Abstract

Sophie Meyjes – Impact of global variability in zooplankton grazing rates on carbon export flux

The biological carbon pump is a key controller of how much carbon is stored within the global ocean. This pathway is influenced by food web interactions between zooplankton and their prey. In global biogeochemical models, Holling Type functional responses are frequently used to represent grazing interactions. How these responses are parameterised greatly influences biomass and subsequent carbon export estimates. The half-saturation constant, or k value, is central to the Holling functional response. Empirical studies show k can vary over three orders of magnitude however, this variation is poorly represented in global models. This study derives zooplankton grazing dynamics from satellite observations, resulting in global distribution maps of the grazing parameter k. The impact of these spatially varying k values on plankton biomass and carbon export flux estimates is then considered. This study finds large spatial variation in k values across the global ocean, with distinct distributions for micro- and mesozooplankton. High half-saturation constants are generally associated with areas of high productivity. Grazing rate parameterisation is found to be critical in reproducing satellite-derived distributions of nanophytoplankton biomass, highlighting the importance of top-down drivers for this size class. Locally optimised grazing dynamics modifies total carbon export by >16%, with increases in faecal pellet export and decreases in export from algal aggregates. This study highlights the importance of grazing dynamics to both community structure and carbon export, with implications for modelling marine carbon sequestration under future climate scenarios.

Joshua Lanham – Regimes of seasonal CDW intrusion onto the circumpolar Antarctic shelf in a high-resolution model

Basal melting of Antarctic ice shelves is driven by heat delivery from Circumpolar Deep Water (CDW). However, the mechanisms and routes of CDW inflow to the shelf are often uncertain. The formation of Dense Shelf Water (DSW) at the ocean-ice interface is thought to be a key factor in modulating the amount of CDW that can cross the shelf break. Coarse resolution climate models struggle to explicitly resolve DSW due to its highly localised formation areas. We classify water masses using an optimum multiparameter (OMP) analysis in an eddy-resolving formulation of MITgcm (SOHI). The analysis successfully reproduces the key features of the known near-shelf water mass configuration, including the sites of DSW formation. We identify five major circumpolar regimes of seasonal CDW intrusion on the shelf. The East Antarctic shelf region is largely characterised by a seasonal CDW cycle which is directly driven by the strength of the along-slope easterly winds. In West Antarctica, CDW concentrations on the shelf are primarily determined by the strength of the undercurrent, which is itself mediated by the seasonal strength of westerly winds associated with the Amundsen Sea low pressure system. Shelf regions of DSW production either show no significant CDW seasonality, or exhibit a CDW cycle which is an inverse of the cycle of DSW production. The distinction between these two regimes appears to be determined by shelf geometry. Finally, we find that shelf-ward CDW inflow can occur along the western edge of canyons during periods of DSW overflow.