Fine-scale nutrient observations confirm simulated biogeochemical variability in estuarine, coastal and shelf systems

Fine-scale ( [ view full abstract ]

Fine-scale (<100m) biogeochemical models demonstrate that coastal waters, and in particular estuaries, have strong spatial gradients in water quality and high temporal variability. Diurnal and tidal fluctuations in biogeochemical properties are difficult to resolve using traditional bottle sampling techniques, and resulting data sets are often aliased by local variation which confounds interpretation. Whilst continuous temperature, salinity, chlorophyll, turbidity and oxygen sensors based on electrical and optical properties have been deployed for some time, continuous nutrient sensors are still relatively novel. In this paper we present data from 3 continuous nutrient observing systems deployed in an estuary, on a coastal glider and on a shelf mooring. Nitrate and phosphate data collected in the Derwent and Huon Estuaries with fine spatial and temporal (<100m; <1 hour) resolution characterise the natural variability in estuarine nutrient concentrations and identify surface nutrient plumes associated with point source discharge. Glider data provide a cross section of nitrate concentration and capture a nutrient enrichment event resulting from the passage of an ex-cyclone at Heron Island. Moored time series of optically determined nitrate were used to determine water mass nutrient relationships and identify cryptic upwelling events off the Queensland shelf. These data sets offer unique insights into the biogeochemical dynamics of these regions, and allow for the first time, a rigorous assessment of the spatial and temporal variability in the nutrient dynamics simulated by the CSIRO Environmental Modelling Suite (EMS) biogeochemical model.