A lake's water quality can refer to water clarity—how many particles are suspended in the water and how far light can penetrate down into the water. Water clarity affects the ability of fish to find food, how deep aquatic plants can grow, dissolved oxygen content, and water temperature. Water quality can also be used to describe how well the lake can support plants, fish, and other parts of a healthy lake ecosystem. Nutrients—like phosphorus—can dramatically affect water quality and what species can survive in the lake.
Lakes can be divided into three categories (or trophic states) based on a lake's water clarity and nutrient levels. These trophic states can give you an idea of what features a lake is likely to have (clear waters, supportive of many, or few aquatic plants or fish).
Water clarity can be influenced by polluted runoff from across a lake's watershed and from decisions made on the lake's shoreline. How lake front property owners take care of their shorelines can dramatically affect whether a lake will be prone to algae blooms, invasive species, and what types of fish can survive in the lake.
Water Paths and Water Quality
The path that water takes to enter a lake is very important to water quality. Water that moves through the soil and into groundwater is filtered during its passage. This filtering removes nutrients, such as phosphorus, and allows them to be reused by plants on the land. In contrast, surface runoff pathways do not allow for this removal and can move water with higher concentrations of phosphorus to the lake.
Because phosphorus is a critical nutrient for biological growth in lakes, if we increase the phosphorus concentration, we will increase the amount of algae in the lake. This leads to a greener color to the water and makes it more difficult for light to penetrate the water.
Lakes in watersheds with conditions that lead to more surface runoff, such as urban areas or areas with more agricultural activity, generally have more algae. Other factors, such as the area of the land that drains water to the lake and the depth of the lake also influence the phosphorus concentration in the lake.
In all cases, if the phosphorus moving into the lake increases, the amount of algae produced will also increase.
The land “behind” the lake: watersheds and lake water quality (PDF 639 KB)
Why Water Clarity Is Important
Water clarity is often used as a measure of a lake’s productivity level and an indicator of ecosystem health. Water clarity is a measure of the amount of particles in the water, or the extent to which light can travel through the water. Water clarity affects:
The ability of fish to find food
The depth to which aquatic plants can grow
Dissolved oxygen content
And water temperature
There are many ways to express water clarity, including Secchi disk depth, turbidity, color, suspended solids, or light extinction. Chlorophyll, a pigment found in all plants, is often used to determine the amount of algal growth in the water and is related to water clarity as well.
What Harms Water Quality
Nutrients such as phosphorus and nitrogen come from sediments (eroded soil), agricultural fields, manure, pet wastes, improperly maintained septic systems, fertilizers, grass clippings, and leaves.
Phosphorus, whether from natural sources or commercial fertilizers, is plant food. Too much phosphorus in our lakes causes excessive aquatic plant growth and algae blooms (where lakes turn green).
Excess algae can cloud water which can block sunlight from penetrating through the water and make it impossible for bottom-rooted plants to grow. When algae, plants, and other organic materials decay at the bottom of lakes this depletes oxygen in the water, making it difficult for fish and other aquatic life to survive. Reduced oxygen levels also contribute to winter fish kills in shallow lakes.
Most Wisconsin lawns and soils already contain adequate—and often excessive—amounts of phosphorus.
Sediments can be eroded from construction sites, developed areas, and cropland. Sediment—soil particles that end up in the water—reduces water clarity causing the water to become cloudy or “turbid.” Sunlight cannot penetrate as deeply into turbid waters, which restricts aquatic plant growth to smaller and shallower areas.
Turbid water conditions can also affect fish by damaging gills and impacting their ability to find food. Some species of fish are unable to tolerate persistent turbid water conditions, and may no longer be able to survive in the lake.
In addition to the impact the sediment particles can have themselves; sediment runoff can pick up and transport additional pollutants such as metal flakes, debris, toxins, and even more phosphorus into our lakes.