There are a number of zooplankton parameters proposed as indicators to evaluate environmental status of marine ecosystems. Mean size and total stock (MSTS) is the only zooplankton-based and HELCOM (Baltic Marine Environment Protection Commission - Helsinki Commission)-approved core indicator. MSTS was developed to evaluate the environmental status of the Baltic Sea based on total biomass (or abundance) and mean body weight of mesozooplankton. This indicator reflects status of the food web and zooplankton biodiversity. Both are qualitative descriptors for determining good environmental status (GES) as defined by the EU Marine Strategy Framework Directive 2008/56/EC. However, the existing indicator concept is applicable to the extent that it characterizes off-shore pelagic habitats, while use of MSTS for coastal habitats remains challenging. In this case study, we aimed to assess and discuss performance of MSTS applied to mesozooplankton data from the shallow Gulf of Riga. Both off-shore and coastal communities were included in the study. MSTS responses to variable environmental factors (temperature, salinity and riverine runoff) were analysed. Temporal variations in temperature revealed response in mean size, whereas salinity covaried with total stock (both - biomass and abundance). However, spatial variations of MSTS parameters stayed unexplained. The results demonstrate difficulties with and provide possible solutions for MSTS-based assessment, with a particular emphasis on coastal waters. The use of mesozooplankton abundance as a determinant parameter for assessment of coastal waters and substitution of the corresponding 99% confidence interval of the mean as an assessment value instead of the mean have been considered.
Keywords: Baltic Sea; Coastal; Environmental status; MSFD; MSTS; Mesozooplankton.