Macroplankton

Macroplankton is a size class of plankton that comprises organisms larger than most microplankton. The category typically includes individuals ranging from a few millimeters to several centimeters in length, encompassing many gelatinous and crustacean species. Because size classifications in planktonology vary, macroplankton is sometimes described relative to nearby fractions such as mesoplankton, with differences arising from methodological choices in sampling and taxonomy.

Common constituents of macroplankton include gelatinous zooplankton such as medusae (jellyfish), siphonophores, salps, and other pelagic tunicates, as well as larger crustaceans like krill and amphipods. Also present are various arrow worms (chaetognaths) and certain large zooplankton species that inhabit the upper water column. These organisms are typically more mobile and often more visually conspicuous than smaller plankton fractions.

Ecologically, macroplankton occupy mid to upper oceanic layers and serve as important prey for a range of predators, including fish, seabirds, and marine mammals. They can influence predator–prey dynamics and contribute to nutrient and carbon transport through processes such as vertical migration and the production of fecal pellets that sink through the water column. The abundance and composition of macroplankton show substantial seasonal and regional variability, guided by productivity, water mass structure, and climatic conditions.

Study of macroplankton relies on nets with larger mesh sizes, specialized sampling gear (such as bongo and multinet rigs), and in situ imaging or video. Challenges include fragility and rapid personal changes in gelatinous taxa, which can complicate identification and quantification but are essential for understanding broader marine food webs and carbon cycling.