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Fine-scale vertical relationships between environmental conditions and sound scattering layers in the Southwestern Tropical Atlantic
Ocean dynamics initiate the structure of nutrient income driving primary producers, and these, in turn, shape the distribution of subsequent trophic levels until the whole pelagic community reflects the physicochemical structure of the ocean. Despite the importance of bottom-up structuring in pelagic ecosystems, fine-scale studies of biophysical interactions along depth are scarce and challenging. To improve our understanding of such relationships, we analyzed the vertical structure of key oceanographic variables along with the distribution of acoustic biomass from multi-frequency acoustic data (38, 70, and 120 kHz) as a reference for pelagic fauna. In addition, we took advantage of species distribution databases collected at the same time to provide further interpretation. The study was performed in the Southwestern Tropical Atlantic of northeast Brazil in spring 2015 and autumn 2017, periods representative of canonical spring and autumn conditions in terms of thermohaline structure and current dynamics. We show that chlorophyll-a, oxygen, current, and stratification are important drivers for the distribution of sound scattering biota but that their relative importance depends on the area, the depth range, and the diel cycle. Prominent sound scattering layers (SSLs) in the epipelagic layer were associated with strong stratification and subsurface chlorophyll-a maximum. In areas where chlorophyll-a maxima were deeper than the peak of stratifications, SSLs were more correlated with stratification than subsurface chlorophyll maxima. Dissolved oxygen seems to be a driver in locations where lower oxygen concentration occurs in the subsurface. Finally, our results suggest that organisms seem to avoid strong currents core. However, future works are needed to better understand the role of currents on the vertical distribution of organisms.
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Publisher's official version | 24 | 3 Mo | ||
S1 Fig. Mean Composite profile acoustic profiles in spring 2015 and autumn 2017 for each diel period (day–dark colours; night–shaded colours) and system. | - | 1 Mo | ||
S2 Fig. Example of a CTD profile from WBCS spring 2015 (ABRACOS I) with the representation of the thermohaline structure defined from temperature (ɵ), salinity, density (σ) and buoyancy .... | - | 1 Mo | ||
S3 Fig. Examples of MVBS (dB re 1m-1) profiles at 38, 70 and 120 kHz from autumn 2017 (ABRACOS 2) in the epipelagic zone of the SECS. | - | 722 Ko | ||
S1 Table. Cross-correlations and the vertical shift of maximum correlation between the stratification (N2) and chlorophyll (mg. m-3), and also between the MVBS (dB re 1m-1) .... | - | 18 Ko |