Ecological quality assessment under uncertainty for transitional and coastal waters: Central Eastern Adriatic Sea study

Abstract

The ecological quality assessment introduced by the European Union-Water Framework Directive (EU-WFD, 2000), based on the repeated measurements sustained over a historical time frame, are often challenged with a limited spatial and temporal frequency of data. Instead of dealing with statistical measures in terms of geometric means and/or some quantiles of quality descriptors, this paper proposes an alternative approach, which is based on quantifying the uncertainty using the full probabilistic description for selected quality indicators at the reference and impacted site. The impacted site(s) represents the water body to be evaluated for the ecological status as required by the WFD. The ecological quality ratio, representing the amount of deviation from a reference condition, is described as a probability of exceedance from the reference condition, named the risk of deviation. The ecological quality ratio is derived through the distributional approach enabling to estimate the probability of being in each of five quality classes for considered transitional and coastal water bodies. The classification boundaries for five quality classes are determined based on the trophic status and related coastal water quality conditions. The ability to estimate the probability that impacted site (or a water body under assessment) could be in any of five quality classes enables decision-makers, responsible for the WFD implementation, to select the Program of Measures, which will be specifically tailored to reduce the ecological quality ratio probability of being below a good quality class. The calculation procedure is semi-analytical and large number of sites and available data could be analyzed relatively fast. The freshwater flow over the salinity gradient in the estuary is analyzed to evaluate the impact on quality indicators in transitional ecosystems. The developed methodology is presented in the case study of coastal and transitional water bodies in the Central Eastern Adriatic Sea.