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CC.4. Dissolved Oxygen

Dissolved oxygen (DO) refers to the concentration of oxygen gas incorporated in water. Oxygen enters water by direct absorption from the atmosphere which is enhanced by turbulence (Image CC.4-1). Water also absorbs oxygen released by aquatic plants during photosynthesis. Sufficient DO is essential to growth and reproduction of aerobic aquatic life (e.g., see Murphy, 2006; Giller and Malmqvist, 1998; Allan, 1995). Information on how to measure DO is provided in CC.4.2.

Advice for deciding whether to include depleted or excessive DO as a candidate cause is provided in this module. You may go directly to a specific section of interest by clicking on the topic outline in the right navigation bar.

CC.4.1. What to Consider When Determining if DO Should be Included as a Candidate Cause

Low and excessive DO is addressed in this module as a proximate stressor that should be listed as a candidate cause when potential human sources and activities, site observations, or observed effects support portions of the source-to-impairment pathways in the conceptual model for DO (Figure CC.4-1). The most common problems associated with DO relate to depletion.

CC.4.1.1. Checklist of Sources, Site Evidence, and Biological Effects

A checklist is provided below to help you identify key data and information useful for determining whether to include DO among your candidate causes; the title for each column is linked to more detailed descriptions. The list is intended to guide you in collecting evidence to eliminate or enhance support for DO as a candidate cause. You also may be aware of other situations when DO should be eliminated or included as a candidate cause; please send us your insights using the comment section.

Consider listing DO as a candidate cause based on the presence of sources and activities, site evidence, and biological effects

Consider contributing, modifying, and related factors as candidate causes when DO is selected as a candidate cause:

• Temperature
• Nutrients
• Sediments
• Ammonia
• Pathogens

Consider not listing (eliminating) low DO as a candidate cause when you have evidence about turbulence and DO site data:

• High turbulence creates consistent aeration, so low DO is an implausible mechanism, or
• DO concentrations measured continuously over time at the site may confirm that DO concentrations are the same as or greater than DO at sites where biological impairment is not observed (lack of co-occurrence).

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CC.4.1.2. Sources and Activities that Suggest Listing DO as a Candidate Cause

The amount of DO in surface waters is influenced by numerous human activities, both in waterbodies and in their associated watersheds. The more extensive the relevant sources and activities, the more likely low DO will impair surface waters.

Impoundments — Impounding water may elevate or depress downstream DO, depending on impoundment design and operation. If water is released from the top of an impoundment or dam, the water may be warmer and thus less able to hold oxygen, but the large impoundment surface area and the increased turbulence over a spillway and downstream may enhance aeration. Water released from the bottom of a dam is often cooler and DO saturation is higher in cold water, but oxygen deficits may occur in these deeper reservoir waters. Upstream of dams, water is moving more slowly and DO may be low in subsurface waters from lack of turbulence and, at greater depths, from lack of light for photosynthesis.

Municipal waste treatment plants — Municipal waste treatment plants (also referred to as public-owned treatment works, or POTWs; see Image CC.4-2) process municipal wastewater, and are operated under permit limits designed to protect receiving waterbodies from excess inputs of nutrients and organic matter. However, during storms, excess flow may be diverted into combined sewer overflows (CSOs) that deposit untreated municipal waste directly into streams. Episodic treatment failures may also occur.

Septic seepage and failed package plants — Seepage from failed septic tanks or their leach fields and emissions from poorly functioning package sewage treatment plants may contribute significant amounts of nutrients and organic matter creating biological oxygen demand (BOD).

Industrial point sources — Some industries release organic chemicals that require oxygen for decomposition (BOD). These end-of-pipe discharges (Image CC.4-3) are regulated under permit limits to protect receiving waterbodies, but in cases where the original system design is not adequate, or problems in operation lead to inadequately treated discharges, oxygen depletion may result.

Agricultural and urban runoff — Nutrient runoff from agricultural or residential fertilizer applications (Images CC.4-4 and CC.4-5) can increase the amount of algae and macrophytes in water, leading to both higher oxygen inputs during the day and increasing oxygen demands from respiration at night. When plants die, they are decomposed by bacteria and fungi that consume oxygen. Organic matter washed into streams from animal wastes or landfills can also increase oxygen demand.

