Nutrient and oxygenation conditions in transitional and coastal waters: Proposing metrics for status assessment

In response to the requirements of the Water Framework Directive, a methodology is proposed for assessing the physico-chemical status, specifically nutrient and oxygenation conditions, of the transitional and coastal waters. The method involved several steps: firstly reference conditions were established according to the near-natural reference site criterion and taking into account the main anthropogenic pressures affecting each water body. Secondly, the data were analyzed by bootstrap statistical techniques; the use of percentiles rather than mean values is proposed, in order to minimize the standard sampling error. Finally, the quality ratios for each parameter were calculated taking into account variations in salinity and each status boundary defined as a 50% deviation relative to the next higher class. The final physico-chemical status of each water body was then defined according to the “one out all out” principle. The method was applied to coastal and transitional waters in the central part of the Cantabrian Sea (NW Spain). The results obtained reveal good physico-chemical conditions in most of the water bodies under study, and also indicate strong relationships between the anthropogenic pressures analyzed and the status of each water body.