Nutrient transport through estuaries: The importance of the estuarine geography

• Nutrient retention in estuaries depends on the “geography” of an estuary.
• This geography is being modified greatly by human activity.
• Changes in geography and inputs contribute to changing fluxes to coastal waters.

We consider here first the nature of nutrient transport through estuaries and in particular the capacity of estuaries to modify that flux. We then focus particularly on the Wash system in the UK as an example of a particular type of “small estuary”, and also consider some tropical estuarine systems in Malaysia. We present nutrient budgets for the Wash system now and create estimates of these budgets 3000 years ago. These show that currently the system is a small sink for fluvial nitrate (∼30% removal) largely due to denitrification and dissolved inorganic phosphorus (∼24%) by burial. 3000 years ago, prior to large scale human intervention to reclaim wetlands for agriculture, the Wash system was much bigger and operated as a sink for all terrestrial nitrogen (albeit with much lower inputs) and also a sink for nitrate from the North Sea, predominantly via organic nitrogen burial. A similar change with time is evident in the phosphorus flux. We suggest that this change in function of this estuarine system has been replicated in many other estuarine systems. Given the key role of benthic processes of burial and denitrification we go on to suggest that the classification of estuaries in terms of area and river flow may offer a route to a typology of estuarine nutrient retention efficiency.