The health of any pond ecosystem relies heavily on the concentration of dissolved oxygen (DO) within the water. DO is the measure of oxygen gas available to aquatic life and is the most important water quality parameter for pond managers to monitor. Concentrations below 3 parts per million (ppm) can stress most warm-water fish species, while levels below 2 ppm may be lethal for some. This dissolved gas is necessary for the respiration of fish and other aquatic organisms, including the beneficial aerobic bacteria that break down waste. Insufficient DO is a frequent cause of fish kills and the buildup of organic sludge.
Harnessing Aquatic Plants for Oxygen Production
The biological process of photosynthesis provides a direct and natural source of oxygen to the water column. Submerged, or true oxygenating, plants use sunlight and carbon dioxide to produce and release oxygen directly into the surrounding water during the day. This release occurs through the leaves and stems that are fully underwater, creating tiny bubbles that dissolve into the pond.
Effective submerged plants include Hornwort (Ceratophyllum demersum), Anacharis (Egeria densa), and Cabomba (Cabomba caroliniana). These plants not only oxygenate the water but also absorb excess nutrients, which helps to limit the growth of nuisance algae. A healthy density of oxygenating plants is recommended to maintain clarity and balance.
It is important to manage the growth of these plants and remove dead or decaying plant matter regularly. While plants produce oxygen during the day, they consume it at night through respiration, which can lead to low DO levels before dawn. Removing spent foliage prevents this organic material from decomposing on the pond floor, a process that consumes oxygen and counteracts the daytime benefit.
Increasing Surface Agitation and Flow
The physical exchange of gases between the atmosphere and the water surface is a continuous process dramatically improved by movement. Oxygen from the air dissolves into the water across the surface. Increasing the amount of surface area that comes into contact with the air, or the turbulence at that boundary, significantly accelerates this gas transfer.
Natural landscaping features like waterfalls, streams, and fountains are highly effective methods for maximizing this exchange. A waterfall or fountain creates a large volume of water droplets and splash zones, which drastically increases the total surface area exposed to the atmosphere. The impact of the falling water also breaks the surface tension, allowing new water layers to absorb oxygen more readily.
Water that is shallow and fast-moving, such as through a constructed stream bed, will absorb more oxygen than water that is deep and still. Positioning a fountain’s spray pattern to maximize the height and travel of the water droplets maximizes the potential for oxygen absorption. Furthermore, these features help to circulate the entire water body, preventing stratification where oxygen-rich water sits atop oxygen-depleted water.
Managing Organic Load to Preserve Oxygen
Preventing the consumption of dissolved oxygen is just as important as actively adding it. Oxygen is primarily depleted by the decomposition of organic matter, which creates a high biological oxygen demand (BOD). This organic load consists of fallen leaves, uneaten fish food, fish waste, and dead algae or plant material that accumulates as sludge on the pond bottom.
Routine maintenance is necessary to keep the BOD low, which includes manually removing sludge from the bottom using a pond vacuum or net. Beneficial aerobic bacteria play a role in breaking down this waste, but they require sufficient oxygen to do so efficiently. Adding bacterial treatments can enhance this natural decomposition process, reducing the amount of oxygen-consuming muck.
Limiting the fish population size is another effective management strategy, as a high density of fish increases waste and the overall oxygen demand. Water temperature has an inverse relationship with its capacity to hold dissolved oxygen, meaning warmer water holds less gas. Providing shade over a portion of the pond helps keep the water cooler, ensuring it retains a higher saturation of oxygen throughout the day.