Dissolved Oxygen Basics for Pond Owners

Dissolved oxygen (DO) is oxygen gas (O₂ ) that is mixed with, but not chemically bonded to, water molecules. DO has the same chemical formula as the oxygen in the air we breathe – O₂ – but instead of being a gas in the atmosphere, it is a gas dissolved in water. DO is important because nearly all aquatic plants and animals breathe or absorb it to live and it plays a role regulating the release of phosphorus from pond sediments.

DO is measured in milligrams per liter (mg/L) which is equivalent to parts per million (ppm).  Levels can vary between 2 mg/L or less in ponds suffering from low dissolved oxygen to 14 mg/L or higher in ponds that are considered saturated. Fish like largemouth bass and bluegills prefer DO levels above 5.0 mg/L but can tolerate lower levels for limited periods. Fish may suffocate rapidly when DO levels fall to 2.0 mg/L or lower¹.

Sources of dissolved oxygen

DO in ponds can come from the atmosphere, living things in a pond, surface and groundwater inflows, and – if installed -- mechanical pond aerators.

Because air contains a higher concentration of oxygen gas than water, oxygen will naturally move from atmosphere into pond water unless the pond is already saturated with DO or covered by ice. This process is called atmospheric diffusion. On windy days, surface waves also mechanically move oxygen from the air into the water, a process called aeration.

As you may remember from science class, oxygen gas is a byproduct of photosynthesis. Just as plants on land produce oxygen that terrestrial animals breathe, aquatic plants and algae produce DO that fish and other aquatic creatures utilize. 

Incoming surface water from streams and groundwater from springs can also add DO to ponds. Streams are a richer source, often containing over 6mg/L of DO unless they are polluted with excessive nutrients or other organic waste. Data on DO levels in groundwater is limited, so it's unclear how much it contributes to spring-fed ponds but in some cases could be important³.

Fountains and subsurface diffusers are mechanical devices that aerate ponds. Fountains serve a mostly decorative function, but can add some DO to the pond's surface layers. Subsurface diffusers circulate DO throughout a pond's entire depth which benefits fish populations and may also reduce weeds and algae growth by minimizing the release of phosphorus from pond sediments.

DO levels in relation to temperature and water depth

The amount of DO pond water can hold increase as the temperature of the water declines. In other words, cold pond water can hold more DO than warm pond water.  Pond water at 40°F in March may hold over 12 mg/L of DO, but the same water will only hold around 8 mg/L of DO in July when the water temperature is at 80°F.

Because water density decreases as its temperature increases beyond around 39°F, deep ponds may form a warm surface layer and a cold bottom layer by late spring. These layers may remain unmixed for days or longer in the summer.  This process is better understood in larger, deeper lakes and is called thermal stratification. It happens when calm weather allows bright sunlight and warm air to rapidly heat the surface of a pond, lowering its density enough so it essentially floats on the colder water below. Scientists and lake managers call the upper layer the "epilimnion" and the lower layer the "hypolimnion." DO levels may diverge within and between a pond's warm surface water and cold bottom depths. Surface water may experience daily DO fluctuations that are explored next. DO levels in bottom water may gradually decline to near zero which can potentially lead to a summer fish kill and the undesirable release of phosphorus from pond muck. 

Daily and seasonal DO cycles

During the day, especially in summer when algae and aquatic plant populations are highest, DO levels will increase because photosynthesis produces oxygen gas as a byproduct (Figure 2). In the evening, after the sun sets, photosynthesis stops and DO levels gradually decline overnight as bacteria, plants, and fish consume the oxygen that was produced during the prior day.  Just before sunset during midsummer is a good time to take a DO measurement to discover your pond’s peak DO levels. Measuring just before sunrise the following day should reveal what the lowest levels are. 

Figure 3 shows how DO levels vary annually in lakes located in regions with distinct seasons. Ponds have not been studied as much as lakes, but they also appear to stratify during the growing season unless they are extremely shallow or prone to wind induced mixing⁴. DO levels are equal in a pond during the spring until stratification occurs. If the layers are separated long enough, oxygen levels will begin to diverge until mixing occurs. In a deep pond, or one that is protected from wind – perhaps nestled behind a row of trees or covered by a bed of thick emergent or floating aquatic plants – the layers may not mix again until cold weather lowers the temperature of the epilimnion (warm upper layer) closer to that of the hypolimnion (cold bottom layer). This mixing is called "turnover". DO levels may stay fairly equalized throughout winter unless an ice cover forms.  Ice stops the diffusion of oxygen from the atmosphere but, unless the ice is covered by deep snow, still allows light to reach plants and algae that photosynthesize and release oxygen all winter. Since the ice prevents wind mixing, the oxygen levels in the top of the pond may be greater than levels in the bottom where it gets consumed by bacteria living in decomposing muck and sediment.

Fish kills

A fish kill is when many fish in a pond die quickly. They happen most often in the summer on ponds that are relatively shallow and warm. They are also known to occur in the winter on ponds that are completely covered by ice and snow. Suffocation is the cause of death in both seasons, but the mechanisms that create the low DO conditions are different.

