Phosphorus Nutrient Loading

Adam Slaminski
04/13/11
Phosphorus is an important element found in all living things. Phosphorus is commonly used as a major component in fertilizer, it helps replace the natural phosphorous used by the plants. Phosphorous is seen as a good thing when trying to grow crops for farming and other agricultural purposes. Phosphorous is still an important part of the aquatic ecosystem but there is a fine line between healthy levels and too much. When an aquatic system receives to much phosphorous, it triggers plant growth similar to the way it affects plant growth in terrestrial systems. However plant growth is not necessarily a good thing in aquatic systems.
Excessive phosphorous levels can lead to massive algae blooms and aquatic plant growth that can completely change the structure of the water body. This rapid plant growth usually occurs in the littoral zone of the lake which is the area of the lake that gets light to the bottom and is where rooted plants grow. The algae blooms usually occur in the limnetic zone which is the lighted zone over deeper water away from shore. When this rapid growth occurs it can change the tropic state of the waterbody, by changing the water temperature. Waterbodies that have cool water and are thought to be productive which would fall under the Mesotrophic state can quickly turn to the Eutrophic state which has the warmest water temperatures and is the most productive. More production seems good at first glance but it is the water temperature that has the greatest impact on fish. Fish are sensitive to water temperatures. Fish that are thought to be cold water fish, like trout can’t survive in warm water environments. Even cool water fish like walleye cannot survive in warm water environments for extended periods of time. Water temperature is raised as a result of the increasing plant biomass which blocks out the sunlight from reaching the bottom. These seemingly small changes can have great impacts on entire aquatic ecosystems.
Another problem with excessive phosphorous is that when winter comes around and the ice covers the water, more oxygen is used in the decomposing process for all the dying vegetation, and less oxygen is available for the fish to breathe. When this occurs you get a situation similar to the Big Eau Plain fish kill that happened a few years ago. This was a result of not enough oxygen for the fish so lots of them suffocated to death and piled up on the shoreline in spring when the ice melted.
They came to the conclusion that the excess phosphorus probably came from farmers fields. It is common practice for farmers to spread manure out on fields in late winter and early spring so that when the ground thaws, the nutrients are taken into the soil. The only problem was that the spring before the massive winterkill the snow melted rapidly before the ground had a chance to thaw and most of the manure washed off the fields and into the reservoir. This coupled with the fact that the aerators in the reservoir were not turned on or malfunctioned led to the massive fish kill. To help insure that this kind of thing didn’t happen again, they went ahead and installed a new aeration system and tried to educate farmers about the steps they can take to reduce the amount of phosphorus leaving their fields and entering the nearby water bodies.