Simulated nutrient and phytoplankton dynamics in the East Australian Current system

Abstract

Western boundary currents such as the East Australian Current (EAC) system are oligotrophic environments, and yet phytoplankton blooms still occur frequently. In the EAC however, the spatial scales, the drivers and the extent of the phytoplankton production are understudied. This is partly due to a paucity in both observations and models with which to understand the system. To address this, I develop a 10-year high-resolution numerical simulation of the EAC System that combines ocean physics and biogeochemistry, validated against satellite observations of surface chlorophyll. The key questions are: 1. What are the patterns of phytoplankton variability? 2. What are the conditions necessary for phytoplankton to bloom? 3. How is primary production sustained post-bloom? The first 4 modes of chlorophyll variance represent more than 99.5% of the variability in the system, and both this variability and the observed latitudinal gradient in nitrate distribution can be explained by dynamical forcing. To investigate the conditions that lead to phytoplankton blooms, I first identify the best metrics with which to estimate mixed layer and nitracline depths through a systematic evaluation of a range of commonly used metrics. These are found to be an offset of 0.05 kg m-3 and 1 mmol N m-3 from reference density and nitrate concentrations respectively. The largest phytoplankton blooms (winter-spring transition) are preceded by a nitracline located at the base of the mixed layer and an incident shortwave radiation of ~200 Wm-2. The phytoplankton blooms occur in areas of high nitrate concentration at the nitracline, which increase poleward and away from EAC waters. Post-bloom, nitrate is depleted, and phytoplankton use recycled nitrogen (ammonium) that is generated locally. Regenerated (ammonium-based) production is shown to alternate with new (nitrate-based) production depending on time and space, conditioned by seasonality and the poleward penetration of EAC waters. During non-bloom periods, ammonium constitutes a significant nitrogen source in the system, with ~80% of the total primary production dependent on it. This work is the first systematic investigation of nutrient dynamics in the EAC system and is the foundation for further research into BGC dynamics in what is a hotspot of ocean warming.