In summer, when ponds are stratified, minimal DO may be present in the bottom layer. To survive, fish rely on oxygen in the surface layer.  But because this zone is often very warm (80°F or higher) its ability to hold DO is reduced.  Usually, the DO levels are enough for fish to survive. But if the ecology or stratification of the pond is disturbed, DO levels can drop quickly to levels that fish may not be able to tolerate. An indication of these levels is when fish are seen gasping for air at the pond surface. Ecological disturbances happen when a mass of aquatic plants or algae die --- either naturally or following poorly timed pesticide applications. Bacteria decomposing the dead plants may then consume so much of the remaining DO in the pond that too little remains for fish to survive.

Stratification disturbances are thought to occur during heavy rain when cold stormwater quickly floods a pond causing the warm and cold layers to mix. If the cold layer is oxygen deficient, O2 levels throughout the newly mixed pond may suddenly fall below the levels fish like bass and bluegill need to live. Small ponds may also be susceptible to wind induced mixing throughout the growing season, resulting in multiple stratification cycles, some that may only last a few days⁶.

In winter, DO continues to enter the pond naturally through atmospheric diffusion and as a photosynthetic byproduct from plants and algae remain active. DO may also enter the pond through wave action. But if the pond freezes over, wind induced aeration stops. And if the ice gets covered by a thick blanket of snow that prevents sunlight from reaching underwater plants, DO production from photosynthesis may also cease. Under these conditions, DO levels will gradually drop as fish and other aquatic organisms consume oxygen. Eventually, unless the snow melts or the ice breaks up, DO levels may drop low enough to kill fish. Plowing snow off portions of a pond can lower winter fish kill risk but, if you try it, make sure the ice is thick enough to support the weight of whatever equipment you utilize. Consult the Pennsylvania Fish and Boat Commission’s ice safety chart for more information.

DO and phosphorus

Many pond owners struggle to manage recurrent weeds and algae blooms in their ponds caused by an overabundance of a nutrient called phosphorus. At the beginning of the year, much of the phosphorus is trapped in bottom of their pond because DO has created chemical conditions that bind the phosphorus to minerals in the sediment.  But if deep water DO levels decline, as they typically do in ponds when stratification happens in the spring, the chemical bonds trapping the phosphors break, and it becomes available to fuel plants and algae growth. This process is called "internal loading". Phosphorus returns to bottom sediments when pond vegetation decomposes following the growing season. Subsurface diffusers reduce internal loading by preventing ponds from stratifying which maintains DO levels in bottom sediment so phosphorus stays trapped which helps minimize weed and algal blooms throughout the growing season.

How and when to measure DO

The best place to measure DO is in the field immediately after a water sample is collected. Professionals use hand-held meters to measure DO, but these units are often expensive to purchase and require regular calibration to ensure accuracy. A less expensive option for measuring DO, that you may recall from high school or college, is called the Winkler method or Winkler titration. Kits that utilize this method, or a modified version, are available from online retailers, typically cost less than hand-held meters and, used correctly, are accurate enough to flag low DO levels⁷.

Since DO levels can vary daily and seasonally and at different depths, the only way to reveal your pond’s complete DO cycle is to measure DO frequently over a full year. But this is probably overkill for most pond owners who may only want to determine if DO levels are low enough to stress fish, or absent from the bottom of the pond which indicates conditions may be present for trapped phosphorus to be released. 

DO levels in the surface of the pond, approximately an arm's length underwater, are most likely to reach their lowest levels just before sunrise during late July or throughout August. If levels from this period are consistently above 5mg/L after testing at least once a week over several weeks, it is a good indication that DO is adequate for fish.

The easiest way to collect water from the bottom of a pond is to use a horizontal depth sampler, often called a Van Dorn bottle, named after its inventor. This device uses a manually operated trigger mechanism to snap shut the sampler’s lids once it is lowered to its intended depth.  Because these water sample collectors are expensive, a cheaper option is pulling water from the bottom of the pond with a flexible tube connected to a small hand or electric pump. Taking several measurements each month over the course of the summer can help reveal if DO levels are declining at the bottom of your pond. These measurements can be taken any time during the day since DO levels in the bottom of ponds do not normally fluctuate daily like they do in water closer to the surface.

Resources

Fundamentals of Environmental Measurements, Fondriest Environmental, 2013.  

Factsheet on Water Quality Parameters, Dissolved Oxygen, US EPA, July 2021. [PDF]

Dissolved Oxygen and Water, Water Science School, USGS

Classifying mixing regimes in ponds and shallow lakes, Holgerson, M. A., Richardson, D. C., Roith, J., Bortolotti, L. E., Finlay, K., Hornbach, D. J., et al. Water Resources Research, 58, e2022WR032522. 2022.  doi.org/10.1029/2022WR032522

Limnology (2^nd ed.), Wetzel R.G. Saunders College Publishing, 1983.

Shallow-lake sediments release nutrients by complete destratification events, Jan J., Breton F., Borovec J., Journal of Environmental Quality. 2025. 

Dissolved Oxygen and Biochemical Oxygen Demand, US EPA