Abstract
Mesoscale eddies play a key role in structuring open ocean ecosystems, affecting the entire trophic web from primary producers to large pelagic predators including sharks and elephant seals. Recent advances in the tracking of pelagic predators has revealed that these animals forage in the mesopelagic and the depth and duration of their foraging dives are affected by the presence of eddies. The ways in which eddies impact the distribution of mesopelagic micronekton, however, remain largely unknown. During a multi seasonal experiment we used a shipboard scientific echosounder transmitting at 38 kHz to observe the distribution of acoustic backscattering in the energetic mesoscale eddy field of the northwestern Atlantic. Observations were collected at 24 stations with 6 located in anticyclonic and 7 in cyclonic eddies. The sampled anticyclonic eddies are characterized by intense acoustic backscattering in the mesopelagic and changes in the intensity of acoustic backscattering layers match gradients of surface properties. Furthermore, mesopelagic daytime backscattering is positively correlated with sea level anomaly. These results suggest that anticyclonic eddies in the northwestern Atlantic impact the distribution of mesopelagic micronekton and may have the potential to locally enhance or structure spatially mesopelagic communities.
Highlights
Mesoscale eddies affect primary production by modulating and structuring phytoplankton biomass, phenology, community composition, and diversity (Bracco et al, 2000; d’Ovidio et al, 2010; Mahadevan et al, 2012; Gaube et al, 2014; McGillicuddy, 2016)
Measurements of acoustic backscattering revealed that mesopelagic micronekton at the sampled stations was distributed in deep scattering layers whose composition was investigated using a Isaacs-Kidd Midwater Trawl (IKMT)
Our surveys revealed that the median NASC seasonal anomaly in anticyclones is larger than in cyclones and out-of-eddies stations (Figures 2C,D)
Summary
Mesoscale eddies affect primary production by modulating and structuring phytoplankton biomass, phenology, community composition, and diversity (Bracco et al, 2000; d’Ovidio et al, 2010; Mahadevan et al, 2012; Gaube et al, 2014; McGillicuddy, 2016). While the movement properties of individual mesopelagic organisms are poorly understood, patterns in diel vertical migration suggest that their maximum speed is of the order of 0.1 m/s, which is at least an order of magnitude slower than the typical currents at the peripheries of mesoscale eddies (Chelton et al, 2011a; Bianchi and Mislan, 2016) These relatively slow swimming speeds coupled with eddy surface areas exceeding 20, 000 km can suggest that active escape by micronekton likely has a small effect on the trapping of mesopelagic communities in eddy cores. We analyze the vertical structure of acoustic backscattering inside a number of North Atlantic mesoscale eddies and regions outside the direct influence of eddies to evaluate whether mesoscale eddies have an impact on the distribution of micronekton and if eddies with different polarities present differences in acoustic backscattering
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