Estuarine Fronts Leading to Enhanced Oxygenation and Degraded Acoustic Communications

Estuaries and their nearby coastal waters are commonly characterized by strong currents and salinity stratification. These conditions can produce intense fronts where fresh and saltwater meet and mix. At convergent fronts, denser saltier waters subduct underneath less dense fresher waters creating strong downward vertical velocities. Wave breaking at or near the convergent front produce bubbles that collect at the front forming long white lines of foam visible from shore and even satellite. Little is known about the subsurface structure of these fronts. As part of an ONR funded project called Under Sea Remote Sensing (USRS) we used two REMUS 100 Autonomous Underwater Vehicles (AUVs) to measure subsurface structure of salinity, currents, acoustic back-scatter and dissolved oxygen at these fronts. We will present new high-resolution measurements obtained in April 2019 at a V-shaped tidal intrusion front which formed during flood tide in the James River just south of Newport News (VA) in the lower Chesapeake Bay. Comparisons will be made to prior results we obtained in June 2017 at a C-shaped ebb plume front which formed at the mouth of the Connecticut River in Long Island Sound. Underneath the foam lines of both fronts we found evidence of large submerged bubble plumes using acoustic back-scatter measurements from the AUV’s upward looking ADCP. Bubble dissolution within these plumes was evident from dissolved oxygen changes across the front. The bubble plumes also significantly degraded underwater acoustic communications in the vicinity of the front. Our results reveal the importance of submerged bubbles plumes formed at convergent estuarine fronts for coastal oxygenation with additional impacts on underwater sound propagation. First steps aimed at prediction of these bubble plumes and their impacts will be presented by comparing the observed near surface vertical profile of acoustic back-scatter intensity to a simple advection/diffusion model of bubbles at the front.