Intense mesopelagic acoustic backscattering in northwest Atlantic anticyclonic eddies

Alice Della Penna, Applied Physics Laboratory, University of Washington, Seattle, WA, United States and Peter Gaube, Applied Physics Laboratory at the University of Washington, Air-Sea Interaction and Remote Sensing, Seattle, WA, United States
Mesoscale eddies play a key role in structuring open ocean ecosystems, affecting the entire trophic structure from primary producers to large pelagic predators such as white sharks and elephant seals. Recent advances in the tracking of pelagic predators has revealed that many of these animals forage in the mesopelagic and the depth and duration of these dives are affected by the presence of eddies. The ways in which eddies impact the distribution of micronekton itself, however, remains still largely unknown.

During a multi seasonal experiment in the North Atlantic we used a shipboard scientific echosounder transmitting at 38 kHz to observe the distribution of acoustic backscattering in the energetic mesoscale eddy field in the Northwestern Atlantic. During the four campaigns, we sampled 6 cyclones, 5 anticyclones along with 9 other locations outside of the direct influence of eddies. The sampled anticyclonic eddies are characterized by intense acoustic backscattering in the mesopelagic with the structure of acoustic backscattering layers reflecting gradients of surface properties such as temperature and chlorophyll. Furthermore, mesopelagic daytime backscattering is positively correlated with sea level anomaly and therefore eddy intensity. These results suggest anticyclonic eddies in the northwestern Atlantic may locally enhance mesopelagic communities and that the communities in proximate eddies of opposite polarity are acoustically distinct.