Control of phytoplankton biomass by dilution and mixed layer depth in the western Weddell-Scotia Confluence

Abstract

Hydrographic, nutrient and trace metal (iron, manganese, and aluminum) concentration data, collected as part of a 2-ship survey during austral summer 2004, were used to examine the influ- ence of upwelling and horizontal mixing on phytoplankton biomass in the region of Elephant Island and South Shetland Islands, Antarctica. Temperature/salinity property analysis and changes in trace metal and nutrient concentrations show that horizontal mixing of shelf waters, not upwelling from depth, is correlated with phytoplankton biomass in the upper mixed layer (UML). The interaction between changing UML depth and nutrient and trace metal concentrations in the UML results in a unimodal distribution of phytoplankton biomass centered at intermediate surface salinities of ~34. Principal component (PC) analysis of hydrographic and chemical observations resolved 3 compo- nents that accounted for 99% of the variability in nutrient and trace metal concentrations. The first PC accounted for a conservative loss of nutrients through dilution across a latitudinal salinity gradi- ent. The second and third PCs separated mixed layer depth and nutrient consumption. Although these 2 PCs accounted for just 20% of the variability in the data matrix, they accounted for 65% of the variability in mean phytoplankton biomass, and recreated the unimodal distribution of chlorophyll concentration when modeled across a salinity gradient. We propose that the distribution of phyto- plankton biomass is structured by the horizontal mixing of nutrient rich waters, derived from Weddell Sea Shelf Waters, with Antarctic Surface Water that enhances stratification and shoaling of the UML.