CADDIS Volume 2: Sources, Stressors & Responses
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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 increased turbulence over a spillway and downstream may enhance aeration. Water released from the bottom of a dam is often cooler (i.e., DO saturation is higher), 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 Figure 3) 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 also may 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. These end-of-pipe discharges (Figure 4) 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 (Figures 5 and 6) 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 also can increase oxygen demand.
Devegetated riparian areas: Removing vegetation from the banks of surface waters (Figure 7) 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 (Figure 7) 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 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.
In addition to observations of sources discussed above, observational evidence suggesting that low DO should be included as a potential candidate cause includes 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 (Figure 8) 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.
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 can tolerate very low DO levels (e.g., channel catfish) (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 also may 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 low 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 HBI scores 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 (Figure 9) 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.
- 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, bubbles or emboli block the flow of blood through blood vessels, causing death. In fish dying from this disorder, external bubbles (emphysema) may be seen on fins, skin, around the eyes or on other tissues. Aquatic invertebrates also are 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.
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. 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 Ways to Measure). When not listing low DO as a candidate cause 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, and they show DO concentrations consistent with those found at unimpaired sites, you may choose not to list low DO (lack of spatial co-occurrence). However, 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).