Does the Biogeochemical Effect of Upwelling Vary Depending on the Source Water Depth?.

Supervisor: Toby Tyrell (University of Southampton).
Upwelling systems are the most productive marine ecosystems in the world due to the nutrient fluxes they transport to the surface ocean. These nutrients regenerate at various depths and are carried by the upwelled source water. This study focuses on understanding the role of source water depth on the influence of biogeochemical effects of these nutrients on the surface water of major upwelling systems. The biogeochemical effects of an alkalinity tracer for CaCO3 cycling (Alk*), Dissolved Inorganic Carbon (DIC), Nitrate (N), Phosphate (P) and Silicate (Si) were studied for the Californian Upwelling System (CUS), the Equatorial Pacific Upwelling System (EPUS), the Mauritanian-Moroccan Upwelling System (MUS), the Benguela Upwelling System (BUS), the Omani Upwelling System (OUS) and the Southern Ocean. Data from GLODAPV2_2019 database, cruises from ICON Experiment and GENUS Project, were used to calculate the enrichment effects on the surface water, the molar ratios & Spearman’s correlations among them and their variabilities. Based on the results, the biogeochemical effects of Si & Alk* in the Southern Ocean and all the variables in the EPUS show a clear influence from their respective source water depths. However, the study indicates that, in addition to source water depth, the ‘Southern Ocean Silicon Trap’, regional water masses, conveyor belt properties, CaCO3 cycling, nutrient deficiency and air-sea CO2 gas exchange also act as the drivers which control the biogeochemical effect of upwelling in these systems. Two sample tests conducted for the variabilities of the enrichment effects and ratios as well as heterogeneity calculations among the upwelling systems suggest that these drivers not only modify the biogeochemical effects but also the molecular relationships among them, thus creating an overall heterogeneity among these upwelling systems. Finally, the study concludes with a new perspective to global ocean upwelling: Although source water depth is a significant driver, its influence on the upwelling effect is neither dominant nor homogenous. Rather, it is a single factor of a network of many drivers which interact with each other both regionally and globally within the upwelling systems. Together, they create a heterogeneity to the overall biogeochemical effect of upwelling in the global ocean.