Darwin Harbour is one of Australia's largest deep-water harbours. Urbanisation and industrialisation have lead to increased loads of nutrients, metals and sediment into the harbour. There is a view that because Darwin Harbour is macro-tidal, pollutants will disperse rapidly. Hydrodynamic models have shown however, that the harbour is poorly flushed, and pollutant sources such as industrial discharges including sewage have been identified in recent reports as presenting the greatest management issue for water quality in Darwin Harbour. Because solutes are readily diluted, biota such as sponges that filter large quantities of water and integrate contaminants over time can be robust bioindicators for assessing water quality.