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

Abstract

Zooplankton has been acknowledged as an intermediate link between bottom-up and toptown regulators, thereby indirectly describing trophic interactions in various food webs. We used zooplankton as a core indicator of biodiversity in the species-poor ecosystem of the northeastern Gulf of Riga by evaluating its role as a dietary component of pelagic fishes. Furthermore, seasonal and interannual variation of total abundance and biomass of zooplankton with mean individual weight of a zooplankter was analysed based on field data collected between 1957 and 2013. The dominating species in mesozooplankton were rotifers and copepods. Abundance and biomass estimates of zooplankton indicated the highest values during summer months (June: peak abundance; August: highest biomass). The dominating species during the peak abundance were rotifers and copepods; the biomass maximum was indicated by copepods (in June) and cladocerans (in July and August). When averaged over summers, the total zooplankton abundance was 156.2 ± 2.4 thousand ind/m3, biomass 62.9 ± 1.3 mgC/m3, and zooplankter individual weight 0.433 ± 0.004 µgC/ind. Our study showed that mean zooplankter weight and total zooplankton biomass correlated with the agespecific herring weight data. To conclude, the structure and stock size of the zooplankton community adequately indicated strong effects of zooplankton on fish size and growth.