Connectivity between estuaries influences nutrient transport, cycling and water quality

Abstract

Water quality in coastal and estuarine systems is dictated by nutrient supply and circulation. However, convoluted coastlines, intermittent river discharge, complex currents, and variable nutrient inputs and transformations, confound interpretation of sparse water quality observations. In high use regions with competing human activities, this system complexity precludes identification of factors causing poor water quality, which can lead to conflict between users. High resolution 3D hydrodynamic and biogeochemical models can simulate the dynamics of a region and demonstrate nutrient pathways in convoluted systems. Here, we use a 3D coupled hydrodynamic, sediment and biogeochemical model to investigate the fine-scale (<100m; <1day resolution) nutrient dynamics of temperate coastal waters in southeast Tasmania, Australia. The fate and transport of anthropogenic nutrients between the Huon estuary, the D'Entrecasteaux Channel and the Derwent estuary is of concern and we use the model to explore the regional contribution and connectivity of marine, catchment and anthropogenic nutrient loads. Model skill in the year 2009 is assessed against water quality observations throughout the region and nutrient budgets are evaluated for each sub-region. Results show seasonal variation in estuarine flow with persistent anthropogenic nutrient sources augmenting phytoplankton production, and reducing water quality in summer months. Nutrient-enriched Huon Estuary and D'Entrecasteaux Channel waters are flushed north to the mouth of the Derwent Estuary and then south into the open ocean, with greatest flux in winter. This demonstrates that water quality in the Derwent is not compromised by nutrient influx from the adjacent waters. By using fine-scale 3D biogeochemical models, such as demonstrated here, we can quantify nutrient transport pathways in coastal waters and resolve conflicts between users to support integrated marine management.