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Dissolved Oxygen and Biochemical Oxygen Demand

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Dissolved Oxygen and Biochemical Oxygen Demand This document is Chapter 9 of the Volunteer Estuary Monitoring Manual, A Methods Manual, Second Edition, EPA-842-B-06-003. The full document be downloaded from: http://www.epa.gov/owow/estuaries/monitor/ Voluntary Estuary Monitoring Manual Chapter 9: Dissolved Oxygen and Biochemical Oxygen Demand March 2006 ChapterOxygen 9 Dissolved oxygen concentrations indicate how well aerated the water is, and vary according to a number of factors, including season, time of day, temperature, and salinity. Biochemical oxygen demand measures the amount of oxygen consumed in the water by chemical and biological processes. Photos (l to r) K. Register, R. Ohrel, R. Ohrel Unit One: Chemical Measures Chapter 9: Oxygen Overview Nearly all aquatic life needs oxygen to survive. Because of its importance to estuarine ecosystems, oxygen is commonly measured by volunteer monitoring programs. When monitoring oxygen, volunteers usually measure dissolved oxygen and biochemical oxygen demand. Dissolved oxygen concentrations indicate how well aerated the water is, and vary according to a number of factors, including season, time of day, temperature, and salinity. Biochemical oxygen demand measures the amount of oxygen consumed in the water by chemical and biological processes. This chapter discusses the role of dissolved oxygen and biochemical oxygen demand in the estuarine environment. It provides steps for measuring these water quality variables. Finally, a case study is provided. 9-1 Volunteer Estuary Monitoring: A Methods Manual Chapter 9: Oxygen Unit One: Chemical Measures Why Monitor Oxygen? Of all the parameters that characterize an estuaries, the consequences of a rapid decline in estuary, the level of oxygen in the water is one oxygen set in quickly and animals must move of the best indicators of the estuary’s health. An to areas with higher levels of oxygen or perish. estuary with little or no oxygen cannot support This immediate impact makes measuring the healthy levels of animal or plant life. level of oxygen an important means of Unlike many of the problems plaguing assessing water quality. ■ DISSOLVED OXYGEN (DO) Oxygen enters estuarine waters from the depleted oxygen, low oxygen conditions may atmosphere and through aquatic plant photosyn­ also naturally occur in estuaries relatively unaf­ thesis. Currents and wind-generated waves boost fected by humans. Generally, however, the the amount of oxygen in the water by putting severity of low DO and the length of time that more water in contact with the atmosphere. low oxygen conditions persist in these areas are less extreme. Dissolved Oxygen in the Estuarine DO and nutrients can be connected in another Ecosystem way. When oxygen is low, nutrients bound to bottom sediments can be released into the water DO is one of the most important factors con­ column, thereby permitting more plankton trolling the presence or absence of estuarine growth and eventually more oxygen depletion. species. It is crucial for most animals and plants Other pollutants may also be released from sedi­ except for a small minority that can survive ments under low oxygen conditions, potentially under conditions with little or no oxygen. causing problems for the estuarine ecosystem. Animals and plants require oxygen for respira­ Oxygen availability to aquatic organisms is tion—a process critical for basic metabolic complicated by the fact that its solubility in processes. water is generally poor. Salt water absorbs even In addition to its use in respiration, oxygen is less oxygen than fresh water (e.g., seawater at needed to aid in decomposition. An integral part 10°C can hold a maximum dissolved oxygen of an estuary’s ecological cycle is the break­ concentration of 9.0 mg/l, while fresh water at down of organic matter. Like animal and plant the same temperature can hold 11.3 mg/l). respiration, this process consumes oxygen. Warm water also holds less oxygen than cold Decomposition of large quantities of organic water (e.g., seawater can hold a dissolved oxy­ matter by bacteria can severely deplete the gen concentration of 9.0 mg/l at 10°C, but that water of oxygen and make it uninhabitable for concentration drops to 7.3 mg/l when the tem­ many species. perature increases to 20°C). Therefore, warm An overload of nutrients from wastewater estuarine water can contain very little dissolved treatment plants or runoff from various land oxygen, and this can have severe consequences uses also adds to the problem. Nutrients fuel the for aquatic organisms. overgrowth of phytoplankton, known as a bloom. The phytoplankton ultimately die, fall to Levels of Dissolved Oxygen the bottom, decompose, and use up oxygen in the deep waters of the estuary. Although nutri­ Although we may think of water as homoge­ ents from human activities are a major cause of neous and unchanging, its chemical constitution 9-2 Volunteer Estuary Monitoring: A Methods Manual Unit One: Chemical Measures Chapter 9: Oxygen does, in