Hydrologic Versus Biogeochemical Control of Nutrient Dynamics in a Shallow Hypersaline Coastal Lagoon: Insight From a Coupled Hydrodynamic‐Water Quality Model

Shallow coastal lagoons with restricted connection to the ocean are often productive but can also be sensitive to nutrient enrichment and hydrologic changes. Resolving nutrient dynamics is important for their sustainable management, yet being able to accurately resolve nutrient budgets has remained a challenge due to their complex hydrological regimes and habitat heterogeneity. In this study, we undertake a systematic nutrient budget of a large shallow hypersaline lagoon (Coorong, South Australia), with assistance of a high‐resolution coupled hydrodynamic‐biogeochemical model, to demonstrate the conditions that lead to nutrient retention. Under current conditions, high rates of evapo‐concentration and limited water connectivity have led to a persistent accumulation of nutrients and poor water quality in substantial areas of the lagoon. The interplay between hydrological drivers and biogeochemical processes was quantified using an adjusted Damköhler number, comparing the timescales of nutrient flushing versus processing. This showed a general transition from hydrologic control to biogeochemical control with increasing distance from the main ocean connection, modified by episodes of increased flows and external loads. Whilst water age was a useful indicator of the factors controlling rates of nutrient retention, interannual variability in retention between areas of the lagoon was explained based on river flows and changes in mean sea level. As the system has been affected by reduced flows over past decades, the results provide evidence that increasing river flow to the lagoon would reduce the nutrient retention, and we discuss the potential for net nutrient export to the ocean under sustained high flows. Plain Language Summary Nutrient loads to coastal waters have increased worldwide as a direct consequence of the growing human population and increased development, with the end result being degraded water quality. Coastal lagoons with topographically restricted connection to the ocean are particularly sensitive relative to other coastal ecosystems since poor flushing and evaporation promotes nutrient retention. Resolving processes and factors that “control” nutrient concentrations is therefore important for guiding water quality management and ecosystem restoration. We studied nutrient dynamics in the Coorong, a shallow hypersaline coastal lagoon located in South Australia, using a high‐resolution coupled hydrodynamic‐water quality model. We showed that redu