Development of ecological classification criteria for the Biological Quality Element phytoplankton for Adriatic and Tyrrhenian coastal waters by means of chlorophyll a (2000/60/EC WFD)

This paper describes the data processing that led to the definition of ecological classification criteria for the Biological Quality Element (BQE) phytoplankton in the coastal waters (CW) of the Adriatic and Tyrrhenian seas, according to the Water Framework Directive (2000/60/EC). The chosen metric was the annual geometric mean of chlorophyll a concentrations owing to the log-normal nature of chlorophyll a distribution. The sensitivity of this metric to the gradient of pressures was tested by adopting an empirical statistical approach. The dilution factor (F_dil), which is the share of freshwater in a sample of seawater, was introduced as a rough, but realistic proxy of nutrient loads from the continent. Correlations between F_dil and trophic indicators (i.e. nitrogen and phosphorus concentrations in seawater and the respective N:P ratio) were then evaluated. The F_dil approach was also used to derive reference conditions for each typology of coastal waters. Functional relationships between chlorophyll a, as phytoplankton biomass indicator, and nutrient concentrations, as pressure indicators, were computed by means of regression techniques. The classification scale for the BQE phytoplankton was based on the TRIX scale of water quality conditions. Reference conditions, pressure/impact relationships, boundary setting and classification criterion definition were treated separately and discussed for each of the CW Types: Type I, Type II A Adriatic, Type II A Tyrrhenian and Type III W for both the Adriatic and Tyrrhenian seas. Due to the lack of a functional relationship between the gradient of pressures and chlorophyll a, and a narrow range of annual chlorophyll a concentrations, only one threshold value was set for Type III W instead of the whole classification scale. Specific topics dealing with the log-normal model adopted for chlorophyll a data and the adequacy of classification criteria are discussed in the Appendix.