Pond life

By Angela Hopko

The best way to find a solution to your water quality problems is to obtain a better understanding of the components that impact the delicate balance of a pond's ecosystem. 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 is the primary source of energy.Most of the energy that controls the metabolism of a lake comes directly from the solar energy used 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 three metres in depth more commonly experience problems such as bottom-rooted weeds or benthic algae and need additional consideration when determining the correct water management solution.

Thermal stratification, meaning 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 which are trapped at the pond's bottom. As a result the water becomes stratified, and separated into layers, which 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, and encourages algae growth throughout the warm surface waters.

As water temperature increases, the water's capacity to hold oxygen decreases.

Water at 11°C can hold over 40 per cent more oxygen than water at 27°C.

The second essential factor is the impact of nutrients on the aquatic ecosystem.

There is a direct correlation in 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; one gram of phosphorous will produce one hundred 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.

Vegetative life in the lake, and sediment at the lake bottom, are the primary sources of nutrient. 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 2.5-12cm per year in temperate climates, and at a rate of 6-16cm per year in tropical climates. At a mid point accumulation rate of 7cm per year, a 4,000 cu m lake will lose 300 cu m of water storage capacity in a single year.

The second most common source of nutrients is runoff from surrounding turf areas as well as roads, farms and other outlying areas. The USGA reports that up to four per cent of the fertilisers applied to areas adjacent to ponds and lakes may eventually runoff into the lakes. This runoff of fertilisers into lakes is known as nutrient loading. Leaves, grass clippings, and other materials will also runoff into the lakes, placing additional burdens on the lake's natural clean up processes.

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. Often inlet waters have minimal oxygen and are loaded with phosphorus, an indication of excess phosphorus is foaming water. The third essential factor is the role oxygen plays. Oxygen is important to all forms of life in the lake, and supports the food chain including the natural decomposition process. A lake is supplied with oxygen from several sources but primarily through photosynthesis, wave and wind action.

Immediate reactions to oxygen depletion would be fish kills or odours. Long term issues include nutrient build up, sludge accumulation, and a chemical imbalance in the lake. Oxygen depletion or stress situations occur for different reasons, but most typically happen during: • Late at night and just before dawn • Cloudy and still days • Hot and humid days • When the lake's nutrient content is high • After a chemical application.

Nature has provided a clean up process that will metabolise or decompose excess nutrients, called organic digestion. This involves two types of naturally occurring bacteria present in all lakes and ponds, aerobic and anaerobic.

The most effective of these bacteria are aerobic bacteria and only live in the presence of oxygen. Highly efficient, they are roughly seven times faster in organic digestion than anaerobic bacteria.

Anaerobic bacteria exist in oxygen deficient pond water and soil, are much slower in breaking down nutrients and allow soluble organic nutrients to recycle into the water column. Noxious byproducts such as methane, ammonia and hydrogen sulphide are created by anaerobic decomposition.

Balance is critical to the aquatic ecosystem, without it your pond or lake will suffer. There are many steps that can prevent an imbalance from occurring, and knowing the causes will assist in determining the best solution for your application. Some methods include proper pond construction, chemical applications, and the addition of oxygen through aeration systems and devices.