fact, vary over time. Oxygen levels, in particular, may change sharply in a matter of hours. DO concentrations are affected by physi­ cal, chemical, and biological factors (Figure 9­ Sewage effluent 1), making it difficult to assess the significance of any single DO value. Runoff At the surface of an estuary, the water at mid­ day is often close to oxygen saturation due both Phytoplankton bloom DO from wave action to mixing with air and the production of oxygen thrives on nutrients & photosynthesis by plant photosynthesis (an activity driven by sunlight). As night falls, photosynthesis ceases and plants consume available oxygen, forcing DO DO trapped levels at the surface to decline. Cloudy weather in lighter layer Dead material Lighter freshwater may also cause surface water DO levels to drop settles since reduced sunlight slows photosynthesis. Decomposition Heavier seawater DO levels in an estuary can fluctuate greatly with depth, especially during certain times of HYPOXIA DO used up by the year. Temperature differences between the microorganism respiration surface and deeper parts of the estuary may be Nutrients released quite distinct during the warmer months. by bottom sediments Fish able to Vertical stratification in estuarine waters avoid hypoxia (warmer, fresher water over colder, saltier DO Consumed water) during the late spring to summer period Shellfish unable to is quite effective in blocking the transfer of escape oxygen between the upper and lower layers (see hypoxia Decomposition of organic Figure 9-1). In a well-stratified estuary, very lit­ matter in sediments tle oxygen may reach lower depths and the deep water may remain at a fairly constant low Figure 9-1. Physical, chemical, and biological processes that affect dissolved level of DO. Changing seasons or storms, how­ oxygen concentrations in estuaries. (Redrawn from USEPA, 1998.) ever, can cause the stratification to disintegrate, allowing oxygen-rich surface water to mix with the oxygen-poor deep water. This period of mixing is known as an overturn. DO Concentration (mg/l) When DO declines below threshold levels, which vary depending upon the species, mobile 6 mg/l animals must move to waters with higher DO; 5 mg/l Usually required for immobile species often perish. Most animals growth and activity and plants can grow and reproduce unimpaired 4 mg/l when DO levels exceed 5 mg/l. When levels drop to 3-5 mg/l, however, living organisms hypoxia 3 mg/l Stressful to most aquatic often become stressed. If levels fall below 3 organisms mg/l, a condition known as hypoxia, many 2 mg/l Usually will not species will move elsewhere and immobile support fish species may die. A second condition, known 1 mg/l as anoxia, occurs when the water becomes anoxia totally depleted of oxygen (below 0.5 mg/l) and results in the death of any organism that 0 mg/l requires oxygen for survival. Figure 9-2 sum­ Figure 9-2. Dissolved oxygen in the water. A minimum DO concentration marizes DO thresholds in estuarine waters. ■ of 5 mg/l is usually necessary to fully support aquatic life. 9-3 Volunteer Estuary Monitoring: A Methods Manual Chapter 9: Oxygen Unit One: Chemical Measures Sampling Considerations Chapter 6 summarized several factors that in the estuary. Tidal effects, then, could be a should be considered when determining consideration when collecting and analyzing monitoring sites, where to monitor in the water DO data. column, and when to monitor. In addition to the considerations in Chapter 6, a few additional Where to Sample ones specific to oxygen monitoring are As mentioned previously, estuary presented here. stratification can have an impact on DO levels at different depths. Stratification is especially When to Sample evident during the summer months, when warm In estuarine systems, sampling for DO fresh water overlies colder, saltier water. Very throughout the year is preferable to establish a little mixing occurs between the layers, forming clear picture of water quality. If year-round a boundary to mixing. sampling is not possible, taking samples from Because DO levels vary with depth— the beginning of spring well into autumn will especially during the summer—volunteer provide a program with the most significant groups may wish to collect samples at different data. Warm weather conditions bring on depths. Van Dorn and Kemmerer samplers (see hypoxia and anoxia, which pose serious Chapter 7) are commonly used to collect these problems for the estuary’s plants and animals. kinds of samples. In addition, there are several Because these conditions are rare during winter, water samplers designed primarily for cold weather data can serve as a baseline of collecting DO samples at different depths information. (Figure 9-3). Appendix C provides a list of Sampling once a week is equipment suppliers. generally sufficient to capture the variability of DO in the estuary. Choosing a Sampling Method Since DO may fluctuate Citizen programs may elect to use either a throughout the day, volunteers DO electronic meter or one of the several should sample at about the same available DO test kits (Table 9-1).
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