Mike Bredeson PhD
When you see the word “pollutant”, what comes to mind? Perhaps you imagine toxic pesticides, plastic, or spilled oil. Many are surprised to find that some of the most harmful environmental pollutants are the same substances we depend on for life— nutrients!
What are nutrients? Nutrients are tiny building blocks that make up a living organism. Humans, plants, bacteria, etc. are all made of different combinations of nutrients like nitrogen, phosphorus, carbon, and many more. Nutrients are like a bucket of Legos®— you can create many different things from the same mixture, depending on how you arrange them.
It would be easy to assume that having more nutrients in an environment would be a good thing— more building blocks, right? But too many nutrients can cause serious damage to an ecosystem.
Ecosystems like streams, rivers, and lakes are at their healthiest when the supply of incoming nutrients is limited. These ecosystems have evolved to function by using nutrients which are recycled after a plant, animal or other organism dies and decays. When there are too many nutrients the delicate balance of decomposition and regrowth is disrupted.
But how can excess nutrients be harmful? Let’s use a lake for example. Fertilizers, which we use to grow plants, contain nutrients like nitrogen and phosphorus. These fertilizers commonly flow off residential lawns, golf courses, and crop fields when it rains or as snow melts.
Flowing water, carrying excess nutrients, runs into ponds, streams, and eventually lakes where aquatic plants and algae use the nutrients to grow and expand beyond their normal, limited rate. Excess plant and algae growth leads to many lakes in urban and agricultural areas turning green in summer’s warmth.
One might think that all this extra plant and algal growth would be a good thing. Afterall, plants and algae produce oxygen which fish and aquatic insects need to breathe under water! But having all this extra vegetative growth causes complications.
As free-floating algae gather excess nutrients, they multiply so quickly that it can become difficult to see through what would normally be clear water. This is a problem for two reasons. First, many important fish species (walleye, pike, bass) use sight to capture their prey. Limited visibility makes it difficult for fish to live in places with too much algal growth.
Secondly, water clouded with algae blocks sunlight from reaching a lake’s bottom. When sunlight is blocked aquatic plants, which are normally rooted to lake sediment, cannot grow. As a result, lakes tend to lose aquatic plants which are important for fish habitat and for preventing a buildup of problematic algae. Once a lake receives too many nutrients and becomes algae-dominated, it is extremely difficult to restore water clarity and aquatic plant life.
Finally, the most problematic consequence of too many nutrients entering a lake is something called “winter-kill”. During winter, aquatic plants aren’t pumping oxygen into the water, and thick ice prevents wind from making waves— another method for delivering oxygen to fish and insects.
Organisms living below the ice depend on breathing oxygen which was stored in lake water during the open-water season. In the fall, lake water contains its highest amount of oxygen, but as winter progresses the level steadily falls. Aquatic animals depend on having enough stored oxygen in lake water to keep them alive until spring when plants, algae, and waves work to replenish the supply.
In lakes which receive too many nutrients the abundant algae and plant matter dies back and sinks to a lake’s bottom when winter arrives. This is where the trouble begins. Bacteria, which also use oxygen when they breathe, begin to decompose an abnormally large amount of plant and algal matter. As the bacteria multiply and consume excess dead vegetation they progressively use up oxygen, suffocating fish, and other vulnerable organisms.
There are a few things that we can do to prevent the environmental damage spurred by too many nutrients. First, if you fertilize your lawn, using products such as compost and worm castings is better than synthetics. Synthetic fertilizers are often salt-based which dissolve quickly and transport nutrients in flowing water at a rate too fast for lawns or farmer’s crops to efficiently use. Fertilizers like manure and compost release their nutrients more slowly, so plants can take advantage of the full nutrient load.
For crop farmers, conduct soil tests to make sure you’re not over-fertilizing, apply nutrients when plants are ready to use them, and improve your soil’s health so that more nutrients remain in fields rather than leach away with water.
Finally, the best way to reduce the risk of too many nutrients entering lakes and other natural areas is by recapturing nutrients before they escape. We each have a responsibility to prevent nutrients from leaving our own property.
Establishing nutrient recapture zones of perennial vegetation between lawns and water, croplands and water, and around drain tile inlets removes nutrients and stores them in plant matter. Leaving wetlands intact also plays an important role in reducing nutrients before water is delivered to lakes and streams.
It is remarkable how nutrients which are necessary for the survival of all organisms, can become a pollutant if managed poorly. It is up to us to make sure that these little building blocks of life are used responsibly to promote healthy ecosystems which provide us with water to drink and clean air to breathe—services we cannot live without.