Preliminary results on the influence of seamount topography on hydrography, macronutrient and biomass distribution in the Southern Tropical Atlantic

Seamounts (commonly defined as submarine elevations rising >1000 m above the surrounding seafloor) are found in all ocean basins, including the Southern Atlantic. Depending on seamount topography, they may create distinct hydrographic regimes that provide ecological habitats for elevated primary production and biodiversity due to the generation of internal waves that promote vertical mixing and redistribution of energy and nutrients from deep to surface waters. Here, we present preliminary results from an oceanographic expedition (RV Maria S. Merian – n° 117) in the Brazilian EEZ across two seamounts (i.e., Toninha and Aracati seamounts at 4°S, 36°W and 3.5°S, 37.5°W, respectively). Multiple datasets were acquired to describe the physical-chemical conditions around and above the seamounts, including hydrography (CTD, UCTD, ADCP, velocity shear and temperature variabilities), macronutrients (NO₂^- + NO₃^-, PO₄^3-, SiO₄^2-), and chlorophyll distribution. The different topography of the two seamounts created specific hydrographic patterns at the two sites, resulting in varying macronutrient and chlorophyll distribution. The chlorophyll maximum was aligned with the pycnocline at Aracati, however, a substantial deviation between the chlorophyll maximum and the pycnocline was observed at Toninha. This discrepancy is likely attributable to hydrographic variations, stemming from the shallower depth of Toninha (approx. 80 m), compared to the 250 m of Aracati, influencing internal waves and energy dissipation above the seamount plateau. Surface waters at both seamount sites were characterized by low nitrogen (NO₂^- + NO₃^-) compared to phosphate (PO₄^3-) availability with a N:P ratio of <10, whereas below the chlorophyll maximum the N:P ratios were close to the Redfield ratio of 16. The partial discrepancy between the two seamounts in terms of hydrographic dynamics, nutrient availability and chlorophyll maximum represents an interesting basis for evaluating biological dynamics and phytoplankton community structure. It is speculated and discussed that the ecological dynamics (in terms of phyto- and zooplankton abundance, diversity and function) are distinctively different at the two investigated seamounts, representing a case study for the biological diversity at tropical South Atlantic seamounts.