Anomalous trend of chlorophyll-a as a response to mesoscale phenomena in a Brazilian oligotrophic ria

Saco do Mamanguá is a tropical Brazilian ria characterized by low nutrient availability and subject to seasonal thermocline, which is mainly influenced by the intrusion of South Atlantic Central Water (SACW) on the continental shelf. Sampling was performed in summer (2014, 2015) and spring (2017). The dissolved inorganic and organic nutrients (nitrogen [N] and phosphorus [P]), along with chlorophyll-a (Chl-a), were evaluated to characterize this area. As a result, the general trophic condition of Saco do Mamanguá is a meso-oligotrophic ria with low Chl-a concentrations. Due to the presence of an atmospheric phenomena that causes an extremely dry period during the summer of 2014, a strong thermocline was observed in the water column which was associated with high Chl-a concentrations (median of 6.14 mg m⁻³). These values were much higher than those observed during the summer of 2015 and in the spring of 2017 (median of 1.03 mg m⁻³). The dissolved inorganic nutrients presented low values in all studied periods with concentrations of nitrate (0.00–0.20 μmol L⁻¹) and phosphate (0.00–0.20 μmol L⁻¹). A wide range of dissolved organic compounds were also observed, as shown by the presence of dissolved organic carbon (DOC), which varied from 63 to 212 μmol L⁻¹; dissolved organic nitrogen (DON), which varied from 2.97 to 12.92 μmol L⁻¹; and dissolved organic phosphorus (DOP), which reached a maximum value of 0.63 μmol L⁻¹. DON and DOP represented at least 50% total dissolved N and P. In this region, the biogeochemical cycles of N and P could ensure the availability of regenerated nutrients from organic sources to support primary production of phytoplankton.