Devegetated riparian areas — Removing vegetation from the banks of surface waters (Image CC.4-6) increases surface water runoff and decreases shading. Decreased shading increases water temperatures and plant production. Higher temperatures decrease the solubility of oxygen in water. Plant production increases DO in daylight hours but increases oxygen demand during the night. Subsequent plant decomposition can deplete DO. In addition, reduced turbulence from less woody debris may decrease aeration.

Channel alteration — Stream channel straightening (Image CC.4-6) often reduces turbulence by removing structural diversity, alters curves and riffles, and may deepen the channel, reducing the surface-to-volume ratio and thus diffusion and aeration. Natural inflows of groundwater usually have low concentrations of DO and may at first lower DO concentration in surface waters. However groundwater is also often colder than surface water and may increase DO saturation levels. Changes to local hydrology and surface water temperatures may shift the effect of groundwater inflow on DO.

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CC.4.1.3. Site Evidence that Suggests Listing DO as a Candidate Cause

In addition to observations of sources discussed above, observational evidence suggesting that low DO should be included as a potential candidate cause include the following:

High plant abundance — Large amounts of algae (in the water column or on solid substrates) or aquatic vascular plants suggest the possibility of low DO, due to high plant respiration at night and high oxygen demand for decomposition of plant detritus. When plant abundance, temperatures, and light levels are high and turbulence is low, DO supersaturation may occur during the day with decreased DO at night.

Slow-moving water — Very slow-moving or still water (Image CC.4-7) may have low DO because of lack of turbulent aeration. In addition, slow-moving water tends to warm, reducing saturation levels for DO in the water column. Slow currents also may hamper delivery of oxygen to organisms.

Reduced water volume — Reduced water volume can concentrate fish into pools or other refugia where respiration exceeds oxygen renewal. Water volume can be reduced by removal for irrigation or other uses, by seasonal changes in rainfall, or by loss of suitable habitat due to episodic pollution, temperature increases, or other factors.

Weather conditions, seasons, time of day — Colder water saturates at higher DO levels than warmer water, so DO concentrations at a specific location are usually higher in winter than summer. During dry seasons, water levels decrease and stream flows decline, warming water and reducing turbulent mixing with air. During rainy seasons, oxygen concentrations tend to rise in most surface waters because rain saturates with oxygen as it falls. More sunlight and warmer temperatures also increase plant growth and animal activity, which may increase or decrease DO concentrations and increase diurnal fluctuation. Weather conditions fostering oxygen depletion include long periods of calm sunny weather that promote extensive algal growth, followed by cloudy days and nights when respiring plants consume more oxygen than they produce. DO concentrations tend to be lowest just before dawn.

Presence of organic wastes — Organic wastes are the remains of any living or once-living organism (e.g., dead plants or animals, leaves, animal droppings, sewage). Such organic matter observed within or being released to a waterbody suggests low DO as a candidate cause because organic decomposition consumes oxygen. Excessive organic wastes in water may result in a grayish cast with visible sludge deposits in depositional areas.

Turbidity — Turbidity can limit photosynthesis and may be due in part to suspended organic matter which creates biological oxygen demand.

Bad odor — Water smelling like rotten eggs or sour cabbage can indicate the presence of low oxygen conditions.

Color — The color of water that is low in oxygen may change from light green to pea-soup green, brown, gray or black. Dark sediments due to metal sulfides indicate anoxic conditions.

Embedded Substrate — When rocky substrates become embedded with fine sediments, benthic organisms may be affected by low interstitial DO concentrations.

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CC.4.1.4. Biological Effects that Suggest Listing DO as a Candidate Cause

Oxygen is essential to aquatic plants, animals, and aerobic microbes. Aquatic fauna obtain oxygen by actively moving water across their respiratory structures or by passively allowing currents to deliver oxygen to them. Some organisms require nearly saturated levels of oxygen (e.g., salmonids, riffle invertebrates), whereas others (e.g., channel catfish) can tolerate very low DO levels [for overviews, see Murphy (2006), Giller and Malmqvist (1998), Allan (1995), Nebeker (1972)].

