Plankton consists of organisms that live in the water column and are not strong swimmers. Many plankton species are microscopic or very small, although this category does include jellys. Plankton are an important component of the food web.
In the three lakes, we study two groups of plankton: phytoplankton and zooplankton. Phytoplankton are primary producers, capable of converting the energy in sunlight into starches, creating oxygen in the process. Often colored green, they function somewhat like plants. The other group is zooplankton, which eat the phytoplankton and are in turn eaten by fish. Thus, zooplankton are analogous to animals that graze on plants.
We encounter three types of phytoplankton in our lakes. The first, and the one we don’t talk about very much, is macroalgae. This algae appear as large branching structures that can look like plants, but they don’t have any of the structures found in plants. Macroalgae are rarely a nuisance, although an invasive macroalgae, starry stonewart, is appearing in more area lakes.
Filamentous phytoplankton are long strings of algae. Sometimes these attach to the bottom, sometimes they drape over plants, and sometimes they make up big “clouds” in the water that look like green cotton candy. Filamentous algae can float to the top and look like bubbling green scum. The term periphyton describes filamentous algae that is attached to rocks or plants.
Unicellular phytoplankton consist of individual cells or colonies of cells that float in the water column. These can give the water a green, brown, or sometimes red tint. At high densities, these form algal blooms that can reduce water clarity. Some blooms of blue-green algae (or cyanobacteria) are harmful algal blooms because these algae have the potential to produce toxins.
Unicellular phytoplankton have their own subgroups, including diatoms, green algae, golden algae, and blue-green algae. These algae cycle through growth patterns during the summer, although with warming weather and heavier rainstorms, that typical seasonal progression is often disrupted.
Our studies show that blue-green algae often dominate in our lakes for most of the year. Blue-green algae are lumped in with other algae, but they are actually a bacteria that can photosynthesize, called cyanobacteria. Cyanobacteria are a concern because they can produce toxins that can be harmful to humans and pets. Toxins may occur during a cyanobackteria bloom, so avoid green and scummy water.
Zooplankton are an important link in the lake food web, since they typically feed on algae, and become large enough that fish will eat them. Three key types of zooplankton are in our lakes. Rotifers are the smallest. Cladocera are usually very common. Sometimes called water fleas, cladocera can be voracious consumers of algae. Copepods are found almost exclusively in lakes, not streams.
Zooplankton can be effective at knocking down the amount of algae in lakes, but they are most effective eaters of diatoms and green and brown algae. Unfortunately lakes with alewives (fish that eat zooplankton) often have lower levels of zooplankton and thus higher levels of algae.
You can find more by reading the phytoplankton and zooplankton report from 2016.