Every lake/pond is unique. If the water is clear and has little submerged weed growth, we call it "clean" and its primary productivity is low. You might think of it as starving for plant nutrients. If the water is turbid, due to planktonic algae, its productivity is high. This would be considered bad if you wanted to swim in it. It would be considered good if you were using it for aquaculture. Owner of small farm and fishing ponds with no outflow sometimes add phosphorus to increase productivity and grow more fish.
If you want to have clear water, you must reduce productivity. In short, you need to withhold nutrients that feed the algae. Phosphorus is typically the nutrient that can limit plant and algae growth in freshwater lakes. Reduction in phosphorus can result in clear water.
Phosphorus, nitrogen and other nutrients enter the lake from four sources:
- Watershed runoff.
- Sediment cycling.
- Atmospheric loading.
- Point source (typically sources known by the owner-can be things such as fish food, cleaning stations, fertilizer additions, etc.).
To identify and quantify the sources of phosphorus, it is necessary to do a total nutrient budget. Since most people can't afford that professional service, they typically ask a limnologist or other lake expert to "eyeball it" and give them their recommendation. Some remedial action can be inexpensive and other suggestions can be cost-prohibitive. If the low-cost fixes are not sufficient to bring about the desired improvement in water clarity, consider an alum treatment to reduce phosphorus in the water and reduce its cycling out of the sediments. It is probably cheaper than you think.
Phosphorus binds tightly to the alum, which becomes incorporated into the sediments of the lake. It is totally safe from a human, animal and environmental standpoint. Aluminum, like silica sand, is one of the most common elements on the face of the earth. It is used in cooking and drinking water filtration.