By Clint Calhoun
As you know from my recent posts, lake managers like to use big words to describe things that apply to lakes. Who knew that a four-letter word could be so complicated right? If you’ve been keeping up, we’ve talked a lot about lake ecology in recent articles and some of the things that impact lakes. In this article I’m going to discuss what probably best describes the aging process of a lake. That process is known as eutrophication. This will also be the last article in this series on lakes.
When we look at lakes in terms of overall time, they are what we would call temporary features in the landscape. The Great Lakes didn’t always exist, but were the result of glaciation, where glaciers carved deep gouges into the land which filled with water as the ice sheets retreated. Similarly oxbow lakes in riverine systems are formed in large expansive floodplains where rivers meander. During floods, erosion will wear away the banks connecting the meandering section, isolating large areas of water creating small u-shaped lakes. These lakes can just as easily be re-connected to the main river system in another flood.
All lakes, regardless of whether they are natural or man-made are subject to eutrophication, but the process has nothing to do with the numerical age of the lake. Eutrophication is defined as the excessive addition of inorganic nutrients (nitrogen and phosphorus), organic matter (leaves and woody debris, decomposing matter, etc.), and silt and sediment accumulation. This input of material increases the biological productivity of the lake. The interesting thing is that not all lakes respond the same way to this input of material as there are other factors that come into play, most of which we have discussed in previous articles.
Eutrophication is a progressional process that takes a lake through different phases known as trophic states. Each state is based on the amount of biological activity in the system. Under normal circumstances, the process of eutrophication could take thousands of years in some lakes, while other lakes where human activity is high might only take a few decades.
An oligotrophic lake has low productivity. Such a lake has very little aquatic plant growth which would be necessary to support higher levels of biodiversity. Oligotrophic lakes have oxygen at all depths, have clear water, and can support trout. These lakes have very low nutrient inputs, so their watersheds are relatively undeveloped. A somewhat local example would be Lake Jocassee in South Carolina.
The next level of trophic state is mesotrophy. These lakes have moderate plant productivity. Stratification tends to occur, with the hypolimnion (you learned this word in my last article) possibly lacking oxygen in the summer months. The water is moderately clear and most often supports species such as large and smallmouth bass and perch. Lake Lure has been described in different studies as straddling the line between oligotrophy and mesotrophy. It doesn’t have a lot of larger plant species found in many lakes, but does have some phytoplanktonic (algae) diversity that helps to support zooplankton which are the base of the aquatic food web. Lake Lure is much better suited to support bass and bluegill than trout and this is supported by the water quality data. Mesotrophic lakes have some nutrient input but are typically very desirable lakes for anglers due to the species they support.
Eutrophic lakes have excess nutrients. These lakes often experience algae blooms during the summer months, particularly potentially harmful blue-green algae. These lakes tend to lack clarity due to high chlorophyll and turbidity from silts and fine soil particles. Many of these lakes have large rooted macrophytes (plants) such as cattails, duck potato, or water lily. Because of the large amount of plant diversity, these lakes are often teeming with numerous species of fish, but these populations can be at risk from oxygen depletion when algae blooms occur. Eutrophic lakes are typically found in areas where there is significant human activity such as large-scale farms, urban areas, and subdivisions. Some lakes are naturally eutrophic, not because of pollution but because they lie in naturally fertile watersheds.
Hypereutrophic lakes are typically covered in algal scum during the summer months, no longer support macrophyte diversity, have low dissolved oxygen, and often experience fishkills. Hypereutrophic lakes are products of uncontrolled pollution and nutrient input.
Natural lakes and constructed lakes age by the same processes, but typically at different rates. Reservoirs generally become eutrophic more quickly because they typically have higher sediment and nutrient loads. The process most often occurs from basin fill-in from riverborne silts and clays.
Understanding this process is important because of the indicators that are present with each trophic state. Those indicators are important for lake managers in understanding how the trophic state of a lake may be changing as watershed changes occur. As the climate changes and population growth continues, changes will occur in Lake Lure. How quickly those changes occur will depend a lot on how development progresses in the watershed and how proactive residents of the watershed are as far as implementing best management practices to reduce runoff and protect water quality. We all have a part to play. Until next time!
Clint Calhoun is a naturalist, biologist and Certified Lake Manager and has worked in Hickory Nut Gorge for over 20 years. He is currently teaching biology and earth/environmental science at Lake Lure Classical Academy. Check out Clint’s blog at http://clintcalhounadventures.blogspot.com.