Planktonic copepods are important to global ecology and the carbon cycle. They are usually the dominant members of the zooplankton, and are major food organisms for small fish such as the dragonet, banded killifish, Alaska pollock, and other crustaceans such as krill in the ocean and in fresh water. Some scientists say they form the largest animal biomass on earth. Copepods compete for this title with Antarctic krill (Euphausia superba). C. glacialis inhabits the edge of the Arctic icepack, especially in polynyas where light (and photosynthesis) is present, in which they alone comprise up to 80% of zooplankton biomass. They bloom as the ice recedes each spring. The ongoing large reduction in the annual ice pack minimum may force them to compete in the open ocean with the much less nourishing C. finmarchicus, which is spreading from the North Sea and the Norwegian Sea into the Barents Sea.
Because of their smaller size and relatively faster growth rates, and because they are more evenly distributed throughout more of the worlds oceans, copepods almost certainly contribute far more to the secondary productivity of the worlds oceans, and to the global ocean carbon sink than krill, and perhaps more than all other groups of organisms together. The surface layers of the oceans are believed to be the world's largest carbon sink, absorbing about 2 billion tons of carbon a year, the equivalent to perhaps a third of human carbon emissions, thus reducing their impact. Many planktonic copepods feed near the surface at night, then sink (by changing oils into more dense fats) into deeper water during the day to avoid visual predators. Their moulted exoskeletons, faecal pellets, and respiration at depth all bring carbon to the deep sea.
About half of the estimated 14,000 described species of copepods are parasitic
 and many have adapted extremely modified bodies for their parasitic lifestyles.
They attach themselves to bony fish, sharks, marine mammals, and many kinds of invertebrates such as corals, other crustaceans, molluscs, sponges, and tunicates. They also live as ectoparasites on some freshwater fish.