Consider suboptimal DO as a candidate cause when you see changes in aquatic community structure or acute biotic effects as described below. Please note, however, that observations of these effects do not confirm a causal relationship. In some cases the same observed effect could be caused by other stressors or multiple agents. If you suspect DO as the cause of observed biological impairments, then also consider temperature and sediments, stressors often associated with and contributing to low DO. If nutrients or organic matter are parts of the causal pathway leading to low DO, then excess plant growth, ammonia and pathogens may also be of concern.

Changes in aquatic community structure — Decreases in DO levels can cause changes in the types and numbers of aquatic macroinvertebrates in surface waters. Species that are intolerant of decreases in DO include some species of mayflies, stoneflies, caddisflies, and beetles. As DO concentrations decrease, these organisms often are replaced by tolerant worms and fly larvae. The Hilsenhoff Biotic Index (HBI) is a biotic index based on species tolerances to organic enrichment (Hilsenhoff, 1987; Hilsenhoff, 1982); high values of the HBI may indicate organic enrichment sufficient to decrease oxygen levels. Fish communities also change with DO, but the patterns are not as clear because of fewer species and a smaller range of tolerance.

Acute effects of low DO — Biological effects and environmental changes associated with oxygen depletion may include the following (Meyer and Barclay, 1990):

• Kills of aquatic life (Image CC.4-8) occurring abruptly in early morning, usually between 0200 hrs and sunrise. If the kill is incomplete, it usually subsides soon after sunrise but may resume the following night,
• Kills of aquatic life occurring on cloudy days preceded by several warm sunny days,
• Large fish of a given species die first, whereas small fish may be alive,
• Species with the highest oxygen requirements die, whereas other species are not as significantly affected,
• Body movements to increase water flow may be observed in certain macroinvertebrates (e.g., some stonefly larvae do "push-ups", some caddisfly larvae undulate),
• Fish gulp air at the water surface and stay in shallow water (short film of gasping fish, courtesy of NOAA),
• Decaying vegetation may be abundant, or many dead and dying algae may be detected under a microscope, and
• Zooplankters are dead or dying.

Acute effects of oxygen supersaturation — When aquatic plants are abundant and weather conditions are ideal for photosynthesis, plants may supersaturate the water with oxygen. If the water temperature rises or if the pressure changes rapidly, fish in the area may develop oxygen-related gas bubble disease (Meyer and Barclay, 1990). In fish with gas bubble disease, the bubbles or emboli block the flow of blood through blood vessels, causing death. External bubbles (emphysema) may be seen on fins, skin and around the eyes or on other tissue in fish dying from this disorder. Aquatic invertebrates are also affected by gas bubble disease, but at levels higher than those lethal to fish. Other gasses can result in similar effects so further investigation is needed.

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CC.4.1.5. Site Evidence that Supports Excluding DO as a Candidate Cause

Advice on excluding low DO as a candidate cause is limited to situations in which the physical characteristics of a site enhance DO or when low DO cannot logically account for the impairment. Thus, unambiguous sources and site observations can be used to eliminate DO as a candidate cause. Biological evidence should not be used to exclude DO since several stressors alone or combined may cause similar symptoms of low or high DO. Further investigation will be needed. This type of initial screening saves time only when unnecessary listing of candidate causes is avoided. Early screenings should be conservative because the premature elimination of an actual cause will increase the time and cost of stressor identification.

Sources — Low concentrations of DO are physically precluded by consistent aeration from turbulence (i.e, the causal pathway is interrupted). Spillways, waterfalls, and turbulent flows in streams and rivers naturally aerate water. However, if flow changes during part of the year, DO will be affected and this should be considered. Strong wave action in marine coastal areas may ensure aeration while gentle wave action and riffles may or may not be sufficient depending on the depth of the water and rigor of mixing. Screening in these situations should be complemented with measures of DO concentrations (see CC.4.2). When not listing low DO as a candidate due to turbulence, consider listing altered hydrologic flow or insufficient sediment retention or supply. Both are known to occur below spillways and waterfalls due to retention of sediment behind the dam, and the power of water turbulence below the dam that can remove sediment and dislodge organisms.

