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This column is part two to last week's column. We began with the question, "So how are the lakes in the Detroit Lakes area?" Since this is such a complicated question, I broke it out into two parts.

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There are many factors that go into evaluating a lake. Once you understand some of these factors, you should have a better understanding of your lake, where it came from and where it's headed.

There are natural factors that contribute to a lake's condition, and in most cases around here, there are also human factors that have affected the lake. Last week I explained the natural factors (you can read last week's article at: http://www.rmbel.info/Reports/Static/LL_compare1.aspx). This week, I'll explain the human factors that affect lakes.

After the glaciers receded, they left us our over 10,000 lakes. Lakes go through a natural ageing process where they gradually receive nutrients (phosphorus and nitrogen) and sediment from the surrounding watershed and become more fertile and shallow. This process is called eutrophication. Eutrophication is a natural process that a lake goes through over hundreds to thousands of years.

Humans can speed up the process of eutrophication by adding excess nutrients and sediment quickly, where the lake will change trophic states in a matter of decades instead of centuries. This type of eutrophication is called cultural eutrophication because humans cause it. We have changed the landscape around lakes, which changes their water quality.

Around lakes, we have added a lot of impervious surface. Impervious surface is any surface on land that is impenetrable to water and prevents its absorption into the ground. Examples include rooftops, sidewalks, parking lots, and roads. The more impervious surface in a concentrated area, the less surface there is for rain to be absorbed into the ground. Instead, it ends up running into lakes and streams and carrying nutrients and sediment from the land it flows over.

Land practices such as urban areas, factories, agriculture, animal feedlots contain very concentrated amounts of nutrients. These nutrients wash into lakes and streams during heavy rains or through storm sewers. The additional nutrients that run into lakes and streams cause algal blooms and additional plant growth.

When erosion occurs along a lakeshore or a stream bank of a lake inlet, that extra soil can get washed into the lake. The extra soil particles cause cloudier water and eventually settle on the bottom of the lake making it mucky and less stable. The soil also carries with it nutrients such as phosphorus and nitrogen.

Eutrophication can be slowed if the inputs of nutrients (especially phosphorus) and sediment are slowed. Creating natural vegetation buffers along lakeshores and streams soak up nutrients and filter runoff. When you're planning new construction near water, make sure you prevent erosion and try and minimize creating more impervious surface.

So in summary, lakes start out with a certain natural condition that depends on their location (northern MN or southern MN) their watershed size, and their area, depth and shape. Then we humans add to that by what type of land practices we implement near the lake and upstream from the lake. Lakes that are in more heavily populated areas usually have had more cultural eutrophication than lakes that are in sparsely populated areas.

So how can you tell if the lake's water quality is declining or improving? Most people that have lived on a lake for a long time have observed these changes over time. The best way to determine long-term trends is to have 8-10 years of lake water quality data. Only short-term trends can be determined with just a few years of data, because there can be different wet years and dry years that affect the water quality naturally. In addition, just graphing the data and seeing if the line goes up or down is not enough. The data needs to be analyzed with a statistical test (i.e.: Mann Kendall Trend Analysis) to be confident in a true trend. Luckily, you don't have to be a mathematician to do this analysis, because it is available online.

Many lakes in Becker and Ottertail Counties have been monitoring their water quality yearly for 8-10 years. You can look at their results and trend analyses by visiting: http://www.rmbel.info/Reports/ReportsQuery.aspx.

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.)

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