A Delicate Balance
Understanding water quality problems in lakes and ponds
By Angela Hopko
From a lake manager's perspective, the most significant factors are light and temperature, nutrients, and oxygen.
Sunlight is of major significance to lake dynamics as it's the primary source of energy. Most of the energy that controls the metabolism of a lake comes directly from the solar energy utilized in photosynthesis. Photosynthesis will occur only in the upper layer of the pond, or euphotic zone. This is the area in the water column that sunlight is able to penetrate.
Shallow bodies of water—less than 9 feet deep—more commonly experience problems such as bottom-rooted weeds or benthic algae. Because of this, these need additional consideration when determining the correct water management solution.
Thermal stratification, or temperature layering, impacts water quality in a lake due to its effect on dissolved oxygen levels, the way we measure how water holds oxygen.
As the sun shines on a pond, it warms the surface water. This water becomes lighter than the cooler, denser waters trapped at the pond's bottom. As a result, the water becomes stratified, or separated into layers that do not mix with each other. This area is called the thermocline or metalimnion, and can act as a physical barrier preventing any vertical mixing in the lake. This encourages algae growth throughout the warm surface waters.
As water temperature increases, the water's capacity to hold oxygen decreases. Water at 52 degrees can hold over 40 percent more oxygen than water at 80 degrees.
The second essential factor is the impact of nutrients on the aquatic ecosystem.
There is a direct correlation between the level of available nutrients and the populations of algae and aquatic weeds. Phosphorus has been identified as the single greatest contributor to aquatic plant growth, for example. One gram of phosphorous will produce 100 grams of algal biomass.
The three most common sources of nutrient introduction are bottom silt and dead vegetation in the lake, runoff water from surrounding turf areas, and the sources of incoming water.
Bottom Silt and Vegetation: Vegetative life in the lake and sediment at the lake bottom are the primary sources of nutrients. Although they only have a two-week life cycle, blue-green algae can experience cell division and double their population as often as every 20 minutes. At the end of the cycle, the plants simply die and begin to sink to the lake's bottom, or benthic zone.
Studies at the University of Florida indicate that sediment can accumulate at a rate of 1 to 5 inches per year in temperate climates, and at a rate of 3 to 8 inches per year in tropical climates. At a midpoint accumulation rate of 3 inches per year, a 1-surface-acre lake will lose 80,000 gallons of water storage capacity in a single year.
Runoff: The second most common source of nutrients is runoff from surrounding turf areas as well as roads, farms and other outlying areas. The U.S. Golf Association (USGA) reports that up to 4 percent of the fertilizers applied to areas adjacent to ponds and lakes may eventually run off into the lakes. This runoff of fertilizers into lakes is known as nutrient loading. Leaves, grass clippings and other materials will also run off into the lakes, placing additional burdens on natural cleanup processes.
Incoming Water Source: Nutrient is also added to lakes and ponds through inlet waters. This inlet water can come from effluent sewage, wastewater treatment plants and leeching from septic systems. Inlet waters often have minimal oxygen and are loaded with phosphorus. An indication of excess phosphorus is foaming water.