Site Observations — When continuous measures of DO are available that document diurnal patterns over a long period of time confirming DO concentrations consistent with that found at unimpaired sites, you may choose not to list low DO (i.e., there is no spatial co-occurrence). However, it is desirable to include DO as a candidate cause and analyze the data more thoroughly in Step 2. We strongly caution against using benchmarks of effects for excluding DO from your initial list of candidate causes, because different species have different oxygen requirements [e.g., some mayflies exhibit effects at 9 mg/L which is well above the U.S. EPA standard of 5 mg/L (U.S.EPA, 1986)].

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CC.4.2. Ways to Measure DO

The concentration of oxygen in water is often reported either as the concentration in mg/L or as the percent saturation. DO concentrations and percent saturation are related, but not equivalent. The saturation level varies naturally, as water can contain more DO at lower temperatures, higher pressures, and lower salinities. For example, 100% saturation occurs at low oxygen concentrations at high elevations compared to low elevations (Hem, 1985).

The Winkler titration procedure was the first recognized method for determining DO concentrations in natural waters (Winkler, 1988, as cited in Mitchell, 2005). More recently, this method was found prone to over-reporting DO under hypoxic conditions, and under-reporting DO under nearly anoxic conditions. Fairly simple and reliable DO measurements can now be obtained with DO meters or field test kits. The electronic meter does not measure oxygen directly; rather, it uses electrodes to measure the partial pressure of oxygen in the water, expressed as a concentration (usually mg/L of water) [see APHA (1998), Mitchell and Stapp (1992), and USGS (1998)]. Percent saturation is calculated by dividing the measured DO concentration by the saturation level and multiplying by 100. Saturation levels can be obtained from U.S. Geological Survey-solubility tables based on water temperature and corrected for different salinities and pressures. Equations for calculating percent saturation can be found at http://waterontheweb.org/under/waterquality/oxygen.html.

Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) are measures of the potential consumption of oxygen by microbial respiration and the oxidation of chemicals in the water. The actual rate of oxygen consumption in a stream is affected by a number of variables including temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water.

The lowest concentrations of DO are usually measured before photosynthesis begins for the day (i.e., just before dawn), and just above the sediments, where most decomposition occurs. Documentation of DO concentrations over a 24-hour period may be useful for identifying diurnal patterns and may reveal information about DO depletions.

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Allan, JD. (1995) Stream ecology: structure and function of running waters. London, UK: Chapman and Hall Publishers.

APHA (American Public Health Association); AWWA (American Water Works Association); WEF (Water Environment Federation). (1998) Standard methods for the examination of water and wastewater, 20th edition. Washington, DC: American Public Health Association.

Giller, PS; Malmqvist, B. (1998) The biology of streams and rivers. London, UK: Oxford University Press.

Hem, JD. (1985) Study and interpretation of the chemical characteristics of natural water, 3rd edition. U.S. Geological Survey Water-Supply Paper 2254.

Hilsenhoff, WL. (1982) Using a biotic index of to evaluate water quality in streams. Tech. Bull. WI. Dept. Nat. Resour. No 132 22pp.

Hilsenhoff, WL. (1987) An improved biotic index of organic stream pollution. Great Lakes Entom 20:31-39.

Meyer, FP; Barclay, LA. (1990) Field manual for the investigation of fish kills. Washington, DC: U.S. Fish and Wildlife Service Resource Pub. 177.

Mitchell, MK; Stapp, W. (1992) Field manual for water quality monitoring, 5th ed. Dexter, MI: Thompson Shore Printers.

Mitchell, TO. (2005) Luminescence based measurement of dissolved oxygen in natural waters. HACH Environmental. Available at www.hachenvironmental.com. Available at www.hachenvironmental.com.

Murphy, S. (2006) USGS water quality monitoring, BASIN project. Available at http://bcn.boulder.co.us/basin/data/COBWQ/info/DO.html.

Nebeker, AV. (1972) Effect of low oxygen concentrations on survival and emergence of aquatic insects. Trans Amer Fish Soc 101:675-679.

US EPA (Environmental Protection Agency). (1986) Ambient water quality criteria for dissolved oxygen. U.S. Environmental Protection Agency, Office of Water, Washington, DC; EPA/PB86-208253.

USGS (US Geological Survey). (1998) National field manual for the collection of water-quality data. U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chaps. A1-A9.

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