Zooplankton are microscopic animals that drift within aquatic environments, including oceans, lakes, and ponds. These diverse organisms vary significantly in size, from microns to several millimeters, and encompass a wide range of groups such as copepods, cladocerans, rotifers, and protozoans. Zooplankton play a foundational role in aquatic food webs, consuming smaller organisms and, in turn, becoming food for larger aquatic life.
The Diverse Diet of Zooplankton
Zooplankton exhibit a broad range of dietary preferences, depending on their species, size, and habitat. Their primary food source is phytoplankton, which are microscopic plants and algae that perform photosynthesis. Zooplankton consume various types of algae, including diatoms and dinoflagellates.
Beyond phytoplankton, zooplankton also feed on bacterioplankton and detritus, which are organic particles or marine snow. Some larger zooplankton species are predatory, consuming smaller zooplankton, creating complex trophic interactions. For example, some copepods are selective feeders, choosing prey based on size, shape, and even nutritional value. Environmental conditions, such as nutrient availability and light, also influence the availability and type of food sources, further diversifying their diets.
How Zooplankton Capture Food
Zooplankton use various mechanisms to capture food. Many species, such as copepods and cladocerans, are filter feeders. They use specialized appendages, like hair-like structures called setae, to create water currents that draw food particles towards them, sieving them from the water.
Other zooplankton engage in active predation. These organisms, including certain protozoan zooplankton and some copepods, actively hunt and capture individual prey using grasping appendages or modified mouthparts. They can detect prey through chemical cues or changes in light, then pursue and engulf them, a process known as raptorial feeding. Some simpler organisms, like amoeboid zooplankton, can directly engulf their food.
Ecological Significance of Zooplankton Feeding
Zooplankton feeding habits are central to aquatic ecosystems. They serve as the primary link for energy transfer from microscopic primary producers, like phytoplankton, to higher trophic levels, such as fish and marine mammals. This transfer supports a wide array of aquatic life.
Zooplankton also contribute to nutrient cycling within water bodies through their feeding and waste products. Their consumption of phytoplankton helps regulate algal populations, preventing overgrowth that could lead to nutrient depletion or harmful algal blooms. Furthermore, their feeding plays an indirect role in the biological carbon pump. When zooplankton consume carbon-rich phytoplankton, the carbon is incorporated into their bodies, and their fecal pellets can sink to the deep ocean, helping to remove carbon from surface waters.