Work due 01 Jun 2016

a) Analytical methods

For the assessment of chlorophyll-a concentrations, different analytical methods are generally used. Although these different analytical methods generally give comparable results with reasonable to good correlations between methods, simple fluorometric and photometric methods are less accurate and therefore may be a source of uncertainty. It is recommended to include a description of analytical methods to determine chlorophyll and to account for different analytical methods in the trend analysis.

b) Definition of the growing season

In the current application, two growing seasons are distinguished, one for the northern part of the Baltic Sea (June-September) and one for the southern part of the Baltic Sea, the North Sea, NE Atlantic waters, Mediterranean and Black Sea (May-September). It is questionable whether using one growing season for all waters that range geographically from the Mediterranean, the Black Sea to the North Sea andBaltic Sea, is appropriate. For instance, in the Mediterranean Sea, growing seasons are not relevant whereas annual means may be more appropriate. Moreover, currently only surface concentrations are considered. However, in the Black Sea, not only do the chlorophyll concentrations show peaks in late winter, late spring and autumn, these peaks do not only occur at the surface but also in subsurface layer (BS SoE, 2008). It is therefore recommended to revise the definition of the summer period to a definition of the phytoplankton growing season that takes into account the differences between regional seas, for instance by applying the seasonal periods that were defined for the WFD or by RSCs. Adjustments of the definition of the growing season per subregion are currently being evaluated and will be taken into consideration in the future revision of the indicator methodology.

c) Classification and geographical aggregation

In the current trend analysis stations are aggregated per country and attributed to a regional sea, and there is no distinction between sampling stations that are strongly influenced by anthropogenic eutrophication (e.g. transitional and coastal waters) and sampling stations that are not or only to a minor extent influenced by eutrophication. It is therefore recommended to replace the classification system per regional sea by a classification system that accounts for the geographical differences within the seas. The threshold/targets established by the RSCs are already being evaluated and will be taken in consideration in future improvements of the indicator methodology. As the implementation of the MSFD is still in progress, a future option might be to apply the environmental targets that are being developed under the MSFD.