We’ve completed our visits with lake phytoplankton and are ready to begin with a new group – the zooplankton. The term ‘zooplankton’ is a general one that refers to all of the very small swimming life found in lakes, ponds, and marine environments. Most zooplankton are microscopic, but some are large enough to be easily seen – think spiny water flea. The image above shows a mixed zooplankton sample with some common taxa –  Copepods: 1 Calanidae, 2 Temora stylifera, 3 Calocalanus, 4 Clausocalanus, 5 Oncaeidae, 6 gastropod larva, 7 doliolid, 8 fish egg, and 9 decapod larva. (Image: A. Zingone, D. D’Alelio, M. G. Mazzocchi, M. Montresor, D. Sarno, LTER-MC team , 2019. Creative Commons.org/license)

All of zooplankton can swim, but with their small size, they pretty much go where the water takes them. To learn more about their swimming moves, take a look at this Scientific American article.

spiny waterflea, showing size on human fingernail

Spiny water flea is a large zooplankton you can see with the naked eye. For more information on this invasive species, see the VT Invasives website. Photo courtesy of the NY DEC.

Zooplankton are not photosynthetic and must eat in order to survive. Anything small enough to fit into their mouthparts is fair game – bacteria, detritus, phytoplankton, other zooplankton. Zooplankton can be so abundant and so hungry that they can eat almost all of the phytoplankton, leaving the water very clear. This ‘clear water phase’ is an important annual spring event in many lakes. With that big of an appetite, you can see why zooplankton are an important link in the aquatic food web. They eat algae, use it to produce baby zooplankton, and in turn are eaten by fish.

The number of zooplankton in a lake is influenced by many different things. Size and depth of the lake are important. Smaller shallower lakes tend to have fewer large zooplankton. The amount of nutrients is important too. Lower nutrients typically mean less phytoplankton grows, meaning less food for zooplankton and therefore fewer zooplankton. Water temperature affects many different aspects of zooplankton life. They can grow more quickly and are physically more active in warmer water. Water temperature can affect swimming speed and how easily an individual zooplankton can avoid being eaten (For a deeper dive into movement, water temperature, and predation see this BBC video clip.)

zooplankton sampling with a plankton net

To learn more about zooplankton, you have to catch them first. Watch the University of Montana‘s Flathead Lake Biological Station team sample for them. Photo snipped from the FLBS video.

There are several taxonomic groups within the zooplankton:

  • Protozoa
  • Rotifers
  • Crustaceans
    • Cladocerans
    • Copepods
  • life stages of several different organisms that spend time swimming in the open waters (fish larvae, mollusc larvae, insect larvae ).

I’ll share more about these individual groups in future posts. Each of them has unique adaptations and behaviors that have helped them survive in the constantly changing conditions of the open water. Like other plants and creatures around the world, they are facing many new challenges – climate change, microplastics, and toxins are just a few. The amazing tangle of life, water chemistry, and physical conditions in even a single lake makes it difficult predict how zooplankton communities will adapt to their changing world. We also can’t easily predict what those changes may mean for the rest of lake life. Years ago, I worked as a research assistant at the Flathead Lake Biological Station in Polson, MT just as they recognizing a series of events that eventually disrupted a major tourist attraction in Glacier National Park. You can read about it here and here. Spoiler: it all hinged on a zooplankton species……..

Additional fun photos and video on zooplankton