Lake Trophic States
Last week I talked about the trophic state index, which I defined as a continuum in lake phosphorus and algal concentration with an assigned number between 0-100. Trophic states are divisions of that continuum. We classify lakes and give names to the different lake types so they can be easily referred to.
I think it's most helpful to understand the concept of trophic states when we break down the word into its components and look at the definitions of the components. The root "trophic" means nutrients or nutrition; therefore, lakes are classified based on the amount of available nutrients for organisms (Phosphorus and Nitrogen). More fertile lakes have more nutrients and therefore more plants and algae. Most of the lakes in Minnesota were formed as the glaciers receded, carving out basins in the landscape. In these newly formed lakes, the edges and bottom are exposed rock, which doesn't erode very quickly, meaning there are not many nutrients available. As a lake ages, sediment from the watershed is washed in, filling in the bottom of the lake. This sediment is rich in nutrients, and therefore also increases the fertility of the lake.
"Oligo" means very little; therefore, oligotrophic means very little nutrients. Oligotrophic lakes are typically in northern Minnesota and have deep clear water, rocky and sandy bottoms, and very little algae.
The fish found in oligotrophic lakes like cold, high oxygenated water, examples include lake trout and whitefish.
"Meso" means middle or mid; therefore, mesotrophic means a medium amount of nutrients. Mesotrophic lakes are typical for central Minnesota and have fairly clear water with aquatic plants and algal blooms in late summer.
Mesotrophic lakes are great fishing lakes and are home to many sport fish such as walleye, perch, smallmouth bass, muskellunge and northern pike.
"Eu" means true; therefore, eutrophic literally means true nutrients or truly nutrient rich. Eutrophic lakes are more typical of southern Minnesota where the soils are more fertile and where there is a lot of farmland. Eutrophic lakes are shallow and have murky water and muddy, soft bottoms. They also have an abundance of plants and algae.
In less eutrophic lakes, common fish include largemouth bass, northern pike, perch and panfish. As a lake becomes increasingly eutrophic, sport fish dwindle and carp abound.
Like I said last week, some lakes may be naturally eutrophic for the past 100 years. Other lakes have gradually increased in nutrients as a result of human activities and have gone from mesotrophic to eutrophic. The divisions of the trophic state index for each trophic state are roughly: 0-30 Oligotrophic, 30-40 Oligotrophic/Mesotrophic, 40-50 Mesotrophic, 50-70 Eutrophic, 70+ Hypereutrophic.
One drawback of defining trophic states as certain divisions in the trophic state index is that each lake is different. The trophic nature of a lake depends on many things including depth, area, watershed size, adjacent land use, and climate. A lake with a TSI of 49 falls on the borderline between mesotrophic and eutrophic, and there is some overlap. Trophic states should be considered a general definition of lake condition.
And again to re-iterate from last week, lake trophic state is not interchangeable with water quality. Water quality is subjective and depends on how you intend to use the water body. Those of you excited for opening fishing weekend are excited about catching as much fish as possible, not swimming in crystal clear water.
For more information on trophic states, visit: http://dipin.kent.edu/trophic_state.htm.
Until next week, enjoy the lakes!
Moriya Rufer is the Lakes Monitoring Program Coordinator for RMB Environmental Laboratories in Detroit Lakes, 218-846-1465, lakes.rmbel@eot.com.