mesoplankton

Mesoplankton are the middle-sized planktonic organisms in aquatic ecosystems, comprising zooplankton with lengths roughly from 0.2 millimeters to 20 millimeters. They lie between microplankton (about 0.02 to 0.2 millimeters) and macroplankton. The mesoplankton community is diverse, but the largest component consists of crustaceans such as copepods (including Calanoida and Cyclopoida), mysids, and amphipods; non-crustacean groups include chaetognaths (arrow worms), appendicularians, and some polychaete larvae and larval tunicates. The exact species composition varies with region, depth, and season.

Ecologically, mesoplankton occupy a key trophic position. They feed on phytoplankton, microzooplankton, and organic particles, and they themselves are major prey for fish larvae, larger zooplankton, and some seabirds. Their feeding activity can influence carbon transfer and nutrient cycling, including participation in vertical carbon flux through respiration and fecal pellet production. Many mesoplankton exhibit diel vertical migration, feeding in the photic zone at night while descending to depth during the day to avoid predators.

Habitat and distribution: They are found in virtually all marine and freshwater systems, from coastal shelves to the open ocean, usually concentrated in the upper mixed layer, though some species inhabit deeper layers. The size range makes mesoplankton detectable with standard zooplankton nets using mesh sizes around 180–200 micrometers.

Life history: Members such as copepods have complex life cycles with multiple naupliar and copepodite stages; generation times vary from days to weeks, enabling rapid population responses to environmental changes.