The mesozooplankton is a size fraction of zooplankton community (0.2 to 20 mm) that is addressed in the HELCOM- monitoring guidelines. Mesozooplankton constitute an important part of zooplankton in the pelagic food webs, since these are the organisms representing the link between primary producers (phytoplankton) and higher trophic levels (zooplanktivorous fish and invertebrates). Mesozooplankton community structure and productivity can be affected by changes in phytoplankton stocks, species/size composition and phenology. Further, alterations in mesozooplankton can influence prey availability for zooplanktivores and, thus, fish stock recruitment, as well as sedimentation of the primary production, which, in turn, may affect food supply to benthic animals and oxygen levels in the bottom water. Published Refereed Current 14.a Zooplankton biomass and diversity Mature Validated (tested by third parties) Multi-organisational International Method Specification of criteria