mesozooplankton

Mesozooplankton refers to the middle-sized component of zooplankton, typically ranging from about 0.2 to 20 millimeters in length. It sits between microzooplankton and macrozooplankton in the pelagic food web and is found in many oceanic environments, including coastal zones and the open ocean, from polar regions to the tropics. Members of this size class contribute substantially to energy transfer between primary producers and larger predators.

Taxonomy and composition are diverse. The largest share is made up of crustaceans, especially copepods (Calanoida, Cyclopoida, Harpacticoida). Other important crustaceans include amphipods, mysids, and ostracods, along with pelagic molluscs such as pteropods. Some soft-bodied or gelatinous forms, including certain larval stages of larger animals, are also encountered within the mesozooplankton size range. The exact species mix varies by region, season, and depth.

Ecology and feeding roles vary within the group. Many mesozooplankton feed on phytoplankton or microzooplankton, while others are omnivorous or carnivorous, preying on smaller zooplankton or detritus. They are a primary food source for many planktivorous fish, squid, seabirds, and marine mammals. A number of species perform diel vertical migrations, feeding near the surface at night and residing deeper during the day, contributing to the ocean’s carbon cycle through respiration and fecal pellet production.

Life cycles are often complex, particularly for crustaceans, with multiple naupliar and copepodite stages. Population dynamics of mesozooplankton respond rapidly to changes in food supply and environmental conditions, making them sensitive indicators of ocean health.