Remineralisation signals in the western South Indian Ocean thermocline: diagnosing local biogeochemical processes from nutrient stoichiometry

The western South Indian Ocean is a highly dynamic region where tropical and subtropical waters converge. As in other ocean areas, the nutrient conditions of its thermocline play a critical role in setting regional productivity, yet they remain largely unstudied. Here, we investigate local biogeochemical processes that alter thermocline nutrient ratios in the western South Indian Ocean by applying a regional optimum multiparameter analysis to WOCE data collected across the northern and southern entrances to the Mozambique Channel, the Southeast Madagascar Current, and the Agulhas Current. We first quantify the relative contributions of three proximate source waters (Equatorial, Tropical, Subtropical) to the regional thermocline (∼100–900 m), along with the nutrient (i.e., nitrate and phosphate) inventories expected from isopycnal mixing of these source waters. We then use the difference between the expected and observed nutrient concentrations and ratios to diagnose the occurrence of local biogeochemical processes such as organic matter remineralization and N₂ fixation. We find that distinct regimes characterize the two entrances to the Mozambique Channel, with nutrient-rich Tropical source waters dominating the northern entrance (>55%) while nutrient-poor Subtropical source waters dominate the south (>80%). At the northern entrance, overlapping nutrient addition and removal processes drive only minor changes to the thermocline nutrient inventory relative to the nutrients supplied by the proximate source waters, with no evidence of local N₂ fixation. By contrast, south of the Mozambique Channel (>25°S), including across the Southeast Madagascar Current and Agulhas Current, a strong local remineralization signal indicates nutrient addition to the Subtropical thermocline. These nutrients have a high nitrate-to-phosphate ratio relative to those supplied by the proximate and underlying source waters, which we interpret as evidence of local N₂ fixation. Our analysis shows that N₂ fixation occurs locally in the subtropical southwest Indian Ocean where it will fuel regional productivity and carbon export.