To address these questions, the collected acoustics and net-tows data were then analyzed by comparing patterns in acoustic backscattering with the locations and properties of mesoscale eddies (rotating currents with typical spatial scales of approximately 100 km and lifetimes of few months) and submesoscale fronts (regions of confluence of water masses having different properties).
We found that (i) mesoscale eddies can create gradients in acoustic backscattering of the same order of magnitude as the ones that can be detected across different ocean basins, (ii) the cores of anticyclonic eddies (i.e. clockwise spinning rotating currents, characterized generally by warmer waters and clearer waters in the North Atlantic) are characterized by particularly strong acoustic backscattering in the mesopelagic (depths 200-1000 m) and (iii) gradients in near-surface properties such as temperature, salinity or density at the peripheries of eddies match gradients in acoustic backscattering deep in the mesopelagic.
Fronts, on average, corresponded to gradients in acoustic backscattering both in the epipelagic (top 200m) and mesopelagic. We also found that, on average, fronts are characterized more often by strong acoustic backscattering in the epipelagic.
This combination of results suggests that even ITLs with strong swimming capabilities are likely to be heavily affected by the presence of strong currents. Such currents can constitute barriers to transport (on fronts), but can also, in the case of the ones at the peripheries of mesoscale eddies, cause the trapping of ITLs inside the core of the eddy. The trapped organisms can be then transported by the movement of the enclosing eddy even thousands of kilometers away from the location of trapping.
In the sampled eddies, near-surface primary production was not directly correlated to mesopelagic acoustic backscattering in the mesopelagic. This is an important and novel result because at larger scales (global and basin scale) recent publications have highlighted a positive relationship between these two variables. Our results indicate that at smaller scales high values of primary production do not correlate with high acoustic backscattering in the mesopelagic. This mismatch suggests that other mechanisms such as trapping and transport of different masses, as well as differences in ITLs community composition are likely to be more important mechanisms at fine scales compared to the bulk value of near-surface chlorophyll. The vertical distribution of ITLs is also impacted by the concentration of primary producers in the water column. Previous studies have found that the locations in the water columns where high concentrations of ITLs (also known as Deep Scattering Layers, DSL) are dictated by light levels, which are in turn heavily dependent on the abundance of phytoplankton. Our results show that, while there is link between near-surface chlorophyll and intensity of acoustic backscattering, the relationship between near surface production and depth of the DSL is present at fine scales. For example, we find that in anticyclonic eddies, that are characterized by clearer and less productive waters, the DSL is deeper than in cyclonic eddies, that are generally characterized by larger phytoplankton concentrations near the surface.
The results of this work have been the object of six published manuscripts[1-6], two presentations at international conferences [8-9], a manuscript in preparation [7]and outreach activities.
[1] Della Penna, A. and Gaube, P., 2019. Overview of (Sub) mesoscale Ocean Dynamics for the NAAMES Field Program. Frontiers In Marine Science,
https://doi.org/10.3389/fmars.2019.00384(s’ouvre dans une nouvelle fenêtre)[2] Della Penna, A. and Gaube, P., 2020. Mesoscale eddies structure mesopelagic communities. Frontiers in Marine Science, 7.
https://doi.org/10.3389/fmars.2020.00454(s’ouvre dans une nouvelle fenêtre)[3] Behrenfeld, M.J. Gaube, P., Della Penna, A.,[...] and Hostetler, C.A. 2019. Global satellite-observed daily vertical migrations of ocean animals. Nature,
https://doi.org/10.1038/s41586-019-1796-9(s’ouvre dans une nouvelle fenêtre)[4] Haëntjens, N., Della Penna, A., Briggs, N., Karp‐Boss, L., Gaube, P., Claustre, H. and Boss, E., 2020. Detecting mesopelagic organisms using biogeochemical‐Argo floats. Geophysical Research Letters,
https://doi.org/10.1029/2019GL086088(s’ouvre dans une nouvelle fenêtre)[5] Schulien, J.A. Della Penna, A.,[...] and Karp-Boss, L., 2020. Shifts in Phytoplankton Community Structure Across an Anticyclonic Eddy Revealed From High Spectral Resolution Lidar Scattering Measurements. Frontiers in Marine Science,
https://doi.org/10.3389/fmars.2020.00493(s’ouvre dans une nouvelle fenêtre)[6] Bolaños, L.M. Karp-Boss, L., Choi, C.J. Worden, A.Z. Graff, J.R. Haëntjens, N., Chase, A.P. Della Penna, A., Gaube, P., Morison, F. and Menden-Deuer, S., 2020. Small phytoplankton dominate western North Atlantic biomass. The ISME Journal,
https://doi.org/10.1038/s41396-020-0636-0(s’ouvre dans une nouvelle fenêtre)[7] Manuscript in preparation - Della Penna, A., Gaube, P., and Rivière, P., Fine-scale fronts impact the distribution of Deep Scattering Layers
[8] Poster presentation - Della Penna, A., and Gaube, P., Mesoscale variability of acoustic backscattering in the North Atlantic, ASLO Science Meeting, 2019
[9] Oral presentation - Della Penna, A., and Gaube, P., Intense mesopelagic acoustic backscattering in northwest Atlantic anticyclonic eddies, Ocean Science Meeting, 2020
