Tiny zooplankton graze most of the ocean's daily plant growth
The biggest grazers in the sea are not whales or fish. Microscopic zooplankton, mostly single-celled hunters, eat an estimated 59 to 75 percent of the ocean's fresh plant production every day. Yet scientists have measured their grazing only about 1,600 times worldwide, against more than 50,000 measurements of the growth they consume.
The name means animal drifter, from Greek words for animal and wanderer. Zooplankton cannot make their own food, unlike the phytoplankton, the microalgae and cyanobacteria that harness sunlight, so they must eat other organisms. They range from protozoans such as foraminiferans and radiolarians to jellyfish, krill, copepods, arrow worms and the larvae of fish. Some drift their whole lives; others pass through a planktonic stage before settling on the seabed or becoming free swimmers. The line between plant and animal blurs at this scale: over half of microscopic plankton may be mixotrophs, drawing energy both from internal plastids and from prey.
Drifting does not mean helpless. Many can swim enough to dodge predators, rising and sinking daily in vertical migrations, or to find more prey. They gather in patches shaped by salinity, temperature, breeding and, above all, the mixing of water that brings up nutrients and feeds phytoplankton blooms. As links between algae and fish, they package organic matter into the ocean's biological pump, and they also concentrate pollutants such as mercury up the food chain. Crustacean zooplankton even harbour the cholera bacterium, which clings to their chitin shells for carbon and nitrogen.
Size matters enormously. Plankton are mostly cold-blooded, and in colder water they grow more slowly but mature larger, a pattern called the temperature-size rule that has long puzzled biologists, since theory predicts the opposite. Oxygen supply may be part of the explanation, but it is hard to separate from temperature in the surface ocean.
Because copepod-dominated mesozooplankton usually live less than a year and are rarely fished, they make sensitive gauges of climate change. Research ships catch them with fine nets, including paired bongo nets lowered to about 200 metres and hauled up through the sunlit zone.
Source: Zooplankton
