Zooplankton are a diverse collection of tiny aquatic organisms found drifting in nearly every water body on Earth. These microscopic animals are a foundational component of aquatic life, playing an integral role in the intricate balance of water ecosystems.
What Zooplanktons Are
Zooplankton are the animal component of the plankton community, distinct from phytoplankton. The term “zooplankton” originates from Greek words meaning “animal” and “wanderer,” reflecting their characteristic movement with water currents. While most are microscopic, some, such as certain jellyfish, can be seen with the naked eye and grow quite large.
These organisms are heterotrophic, meaning they obtain nutrients by consuming other organisms, unlike phytoplankton which produce their own food through photosynthesis. Zooplankton encompass a variety of life forms, including copepods and cladocerans (small crustaceans), as well as rotifers and protozoans. The larval stages of larger aquatic animals like fish, crabs, and mollusks also spend part of their lives as zooplankton.
Their Underwater Lives
Zooplankton inhabit virtually all aquatic environments, from freshwater lakes and rivers to the vast expanses of the ocean. They typically concentrate in the upper water column where their food sources, like phytoplankton, are abundant. While often described as drifting, many zooplankton possess limited swimming capabilities, allowing them to undertake daily vertical migrations. They move hundreds of meters deep during the day to avoid predators and ascend to the surface at night to feed on phytoplankton.
Their feeding strategies are diverse. Many zooplankton are filter-feeders, straining small particles like bacteria and phytoplankton from the water using specialized appendages. Other species are predators, actively hunting smaller zooplankton or even phytoplankton. Some zooplankton are omnivores, consuming a mix of plant-like and animal-like particles.
Foundation of Aquatic Food Webs
Zooplankton occupy a central position in aquatic food webs, serving as a link between microscopic primary producers and larger animals. They act as primary consumers by grazing on phytoplankton, which convert sunlight into chemical energy. They also function as secondary consumers when they feed on other zooplankton. This dual role in energy transfer is fundamental to aquatic ecosystems.
These tiny animals are a primary food source for an extensive array of marine and freshwater organisms. Small fish and the larval stages of many aquatic animals rely heavily on zooplankton for sustenance. Even some of the largest creatures on Earth, such as baleen whales, filter vast quantities of zooplankton, especially copepods, from the water. The abundance and composition of zooplankton directly influence the transfer of energy and nutrients throughout the food web.
Zooplanktons and Environmental Change
Zooplankton are highly sensitive to changes in their environment, making them valuable indicators of aquatic ecosystem health. They respond rapidly to various disturbances, including shifts in water quality due to pollution, nutrient enrichment (eutrophication), and sediment input. Changes in zooplankton abundance and species composition can signal environmental stress. For instance, rotifers are used to assess the ecological impacts of toxicants like endocrine disruptors and lead.
Climate change also significantly affects zooplankton populations. Ocean warming can alter their metabolism and increase energy demands. Ocean acidification, resulting from increased atmospheric carbon dioxide, disrupts calcification in species with calcium carbonate shells or exoskeletons, such as pteropods and some copepods. These impacts can lead to reduced feeding efficiency, impaired reproduction, and shifts in species distribution, affecting fish and marine mammals that depend on them.