By Clint Calhoun
Limnology is a natural science focused on the study of the physical, chemical geological, and biological factors that affect aquatic productivity and water quality in freshwater ecosystems which includes lakes, reservoirs, rivers, and streams. The word “limnology” is derived from the Greek word “lemne” which means pool or marsh.
When we think about the ecosystem of a lake we must think about what an ecosystem is and how it works. Many think of Lake Lure as just merely a body of water that is created by damming up the Broad River. It has fish in it and you can swim in it. It’s a great place for waterfowl and there are these strange jelly blobs that show up in the summer time (go back a few years and read my article on the jelly blobs). That is generally the limit of our thinking when it comes to the lake because that’s what we see. So what is an ecosystem?
An ecosystem is a group of interconnected elements formed by the interactions of a community of organisms with their environment. Lake Lure is a part of the overall ecosystem of the Upper Broad River Watershed, even though it has its own ecosystem within itself; yet without the interconnectivity to the overall watershed, Lake Lure wouldn’t exist as anything more than a pool of water. What components of the watershed impact Lake Lure? That list could be pretty long, but we can probably break it down into some basic components for the sake of this article.
A lake wouldn’t be a lake without water. Where does it come from? Sure we have the river and streams that flow into the lake, but where does that water come from? A watershed is essentially the catchment basin or drainage area for all the water that falls in a certain area. In our case that area is a 94 square mile basin whose boundary is created by the highest ridges surrounding the valley. Every drop of rain, snowflake, or ice pellet that falls on this side of those ridges drains to Lake Lure which is the base of the watershed.
As that water moves it transports sediment which is soil from the upland parts of the watershed. Soil is a necessary component that most all living things have either a direct or indirect association with. It has key nutrients that are necessary for life. Bacteria and fungi thrive in it helping to decay dead organic matter. Plants use those key nutrients provided by the work of the bacteria and fungi to grow, and in the process release oxygen into the atmosphere through photosynthesis which provides the air we breathe. That same oxygen gets dissolved into the water providing life for aquatic organisms. This is a basic picture of just a few basic ecological interactions, but you can see that everything within the watershed is connected.
Particulates are an important aspect of lake ecology. Particulates include soil, leaves, twigs, and other organic debris. Particulates are what cause turbidity in the water column and affect water transparency and light needed for plant growth. Particulates can also have nutrients attached to them that promote algal growth. Particulates, particularly, silts and fine sand can have negative impacts on fish spawning area if numbers are high. Deposits can damage habitat for aquatic insects that fish feed upon. We measure the presence of particulates as TSS or Total Suspended Solids. When we conduct our monthly water sampling, TSS is one of the parameters we measure in the feeder streams, but not the lake. Why you may ask? This is because most particulates usually settle to the bottom of the lake fairly quickly after leaving the feeder streams, so they are no longer suspended. Lighter silts can stay suspended for days but they are only part of the overall total entering the lake, so it’s better to sample where water is moving so that the numbers are more accurate. Of course, TSS numbers are higher during and immediately after storms, so the trends we look for have to do with storm frequency. TSS numbers tend to be higher in the spring and summer when short duration, high intensity storms occur.
Besides visible particulates, there is plenty of dissolved material within the water such as calcium which is necessary for balancing pH. Metals such as aluminum and iron are important for biological function within the lake. Pollutants such as pesticides and herbicides can be found in lake water. Phosphorus and nitrogen are primary plant nutrients that are necessary for plant growth. All of these materials make their way into the lake through precipitation and runoff within the larger watershed.
The characteristics of the watershed influence ecology. The larger the watershed, the more runoff and therefore more sediment and nutrient loading, but also less hydraulic residence time (the average time it takes to replace the lake’s water volume). Steep slopes in our watershed increase surface water runoff velocity which increases erosion and sedimentation potential. The geology of the watershed helps to determine the type of nutrients and minerals entering the lake as well as water clarity. Soil types of the watershed influence infiltration and capture of water and will determine the types of sediments that end up in the lake.
Materials carried by runoff depend on the type of land cover that exists in the watershed. A watershed that is composed of mostly forest and dense vegetation tends to experience less erosion than a watershed that has little forest and lots of impermeable surface. Pool Creek, for instance, has the highest water quality rating in the Upper Broad River Watershed because its sub-watershed is small, has dense forest cover, and little development, so there is less material to runoff and less water to carry it.
Obviously, this is quite a watered down (no pun intended) dissertation on lake ecology and only scratches the surface as it relates to the things happening in our lake, which is why I have decided to use several article to discuss it. Next time we’ll look at how lake morphometry (hey, that’s a big word) and lake processes affect lake ecology.
Clint Calhoun is a naturalist, biologist and Certified Lake Manager and has worked in Hickory Nut Gorge for over 20 years. He is currently the Environmental Management Officer for the Town of Lake Lure. Check out Clint’s blog at http://clintcalhounadventures.blogspot.com.