Mean weight and total biomass of zooplankton as a core indicator of biodiversity of the Marine Strategy Framework Directive: an example of the Gulf of Riga

INTRODUCTION The value of zooplankton as an indicator of ecological processes arises from its position in various food webs. Zooplankton acts as a middle point between top-down (fishes) and bottom-up (phytoplankton) regulators (Jeppesen et al., 2011). Thereby, zooplankton indirectly indicates trophic interactions between phytoplankton/bacterioplankton and zooplankton as well as zooplankton and fishes, hence, eutrophication as well as fish predation on zooplankton (Haberman, 1996). Direct predation pressure from fish can significantly impact on zooplankton communities. For example, predation-induced mortality leads to a high percentage of overall mortality in copepods (Hirst and Kiorboe, 2002; Tang et al., 2006; Martinez et al., 2014). Larger individuals of zooplankton are normally consumed in case of high rates of fish predation, which leads into a situation where domination within zooplankton communities is given to smaller individuals (Haberman, 1996; Brucet et al., 2010; Jeppesen et al., 2011). In addition to the size of zooplankton, top-down predation pressure is also affected by the morphology of various life history stages of zooplankton (Brooks and Dodson, 1965; Otto et al., 2014). A common assumption is that marine zooplankton is bottom-up controlled. Thus, it could be used as an indicator of climate change effects in the open ocean where anthropogenic impact on top of the food chain is considered to be negligible (Adrian et al., 2006; Barton et al., 2013; Daewel et al., 2014). Nevertheless, recently species on lower trophic levels have shown cascading effects in various marine ecosystems due to the overfishing of top-down predators (Casini et al., 2008, 2014). Various studies have focused on long-term dynamics of zooplankton in relation to hydro-climatic conditions in the adjacent sea (Viitasalo et al., 1995; Mollmann et al., 2000, 2008; Kotta et al., 2009). It is common in aquatic ecosystems that hierarchic response takes place along trophic levels, i.e. the intensity of response to eutrophication can vary among trophic levels (Hsieh et al., 2011; Lewandowska et al., 2014). Surprisingly, mesozooplankton has not been included into the European Water Framework Directive (WFD) as a quality element. The importance of mesozooplankton in terms of ecological environmental assessment has been demonstrated in rivers and lakes, and the necessity of including mesozooplankton in the WFD has been outlined (Jeppesen et al., 2011). However, mesozooplankton is included into the EU Marine Strategy Framework Directive (MSFD). On the basis of work carried out by the MSFD HELCOM zooplankton working group, a core indicator of food web structure based on mesozooplankton, i.e. the average size or weight of a zooplankter, was proposed by Gorokhova et al. (2013a). The indicator is also supported by total values of zooplankton abundance and biomass. Thus, the measure captures both zooplankton community structure (by mean weight) and the stock size (by biomass or abundance). In the current study we analysed the use of this indicator of good environmental status in the northeastern Gulf of Riga based on zooplankton data collected mainly from Parnu Bay. The effect of zooplankton as a food source affecting fish growth was evaluated. Seasonal and interannual variation of total abundance and biomass of zooplankton together with mean weight of a zooplankter was analysed based on both seasonal and long-term data. MATERIAL AND METHODS Field data were collected from the northeastern Gulf of Riga, mainly from Parnu Bay, between 1957 and 2013. In total 6746 quantitative samples of mesozooplankton were used (of these 3067 were collected from June to August). Zooplankton sampling and analysis followed the HELCOM recommendations (1988). Samples were collected with a Juday type plankton net (mouth opening 0.1 [m.sup.2]; mesh size 90 [micro]m) with vertical hauls from the seabed up to the surface. …