Eutrophication in transitional waters: an overview

Despite their ecological and economic importance, Transitional Waters (TWs) have fallen behind all the other water categories in respect to the implementation of the Water Framework Directive (WFD) and further, pose some interpretation problems in other existing Water Directives. The problems faced by TWs concerning the implementation of WFD are mainly related to their characteristics, including: the high spatio-temporal variability, the fast response time to perturbations, their high productivity, the fact that primary production in these systems is normally not dominated by phytoplankton, their high socio-economic importance with a long historical tradition, and the strong anthropogenic pressures which exists in these systems. The present review attempts to bring together the main elements characterizing TWs and to identify the current understanding of the process of eutrophication and the problems this raises in establishing reference conditions, in view of the need of implementing the WFD. It is thus necessary to reach agreement on working definitions of TWs that can form the basis for the development of methodologies permitting the establishment of reference conditions; gain an understanding of the processes of eutrophication and the drivers and pressures that play a major role in their evolution; and investigate the use of indicators susceptible of accurately reflecting the ecological quality status of these types of water, as required by the WFD.