ISEA Café: Plankton

Inland Seas volunteers and staff gathered in the Education Center to talk about Plankton. Here’s what we learned about types of plankton, methods for identification, and what counts as a plankton. Have you ever wondered if you qualified as plankton?

A female copepod plankton
A calanoid copepod, about 1.5 mm in length. Relatively abundant in the Great Lakes.

What is a plankton?

The term ‘Plankton’ means drifter. Plankton are extremely weak swimmers, some can’t swim at all and just drift on currents, others rely on gas bladders to regulate their position in the water. For most, their swimming motions prevent them from sinking, rather than provide them locomotion in a particular direction. They feed on things they bump into; very few intentionally move toward forage or prey. Many plankton migrate vertically in the water column on a daily cycle – phytoplankton toward the surface in the day, and zooplankton toward the surface in the evening. Phytoplankton manage this migration by regulating the size of an internal gas chamber. (Phytoplankton make oxygen through photosynthesis and carbon dioxide through cellular respiration so they have ready access to gasses even under water.)

Although they can’t move far, copepods (pictured above) are among the fastest creatures on the planet. This make them very hard to catch, so most zooplankton eaters concentrate on eating cladocerans and rotifers instead. Although the vast majority of plankton are microscopic organisms, some jellyfish – which qualify as plankton – can grow pretty large. The largest was a Lion’s mane jellyfish that was measured at 7’6″ wide and 121’5″ long – that’s a huge plankton!

Identifying Plankton

Microscopic plankton can be difficult to identify; to tell one type from another you need to look carefully at the features, movement, and size of what you’re looking at. Want to brush up on your local plankton identification? Inland Seas has created these plankton identification and fact cards (Plankton ID cards p1, Plankton ID cards P2) to give you an idea of who is swimming in northern Lake Michigan.

Found a species you want to identify? Use this handy image-based key by the University of New Hampshire. Use their Zooplankton Identification Guide for more help.

To find out more about Great Lakes zooplankton, visit this page assembled by students at Central Michigan University, or this page by USGS on Great Lakes copepods that gives incredible detail while also being very accessible.

If you want to brush up on your identification skills, use the Plankton Portal, a fun way to help researchers with their data by identifying plankton.

Some tips from the experts

The volunteers at Inland Seas spend a good amount of time catching, finding, and identifying plankton; here are their tips for figuring out who’s who in lake:

  • Daphnia and Bosmina are more likely to be found near the surface. See if you can find them caught in the surface tension of the sample or attached to any bubbles. (Note: It would be unusual to see Daphnia, so if you think you’ve spotted one, show someone else what you found!)
  • Leptodorans are almost always seen from the dorsal view (looking down on their back). They are entirely transparent except for their eyespot, and generally produce a rhythmic, but not spastic, disturbance in the water as they attempt to propel themselves. Here’s a video. Look for them, they may be more common that we think!
  • Copepods can be divided into males and females; males have a bent right antenna.

What is the difference between algae and phytoplankton?

These two words are used a lot to describe the green stuff in our lakes. Are algae and phytoplankton different things, or are they different words to describe same things?

Phytoplankton is easy to define, so let’s start there. Phytoplankton are planktonic organisms that do photosynthesis. In the Great Lakes the most common phytoplankton are diatoms, dinoflagellates, chlorophytes (green algae), chrysophytes (golden algae), cryptophytes, and cyanobacteria. Cyanobacteria are a type of bacteria that perform photosynthesis. Everything else is a type of protist – not plants, not animals, not fungi, not bacteria, but protists. Let me repeat; phytoplankton are not plants, phytoplankton are protists. They are plant-like because they do photosynthesis, but I think it is time to raise the roof for the other types of life that make oxygen and capture energy for this planet! (Read more about the classifications of living organisms.)

OK fine, phytoplankton are photosynthesizing protists and bacteria, but are they also algae? The simple answer is yes, phytoplankton are also accurately called algae. This is true in the everyday sense, since anything greenish in the water is often called algae, and in the more particular scientific sense. The definition of algae varies a lot, but algae are usually defined as non-plants that photosynthesize. Some definitions of algae include cyanobacteria, and some do not, but cyanobacteria is also called “blue-green algae” and “harmful algal blooms” usually refer to outrageous quantities of cyanobacteria, so when someone, even a scientist, uses the term algae, I wouldn’t rule out cyanobacteria. All of the other phytoplankton in the Great Lakes are safely categorized as algae, and sometimes called unicellular algae, or microalgae.

One more thing…nearly all of the algae or “seaweed” you might find at the ocean such as kelp, nori, dulse, sea lettuce, etc. are also protists, not plants. They are called multicellular algae and although they look like plants, they lack specialized cell types and tissues unique to plants. We also have multicellular algae in the Great Lakes. Chara and Cladophora are protists, not plants!

Invasive Plankton, and other invasive species of note

The following species have been found in the Great Lakes region. Although each is small individually, their collective effects add up to a major problem for the stability and diversity of the local aquatic ecosystem.

  • Spiny Water Flea – a type of zooplankton that has established itself in the Great Lakes, and probably got here in shipping ballast water. This species has a big appetite for domestic zooplankton, and is virtually inedible to native fish. Learn more about Spiny Water Fleas.
  • Zebra and Quagga Mussel Veligers – One of the most destructive invasives in the Great Lakes are the small bivalves called zebra and quagga mussels. These pervasive invasives have a planktonic (veliger) stage at the beginning of their lives. Once they touch down on a suitable spot they begin to filter their food and grow their striped shells. The veligers probably also arrived in shipping ballast water. The massive plankton ‘Doughnut’ or gyre that was discovered in southern Lake Michigan is disappearing, very likely due to quagga mussels. Learn more about plankton levels in Lake Michigan.
  • Rock Snot – an invasive protist to the area that clings to hard substrates and forms colonies that alter habitat and foul water intakes and fishing gear. Looks and feels like white or tan wet wool. Ranges from small cotton ball sized patches to thick blankets and long ropy strings. Not slimy. It is a freshwater diatom. Find out more here. 
  • New Zeeland Mud Snail – These tiny snails (1/8″) are now found in Lake Michigan and have the potential to reach high densities. Large numbers, even if small in size, have the potential to outcompete local invertebrate populations. All the details are here.

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