Quantifying the impact of riverine macronutrient fluxes on the eutrophication status of shallow temperate estuaries

Many temperate estuaries receive high riverine loadings of macronutrients that can lead to eutrophication problems during summer months when phytoplankton blooms develop, causing local ‘undesirable disturbance’ of the ecosystem. Christchurch Harbour is a small shallow microtidal enclosed estuary situated on the south coast of England between Poole Harbour to the west and the Solent to the east. It covers an area of 2.39 km2 , is fed by two rivers, the Stour and Hampshire Avon, and exchanges with coastal waters through a narrow channel known as ‘the Run’ at Mudeford. Under high river flows in winter, the estuary essentially becomes a freshwater lake with zero salinity values throughout the harbour. In summer, reduced river flow rates lead to long water residence times which combined with the shallow water depth and high nutrient loading produce sustained high concentrations (up to 90μgL-1) of suspended chlorophyll, caused by blooms of phytoplankton populations of dinoflagellates and cryptophytes. Recent data will be presented collected as part of a NERC funded Macronutrient Cycles Programme grant aimed at quantifying the fluxes, cycles and transformations of macronutrients in the Christchurch Harbour estuary, to better understand their responses to temporal patterns of change over varying degrees of predictability. Data from both summer boat surveys of the estuary and from our automated water quality and nutrient monitoring equipment deployed at the lowest gauging stations on both rivers and at the estuary mouth will be presented. This combination of monitoring is providing a more detailed understanding of how large changes in river flow influence the flux of macronutrients and their impact on the water quality of the estuary.