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Nonpoint Source Pollution and Lake Ecology

Energy captured from the sun through photosynthesis drives all life on this planet. This statement is true for both terrestrial and aquatic ecosystems.

The Algae
Algae are a diverse group of aquatic organisms that can be microscopic, or in the case of kelp, more than 50 feet long. Some algae are attached to rocks or other submerged structures. This is the slippery green stuff most people call “moss.” Other algae live free-floating in the water, and many species can even “swim.”

Algae, like plants, create their own food through photosynthesis, the process by which the sun’s energy is captured by the pigment chlorophyll and used to combine carbon dioxide and water to form simple sugars. The algae can later break the sugar molecules apart, releasing the stored chemical energy for growth and reproduction. Other organisms are also welcome to those sugars; all they have to do is eat the algae.

Algae have it pretty darn good when it comes to the requirements for photosynthesis. As long as they stay near the surface of the water, they can find plenty of sunlight. There generally isn’t much shade out on the lake. The exception to this is when water clarity is reduced by suspended sediments or excessive algae. The second requirement for photosynthesis, carbon dioxide, readily diffuses into the lake from the atmosphere and is released into the lake via respiration and decomposition. There is plenty of water in a lake, which takes care of the final requirement for photosynthesis.

Algae are necessary for a healthy lake ecosystem, but there can be too much algae. When this happens, the grazers who eat the algae can’t keep up. As the uneaten algae die off, they sink to the lake bottom and decay. The process of decay requires bacteria, which in turn require oxygen. If there is an abundance of dead algae bacteria use up too much oxygen, and there isn’t enough left over for all of the animals, like insects and fish. Too much algae can also give lakes an unpleasant green color, a surface scum, or (in the case of drinking water reservoirs) a “fishy” taste.

Algae in lakes are similar to grass on a prairie. Not every animal species on a prairie eats the grass, but all life on the prairie is ultimately dependant upon it.

The Nutrients
While algae must photosynthesize to store energy, they also have further requirements for growth and reproduction. For these processes, algae have further requirements of their environment. Chief among these requirements are nitrogen and phosphorus, nutrients often responsible for limiting algal growth. If nutrients in the water are insufficient, algal growth will slow or stop entirely until more nutrients become available. An increase in the concentration of nutrients in a lake will probably increase the amount of algae as well.

Nitrogen and phosphorus are natural components of soil but are also frequently applied on land as fertilizer. If these fertilizers are not taken up by terrestrial plants, the nutrients may be transported as nonpoint source pollution to our lakes. Nutrients, especially phosphorus, also attach to soil particles, which wash into lakes as suspended sediment particles during rain events. To truly control algae in lakes, you must control the nutrients.

The Sediments
While nutrients encourage algal growth in a lake, suspended sediments can restrict growth. These soil materials decrease water clarity, limiting the amount of light available to algae. Suspended sediments make a lake brown, reducing its aesthetic appeal. As the sediments settle out of suspension, they fall to the bottom of the lake and eventually fill in its bottom, requiring that the lake be abandoned or dredged. Dredging is the extremely expensive process of scooping out a lake’s bottom and taking the removed soil away. Additionally, suspended sediments must be removed from lake water that is to be used for drinking.

The Watershed
To control nutrients and sediments within a lake, we must look to the watershed, the land uphill from a lake (or stream). All surface water has a watershed, and all land is part of at least one watershed. Tavern Creek and the Lake of the Ozarks each have watersheds. Their watersheds are part of the larger Osage River watershed. The Osage River watershed is a part of the much larger Missouri River watershed. The Missouri River’s entire watershed is included in the even larger Mississippi River watershed.

Water (rain, snowmelt, etc.) moves down the landscape before entering the lake as runoff. Along its way downhill, runoff can pick up nutrients, sediments and other pollutants from the watershed. Those pollutants that enter lakes and streams are called nonpoint source pollution. Nonpoint source pollution comes from a diffuse area in the watershed, rather than a single (point) source. To improve water quality anywhere, you start by addressing human activity in the watershed.


The Lakes of Missouri Volunteer Program

Nutrients and sediments are two of the top three pollutants in lakes, according to the EPA. These pollutants often enter our lakes as nonpoint source pollution, through runoff. The way to reduce the impact of excessive nutrients and sediments on our lakes is to reduce nonpoint source pollution in the watershed. The Lakes of Missouri Volunteer Program uses volunteer monitors to track nonpoint source pollution in Missouri’s lakes by measuring chlorophyll (a measure of algal biomass), nitrogen and phosphorus (nutrients), suspended sediments (soil particles), and water clarity.

Suspended sediments and algae will reduce the clarity of lake water.

Water clarity is measured using a Secchi disk. The Secchi disk is lowered into the water until it is no longer visible. The depth is then recorded. Clear water has a greater Secchi depth than murky water.

Next - Land Use and Water Quality in Missouri's Lakes

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The Lakes of Missouri Volunteer Program is operated by employees of the University of Missouri