Water, water, every where,
In lake or river, I oft sink;
Water, water, every where,
What is it that I drink?
- by a (not-so) Ancient Mariner
It’s easy to look at a river and see the flashy fish swimming through its currents, or the swooping swallows as they flit above the surface to snatch emerging insects. But there is a hidden world in every drop of water that is vital, from the top of the food chain to the bottom.
The group of organisms we call “zooplankton” are primarily composed of animals known as rotifers and the crustacean cladocerans and copepods. Many are only visible by using a magnifying glass or microscope, though some of the larger cladocerans and copepods may be seen as small specks darting through a glass of river water.
Most are parthenogenetic (meaning all-female) during the growing season, producing eggs that are essentially clones of the adult. During times of stress (e.g., too hot/cold, too dry, too little food, etc.), males will develop and they will form dormant structures known as resting eggs that persist until conditions improve. Individuals may live anywhere from a few days to several weeks. This quick generation time makes them ideal test subjects in life table studies investigating the impacts of chemicals on characteristics like growth, reproduction, and mortality.
Zooplankton fill an important niche in the ecosystem. They eat phytoplankton and can help control harmful algae blooms. In turn, they are food for aquatic insects and are the first food source for most species of fish right after they’re hatched, from the smallest minnow to the largest catfish.
Daphnia species, a type of cladoceran zooplankton. Image taken under a compound microscope.
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Disruptions in zooplankton populations are felt throughout the food web. The invasive bigheaded, or Asian, carp are filter-feeders and consume large quantities of zooplankton throughout their life. Their invasion on the Illinois River has caused huge declines in populations of larger zooplankton, leading to declines in our native planktivorous fishes like bigmouth buffalo and gizzard shad. Declines in gizzard shad populations may be felt higher up the food chain, as it is one of the primary forage fish for predators (and important sportfish) like largemouth bass and white bass. Indeed, recent research has linked declines in several sportfish species to the concurrent rise in bigheaded carp populations.
But all hope is not lost.
Large-scale bigheaded carp harvest efforts on the upper Illinois River (representing partnerships between the Illinois Natural History Survey, Illinois Department of Natural Resources, commercial fishermen, and several federal agencies) have resulted in improvements in the gizzard shad community, which might also suggest a rebound in the zooplankton community.
So, next time you’re out on the river and inadvertently take a big gulp of water, channel your inner paddlefish and think fondly of the microscopic cocktail shrimp you just consumed!
