Zooplankton, Grasshoppers of the Lake

Photo: Closeup of Daphnia (Cladocera) by MarekMis (edited)

We have mentioned zooplankton many times in our newsletter, particularly regarding their importance as a trophic link between phytoplankton (like algae) and larger organisms. We haven't yet devoted a newsletter article to zooplankton specifically. It's time to fix that!

Zooplankton are small animals that live in the water column. They can range in size from tiny, single-celled protozoans to much larger animals easily visible to the naked eye. The term zooplankton refers to animals ("zoo") that drift around in the water column ("plankton"). Zooplankton feed on bacteria, phytoplankton (including algae) and even on each other. Individual zooplanktonic organisms are typically short-lived (a few days to a month or more) but serve a vital role in our lakes in their short time.

Venn diagram of the planton

Grasshoppers of the Lake

Often, when talking to a group, I will note similarities between phytoplankton in a lake and grasses on a prairie. Like grasses, phytoplankton (via photosynthesis) use the sun's energy to form simple sugars that are essential for the survival of other organisms. Not all organisms can survive by eating grass (coyote, for a prairie example) or phytoplankton (largemouth bass, as a lake example). To continue the analogy, think of zooplankton as grasshoppers of the lake. Like grasshoppers eating grass, when zooplankton feed on phytoplankton they consume the phytoplankton’s sun-derived energy. As zooplankton and grasshoppers are fed upon by larger organisms, the sun-derived energy moves further through the food web.

Size comparison of zooplankton groups, with a US dime for scale

During summer we can expect to find around 1,500 individual zooplankton in a gallon of Missouri lake water. Ultimately, the number of zooplankton in a lake is largely determined by available food. That means that nutrient rich lakes are likely to have more zooplankton than clear lakes. The size of individuals within the zooplankton community is determined by predator pressure. In other words, the things that eat zooplankton can shape the zooplankton community. If many of the larger zooplankton are eaten by gizzard shad (for example), the lake will be dominated by smaller zooplankton.

Let’s look at three zooplankton types we find in Missouri's lakes.

Rotifers

Of the three groups discussed here, rotifers are the most abundant and typically have more biomass per lake than the other groups (Canfield and Jones, 1996). Rotifers are also the smallest of the three groups, with an individual's length roughly the same as the thickness of one or two sheets of paper (0.2 mm). There are about 2,200 species of rotifers described worldwide and they are more commonly found in freshwater than saltwater. Rotifers have cilia near their “mouths” and use them to create currents that draw in food. While rotifers may be found attached to plants and other underwater surfaces, it is only when they are free-floating that they are part of the zooplankton community.

Photo: Bdelloid rotifer, Credit: Diego Fonteneto (edited)

Copepods

Copepods are crustaceans (like lobster and shrimp) and are much larger than rotifers. There are around 13,000 species known worldwide. Feeding primarily on phytoplankton and smaller animals (like rotifers), they move by paddling their second set of appendages (the word Copepod means “oar feet”). Their distinctively large antennae feature receptors to help them detect food, mates, and predators.

Photo: Several Copepods. The structures on either side of the tail are egg sacs.
Credit: Andrei Savitsky (edited)

Cladocera

Cladocera are also known as “water fleas”. They are crustaceans, like Copepods, and are similarly sized. Around 1000 species have been described worldwide. Freshwater Cladocera are primarily filter feeders, using their rear lower appendages to generate a current that brings phytoplankton and microbes to their filtering parts. The prominent antennae at the head are used for locomotion. Reproduction is typically asexual for the Cladocera, however they will reproduce sexually when environmental conditions are unfavorable for the population. The eggs generated from sexual reproduction are called ephippia and are durable enough to withstand long periods out of water.

A daphnia giving birth. Credit: MarekMis (edited)

The next time you’re out on the lake, take a clear, glass jar with you and fill it with lake water. The odds are very good that you will see the larger zooplankton, like Copepods or Cladocera, swimming around. If you are filtering water for the Lakes of Missouri Volunteer Program, you may even see zooplankton on your finished filter.

For more information:

Canfield, T.J. and J.R. Jones,1996. Zooplankton abundance, biomass, and size-distribution in selected midwestern waterbodies with relation to trophic state. Journal of Freshwater Ecology. Vol 11. (PDF link)