The benthos of an aquatic system are those organisms living on or in the substrata. They often play an important role in the processing of organic matter, and are an important part of the diets of many fishes. This exercise will introduce you to the diversity of benthic macroinvertebrates that inhabit lake (lentic) and flowing water (lotic) systems.

The density and diversity of the benthos community in Lake Winona and the backwaters of the Mississippi River will be less than those of benthos communities in a local trout stream during autumn.

The benthic macroinvertebrates inhabiting Lake Winona and a backwater of the Mississippi River will be examined and compared to those inhabiting Gilmore Creek.

Field collections:

Benthos in soft substrates will be collected with a Petersen dredge (or grab) and an Ekman dredge. The dredges consist of metal "boxes" equipped with jaws for taking a "bite" out of the bottom substrata. The Petersen dredge samples an area of 621 cm2, and the weight of the sampler allows it to work equally well in soft (e.g., silt, sand) and hard (e.g., stiff clay, gravel) substrata. The dredge is lowered into the water via a winch system and allowed to sink into the substrata. When the line is allowed to go slack, the closing mechanism automatically engages. The dredge is at first raised slowly, allowing the jaws to close completely, and then hauled to the surface. The contents of the dredge are then placed into a screen-bottomed wash pail, and the fine sediments are washed from the sample, leaving the organisms behind. The Ekman dredge is much lighter in weight, so works well only in silt or sand substrata. It is lowered and raised by hand-held rope, and a weighted messenger is used to close the spring-loaded jaws.

1) Collect several dredge samples from soft sediments in Lake Winona and the Mississippi River backwater. Place the organisms collected into enamel pans and count and identify the organisms in the field using the resources available.

Invertebrates from hard substrates within the lake and river backwater will be sampled with the "grab-and-inspect" approach. Hard substrates will be collected individually and all visible invertebrates will be removed, identified, and counted.

2) Collect and process invertebrate samples from several different habitats (e.g., submerged rock riprap, submerged woody snag). Measure the surface area of each object sampled to allow for density determinations.

Stream-dwelling invertebrates will be collected with a Hess sampler, an open-ended, screen-sided cylinder with attached net. The sampler is pushed down into coarse substrates, the substrates within the cylinder are agitated to remove invertebrates, and the invertebrates are collected within the net.

3) Collect and process several benthos samples from Gilmore Creek using a Hess sampler (area of 1000 cm2 or 0.1 m2).

1) Compare the types of organisms collected among the different systems (lake vs. backwater vs. stream) and habitats (soft sediments vs. snags vs. riprap).

2) Calculate densities (number per m2) for each sample collected from the different habitats/systems.

3) Calculate a Simpson diversity index (Ds, see Ecology lab manual) for each sample.

4) Compare densities and diversities among the systems (lake vs. backwater vs. stream) with the aid of Kruskal-Wallis or Mann-Whitney tests.

Return to Limnology Lab

Neal D. Mundahl
Winona State University
Winona, MN 55987-5838