Anthropogenic nutrients and eutrophication in multiple land use watersheds: Best management practices and policies for the protection of water resources

Blooms of cyanobacteria (Microcystis aeruginosa) are becoming increasingly recurrent in the A Baxe dam reservoir (Galicia, Northern Spain) as a result of increasing levels of different anthropogenic pressues in the Umia catchment. The aim of this study is to develop a model that allows us to detect the spatial level where the main problems that trigger eutrophication occur. We used Mike Basin coupled with Load Calculator to model and visualize spatially explicit results of stream flow and N and P export. The results indicate increase in nitrogen (N) and phosphorus (P) concentrations that trigger eutrophication.High concentrations of nutrients at the Upper Umia derive from livestock, while at the Lower Umia the origin is sewage, highlighting impacts from diffuse pollution and point source pollution in different areas of the basin. Cyanobacteria blooms result from the influence of both contaminant sources, being also triggered by local environmental conditions such as temperature, solar radiation or flow rate. This model will be useful for predicting possible changes, alterations and evolutions that occur in the watershed, that can help ensure compliance with the Water Framework Directive. In addition to the model developed, the results indicate the need for the implementation of management practices in order to reduce the blooms of cyanobacteria (green systems for wastewater in rural areas, riparian forest restoration, etc.). Some sub-basins, such as the Gallo sub-basin, require immediate action since they are major contributors of nutrients to the main river as a result of the low carrying capacity of local waste water treatment plants.