Reference conditions for phytoplankton at Danish Water Framework Directive intercalibration sites

Phytoplankton is one of the biological quality elements included in the EU Water Framework Directive (WFD). Classification of water quality according to the WFD is based on the deviation of the present conditions from reference conditions. Given the lack of data from pristine conditions, this study used approximately 100-year-old measurements of Secchi depths from Danish waters in combination with relationships between Secchi depth and chlorophyll a (as a proxy for phytoplankton biomass) obtained from recent monitoring to calculate 'historical' or reference chlorophyll a (Chl-a) concentrations. Historical Secchi depth data were available for 9 out of the 11 Danish WFD intercalibration sites. At eight of the sites, reference summer (May-September) Chl-a concentrations were in the range 0.7-1.2 mu g l(-1). At one site, west of Bornholm in the western Baltic Sea, historical Secchi depth measurements date back to only the late 1950s corresponding to a calculated Chl-a concentration of 1.3 mu g l(-1). This value cannot be considered representative of reference conditions.