Why are Some Lakes Clearer than Others?

posted Jun 30, 2016, 9:20 AM by Joseph Bickard

June 27, 2016 - Augusta-With summer upon us, it is finally lake season again in Maine. Even the most timid among us are now venturing – slowly but surely – into the water. You may have noticed that not all lakes are created equal: some Maine lakes are incredibly clear, while others are so murky you can barely see your feet while swimming. So why are some lakes clearer than others?

One culprit is our forests. Forests “leak” nutrients into streams, which eventually drain to lakes. These nutrients are responsible for reducing water clarity in our lakes. Forests leak nutrients through debris and runoff.

Forest debris – mainly consisting of leaves and soil – washes into streams during rainstorms. Once this debris reaches streams, it provides a feast for a myriad of insects. Stonefly larvae eat this debris, shredding it into small particulates. The remaining particulates are collected and eaten by caddisfly and blackfly larvae. Some caddisflies (known as hydropsychids) even weave nets to catch as many particulates as possible.

These insects act like a natural filter, preventing this leafy debris from making it downstream to lakes. Healthy, natural streams with lots of places for these insects to hide tend to be very effective at processing this leafy debris before it can foul the water. Streams that have been polluted or canalized have far fewer insects, and therefore will allow more leafy debris to travel downstream to lakes.

What do leaves in streams have to do with lake water clarity? If you drink tea, then you know that bits of leaves left in water for long enough will start to add color to the water. This color comes from chemicals (known as dissolved organic carbon, or DOC) that slowly leach out of the leaves. Just like with tea, stream and lake water will become darker if you add more leaves or leave the leaves in for longer.

The amount of leaves added to a stream depend on the size of its watershed, the area of land that drains into the stream. Lakes with large watersheds receive more inputs of leaf matter into their streams, and are therefore more likely to have a darker, murkier water color. The inverse is one reason why many lakes in Acadia are so clear – their mountainous watersheds are very small, so Acadian lakes relatively few leaves. Lakes with smaller watersheds and healthy streams tend to have the lowest levels of leaf inputs, and are therefore the clearest.

Dissolved chemicals from leaves are not the only culprit behind murky lakes. The more known (and more pesky) cause of murky lakes is algae. Algae come in many different forms, but they all rely on a combination of sunlight and nutrients to grow. The main nutrients that give algae a boost are nitrogen and phosphorus. If these two sound familiar, it is because they are the two base ingredients for plant fertilizers.

Young, developing forests have the highest need for nutrients, so they tend to release relatively few nutrients into streams. Older forests, which are growing less than their younger counterparts, tend to leak more nutrients. Fresh clearing of forests can lead to soil erosion, which allows nutrient-rich organic soil to wash downstream. A buffer strip between active logging and streams can help reabsorb some of the eroded nutrients, preventing them from reaching the water.

Dissolved nutrients in streams can be intercepted and absorbed by a variety of helpful organisms before reaching lakes. Among these organisms are aquatic mosses and algal mats, which absorb nutrients and use them to grow. As with the insects I mentioned above, healthy streams with healthy mosses and algal mats can absorb nutrients before they reach our lakes, starving lake algae and keeping lakes clear.

The forest is not the only source of nutrients and DOC in lakes – these chemicals are also produced in smaller amounts by in-lake plants and bacteria, particularly in boggy areas. Nutrients also can come from human sources, including fertilizer runoff and sewage.

Zachary Wood, Bangor Daily News, June 2016