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MEchanisms of Coupling of Ocean Dynamics and Intermediate trophic levels: High Resolution study

Periodic Reporting for period 1 - MECODIHR (MEchanisms of Coupling of Ocean Dynamics and Intermediate trophic levels: High Resolution study)

Reporting period: 2018-10-01 to 2020-09-30

The open ocean is the largest biome on Earth, yet it is the least protected. A major obstacle to its conservation lies in the fine-grained understanding of how marine organisms are affected by the ocean dynamics. In the last decades, remote sensing and bio-logging drastically increased our understanding to how phytoplankton (that can be observed from space as ocean color) and large marine animals (that can be followed with sensors directly attached to them) responds to oceanic turbulence down to the mesoscale (few weeks-months, 50-300 km) and submesoscale (1-50 km, few hours to few days). A major knowledge gap still concerns the so-called "intermediate trophic levels" (ITLs, i.e. zooplankton and micronekton,) and how mesoscale currents (such as fronts and eddies) affect them. The MECODIHR project aimed understanding of these important yet poorly understood animals and how they interact with their environment at the (sub)mesoscale (including meso and submesoscale). Using echosounders (sonars that can be used to describe the spatial structure and abundance of small animals in the water column), mid-water trawls, optical sensors and satellite observations, we addressed the following questions focusing on the northwest Atlantic:
Q1 – What is the (sub)mesoscale distribution of intermediate trophic levels (ITLs)?
Q2 – What are the physical, biogeochemical, and ecological mechanisms controlling the spatial distribution of ITLs at the oceanic (sub)mesoscale?
Q3 - How does the influence of (sub)mesoscale processes on ITLs changes in respect to the seasonal variability of submesoscale features and of primary production?
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
[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
[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
[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
[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
[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
[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
To date, our comparison between mesoscale eddies and patterns in acoustic backscattering is the largest study of ITLs and mesoscale eddies in terms of sample size and one of the few addressing the question of eddy polarity (i.e. the sense of rotation of the sampled eddies). Our findings also challenged the assumption that higher values of near-surface chlorophyll match to more intense acoustic backscattering in the deep ocean, suggesting different directions for future studies.
Example of an echogram collected in the North Atlantic and sample from the mid-water trawl.