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 plankton
Grasshoppers of the LakeOften, 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.
RotifersOf 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)
CopepodsCopepods 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.
CladoceraCladocera 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:
Photo: A group playing Flipping Lakes. Image from YouTube
Educators in the Netherlands have created a card game that makes learning about lake pollution fun.
In this turn-based game, called Flipping Lakes, players construct their watershed and flip cards to introduce random pollution events. Your job as a lake manager is to keep pollution out of your lake, located at the bottom of the watershed. You can spend your limited money on preventative actions like lake dredging, public education, treatment plant construction, etc. Different actions have different costs, so you must choose wisely!
From the website:
Flipping Lakes is a game in which you, as a player, take on the role of a catchment water manager. As the water manager you are tasked to prevent a recreation lake from “flipping” from a clear and biodiverse state to a turbid and algae rich state. During the game, your catchment slowly becomes polluted by a diverse range of sources, such as farmland or sewers, and the effects of those are worsened by societal or climatic scenarios. To counter the impact of pollution and to keep your lake clean, you need to implement measures and use the intrinsic properties of the catchment. It’s time to gear up. The battle is on!
The game is free and you can print out everything you need to play.
Flipping Lakes looks to be a fun way to convey lake pollution prevention concepts at a lake board meeting, in a classroom setting, or with a service club.
You can download the game at https://nioo.knaw.nl/en/flipping-lakes
Game cards from Flipping Lakes