Modelling the Impact of Macronutrients on the Eutrophication Status of Small Estuaries

Eutrophication in temperate estuaries is a growing issue that has been extensively studied and is well constrained in large systems such as Chesapeake Bay and the Baltic Sea. However, understanding of eutrophication and its associated problems in small shallow estuaries is lacking. Christchurch Harbour is a small microtidal lagoon-type estuary in Dorset, UK, with previous occurrences of oxygen undersaturation in summer months when river flow is low. The aim of this project is to predict the impact of macronutrients on the eutrophication status of Christchurch Harbour and ultimately provide insight to the effect of future climate change scenarios on water quality in small estuaries. Modelling water quality at such small scales has rarely been carried out, so a depth-average barotropic hydrodynamic model of the harbour has been developed in MIKE 21 to investigate the physical processes controlling circulation and flushing times in the estuary. The model, validated against water level, predicts the physics of the estuary well. Variable river flow simulations indicate a switch in circulation controls from tidally- to fluvially-driven which is supported by flushing time estimates using a particle tracking tool in MIKE Zero. The particle tracking results also indicate neap flushing times are longer than those at springs when river inputs are low, but when river fluxes are high, flushing times during spring tides are longer than at neaps. To identify the effect of these physical drivers on the biogeochemistry of the harbour, a water quality model has been coupled to the hydrodynamic model. By including nutrient concentrations in the model, the eutrophication status can be predicted over varying conditions. This will improve understanding of the system’s ability to recover from extended periods of low riverine discharge and high nutrient inputs, and allow for better understanding of the controlling factors of changes in water quality in small shallow estuaries in the UK.