Lake Organism

Emily Wolf, former Regional Extension Educator, currently a high school science teacher in Osakis, MN

What is it? A shriveled shrimp? An alien life-form? A vital part of the aquatic food web? The answer of course is the last one, a vital part of the aquatic food web. Macroinvertebrates, such as the sideswimmer pictured here, are small animals without backbones, just visible to the naked eye. These creatures are key in sustaining the quality of surface waters in Minnesota.

Why are they important? Macroinvertebrates are good indicators of pollution levels due to their relatively long life spans and functional roles in aquatic food webs. They provide better insight into aquatic ecosystem health than traditional water chemistry measures because they integrate the effects of disturbances or pollution over time. Studies have shown benthos to be useful in water quality assessment due to their diversity, wide distribution, and varying tolerances to pollution.

Water quality affects macroinvertebrates both directly and indirectly. Most species of mayflies typically do not tolerate acidic conditions, while other insects, such as caddis flies, are less sensitive. Amphipods, such as the sideswimmers mentioned earlier, are known to be negatively affected by increases in salinity (salt concentration), and decreases in pH. Deviations from normal patterns of reproduction and growth of certain amphipods have been studied as a result of temperature and water level changes. Temperature, dissolved oxygen, and pH can further influence absorption of metals and other pollutants, indirectly impacting organism health. Poor water quality can reduce macroinvertebrate diversity and alter community structure, which causes changes throughout the aquatic ecosystem.

Sensitive Organisms or Tough Bugs?

Using macroinvertebrates in monitoring is not a foolproof measure of water quality. Recent studies suggest that the commonly found macroinvertebrates have become tolerant of environmental disturbances. If you consider Minnesota’s history, such as the extensive logging in the late 1800s, the use of waterways as sewage conveyances or worse in the 1900s, and the growing influence of urban runoff, it seems miraculous that anything but the toughest of the tough are left.

Take a closer look at the shoreline. Why do fish and wildlife spend so much time in the nearshore area of lakes and rivers? That’s where the food is. Aquatic insects, snails, clams, leeches and other invertebrates are bountiful in the near-shore area among lush vegetation. Imagine the underwater forest-like habitat of submerged and emergent vegetation, filled with swimming, crawling, and burrowing creatures. It’s the perfect place to raise a fish with ample food and protection. Preserving this area or re-establishing vegetated buffers provides habitat for these macroinvertebrates, which in turn, serve as food for many species of fish and wildlife.

Macroinvertebrates also play a particularly important role in the “green-ness” of many lakes. Several macroinvertebrates are the herbivores of the water, consuming algae at incredible rates. A healthy macroinvertebrate population in combination with (lower) phosphorus loads is nature’s way of controlling algae blooms. Researchers have used this function to our benefit. The herbivores serve as prey for larger macroinvertebrates and fish fry. With this understanding, manipulations of fisheries have been used to alter the numbers of macroinvertebrate herbivores, improving water clarity of some shallow lakes.

The diversity of macroinvertebrates is simply astounding. Have you ever seen a leech with six or eight eyes, or one that was twelve inches long? Or observed the jet propulsion of the small dragonfly nymph? Before you begin to wonder if I’m describing creatures released from Area 51, I should tell you these are in fact inhabitants of Minnesota lakes. Perhaps it’s a good thing that winter is nearly here; that way you may be able shake off the hee-bee-gee-bees before going for a swim next summer. Just remember when you step into that cool water, you are not alone….

References:

• Batzer, D.P., B.J. Palik, and R. Buech. 2004. Relationships between envi ronmental characteristics and macroinvertebrate communities in seasonal woodland ponds of Minnesota. Journal of the North American Benthological Society 23(1): 50-68.
• Covich, A.P., and J.H. Thorp. 2001. Introduction to the subphylum crus tacea. Chapter 19 in J.H. Thorp, and A.P. Covich [eds.], Ecology and Classification of North American Freshwater Invertebrates. Academic Press Inc.
• Hellawell, J.M. 1986. Biological Indicators of Freshwater Pollution and Environmental Management. Elsevier Applied Science Publishers.
• Hilsenhoff,W.L. and R.L. Hine. 1982. Using a biotic index to evaluate water quality in streams.Wisconsin Department of Natural Resources Technical Bulletin Number 132:1-22.
• Johnson, R.K.,T.Wiederholm, and D.M. Rosenberg. 1992. Freshwater biomonitoring using individual organisms, populations, and species assemblages of benthic macroinvertebrates. Pages 40-125, Chapter 4 in D.M. Rosenberg and V.H. Resh [eds.], Freshwater Monitoring and Benthic Macroinvertebrates. Chapman and Hall.

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