Ecosystem service evaluation of streams for nutrient and bacteria purification in a grazed watershed

The importance of sustainable development of river systems based on ecological and socio-economic criteria is widely recognized. The "Integrated Valuation of Ecosystem Services and Tradeoffs" (InVEST) model, an ecosystem based model which couples biophysical and economic models, is used to examine the nutrient and bacteria mitigation processes in the ungauged grazed area draining to the Gulf of Kalloni, a semi-enclosed bay in Lesvos, Greece. Point sources and grazing land loads are neglected by the InVEST model, leading to poor estimation of nutrient loads at this site. We include additional terms to account for these sources. We also apply climate change scenarios to assess their likely impact on nutrient export and hydrology over the drainage area. We show that the nutrient mitigation capacity is currently exceeded in four sub-basins with a compensation cost of up to 20 million dollars over a 20 year period.