AWRI Education

Instructor's Manual - Plankton Sampling

What are plankton?

Plankton, a word loosely meaning, "to drift", are distributed throughout the lake. They are found at all depths and are comprised of both plant (phytoplankton) and animal (zooplankton) forms. Plankton show a distribution pattern that can be associated with the time of day and seasons.

There are three fundamental sizes of plankton: nannoplankton, microplankton, and macroplankton. The smallest are nannoplankton that range in size from 5 to 60 microns (millionths of a meter). Because of their small size, most nannoplankton will pass through the pores of a standard sampling net. Special fine mesh nets can be used to capture larger nannoplankton.

Most planktonic organisms fall into the microplankton or net plankton category. The sizes range from the largest nannoplankton to about 2 mm (thousandths of a meter). Nets of various sizes and shapes are used to collect microplankton. The nets collect the organisms by filtering water through fine meshed cloth. The plankton nets on the vessels are used to collect microplankton.

The third group of plankton, as associated with size, are called macroplankton. They are visible to the unaided eye. The largest can be several meters long.

How are plankton sampled?

using plankton netThe plankton net or sampler is a device that makes it possible to collect both phytoplankton and zooplankton samples. For quantitative comparisons of different samples, some nets have a flow meter used to determine the amount of water passing through the collecting net.

The plankton net or sampler provides a means of obtaining samples of plankton from various depths so that distribution patterns can be studied. Quantitative determinations can be made by considering the depth of the water column that is sampled. The net can be towed to sample plankton at a single depth (horizontal tow) or lowered down into the water to sample the water column (vertical tow). Another possibility is oblique tows where the net is lowered to a predetermined depth and raised at a constant rate as the vessel moves forward.

What is commonly found in plankton samples?

The base of the food chain for Lake Michigan is plankton. The phytoplankton are the producers and they are typically green algae, cyanobacteria (previously known as blue green algae), and diatoms.  Cyanobacteria, such as Microcystis, prefer warm water and high nutrients.  Some cyanobacteria can fix nitrogen and produce toxins.  There are times when Spring Lake and Muskegon Lake experience algal blooms.  During these episodes, the lakes turn green as if green paint had been spilled.  Crustaceans such as water fleas (Daphnia), cyclops, and copepods are representatives of the consumers or zooplankton found in samples.  Examples of species collected in plankton tows can be found in the plankton data sheet and the drawings in Appendix D.

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Page last modified March 11, 2014