Tracking episodes of nitrate depletion that lead to productivity using Lagrangian particles: application to the Great Australian Bight

Productivity, or events of phytoplankton increase, is often studied in an Eulerian framework. This approach can reveal where and how often these events occur; however, it does not provide information on the mechanism (e.g. vertical or horizontal advection of nutrient rich water) providing the necessary conditions for phytoplankton growth, or the nutrient sources. In contrast, examining productivity events with a Lagrangian framework, that is, following water parcel trajectories, allows deciphering nutrient sources and mechanisms that lead to phytoplankton growth (e.g. upwelling, eddies). This study simulates backward and forward trajectories of water parcels to examine the sources of nitrate that lead to phytoplankton increase, and the mechanisms by which nitrate is supplied into the Great Australian Bight (GAB), a highly productive semi-enclosed sea south of Australia. Once water parcel trajectories are found, nitrate and phytoplankton concentrations are interpolated onto the trajectories from outputs of an eddy-resolving hydrodynamic model coupled with a biogeochemical model. We find that nitrate concentrations along water parcel trajectories in this region are expected to differ considerable after 81 days; although currents can carry water from the Pacific, Indian, and Southern Oceans into the GAB within this time frame, most of the nitrate supplied to the GAB cannot be tracked back to a specific ocean. The largest nitrate input to the GAB occurs in the upper 100m; however, a substantial amount comes from deep waters due to topographically induced upwelling when flow crosses deep isobaths (~2000m). Episodes of nitrate depletion that lead to productivity have both periodicities of few days, and a strong annual oscillation that have persisted over the last ten years. Our study shows how a Lagrangian approach can be used to characterize productivity episodes while deciphering the sources and mechanisms by which nutrients that trigger them are supplied.