Water Quality Notes: Dissolved Oxygen1 - Polk.WaterAtlas.org

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Water Quality Notes: Dissolved Oxygen 4Aquatic invertebrates also differ widely in whereand how they obtain oxygen. Table 4 lists severalcommon orders of aquatic insects and their sources ofoxygen and respiratory options. Some aquatic insectsare capable of obtaining oxygen directly from theatmosphere or from plants. Dissolved oxygenconcentrations in water are of little consequence tothese organisms. Others have cutaneous respiratorysystems, gills, or hemoglobin that transfers dissolvedoxygen from the water column into their bodies forrespiration. The amount of oxygen dissolved in thewater is important for these species. In addition,these organisms vary in their abilities to adapt to- andin their tolerance of- low DO conditions. Many of thesediment dwelling chironomid larvae andpolycheate/oligocheate worms are commonly foundin areas with very low DO concentrations.Table 4. Several common orders of aquatic insects and theirmodes of obtaining- and sources ofoxygen for respiration.OrderDipteraColeopteraHemipteraColeopteraDipteraColeopteraDipteraLepidopteraColeopteraDipteraEphemeropteraOdonataPlecoptera,LepidopteraMegalopteraNeuropteraTricopteraRespiratoryOptionAtmosphericbreathers.Spiacles in contactwith theatmospherecontinuously orperiodically.Plant breathers.Spiracles modifiedto piercesubmergedportions ofaerenchymatissue in wetlandplants.Use air-storageorgans. This istypically a gasbubble that istransported withthe organism.Breathe throughgills, physicalstructures thatfacilitate O2transfer from thewater.OxygenSourceAtmospherePlantsAtmosphereor dissolvedDissolvedoxygenTable 4. Several common orders of aquatic insects and theirmodes of obtaining- and sources ofoxygen for respiration.OrderDipteraLepidopteraPlecopteraTricopteraHemipteraDipteraRespiratoryOptionDiffusion of O2through thecuticle. Becauseof respirationneeds, onlypractical fororganisms with ahigh surface areato volume ratio.Hemoglobin.Specialized formof hemoglobin thatreleases O2at lowexternal O2pressures, suchas found in lowDO waters andmud.OxygenSourceDissolvedAtmosphereor dissolvedMeasurement Timing and MethodsDissolved oxygen measurements are especiallyuseful when fish kills are first detected, or when fishare seen gulping for air at the water surface. Timelymeasurements of this type are often indispensible indetermining the cause of death (i.e. lack of DOversus poisoning, etc.). In addition to helping definecause and effect associated with fish kills, monitoringof DO is often necessary for managing aquaticsystems for their intended use. Dissolved oxygenconcentrations taken at sporatic intervals of time aredifficult to interpret due to the diurnal oxygen cycle.For this reason, a DO monitoring program should betailored for individual needs. Since the lowest DOlevels typically influence what species can live in agiven waterbody, one might be interested indetermining the lowest DO concentrations over aspan of time by taking measurements before dawnevery morning or over a series of mornings.Conversely, taking samples throughout the day willgive an idea of the daily fluctuations in DOconcentrations (minimum, maximum, mean,median), which are related to photosynthetic activity,respiration, and atmospheric transfer of oxygen.Measurements taken in the early afternoon will likelyrepresent the highest achievable DO concentrations in
Water Quality Notes: Dissolved Oxygen 5a given water body with regard to photosyntheticorganisms.Dissolved oxygen can be measured by twocommon techniques, each with advantages anddisadvantages. The first method is called aniodometric method or Winkler titration and thesecond method uses a gas-permeable membraneelectrode. These methods are discussed in thefollowing sections.Iodometric MethodThe description of this method was taken fromStandard Methods for Examination of Water andWastewater (APHA et. al, 1995) . This referenceshould be consulted for more detailed informationwhen needed. The iodometric method can be one ofthe most precise and reliable procedures for DOanalysis. This is a titration-based method based onthe reaction of DO with divalent manganese (Mn ++ )ions. Briefly, after adding a known amount ofdissolved divalent manganese ions and iodide to aknown volume of sample, the pH is increased byadding base and the bottle is then stoppered. Aninsoluble precipitate of manganous hydroxide forms.Dissolved oxygen present in the sample oxidizes anequivilent amount of the manganous hydroxideprecipitate, forming chemically different hydroxides.The sample is next acidified, which causes theprecipitate to dissolve, releasing the iodide originallyconsumed in the solution. The amount of iodinereleased is then measured by titration with a standardsolution of thiosulfate (Na 2S 2O 3) and a starchindicator. The solution is blue as long as I 2is present.When all of the I 2has been removed from solution bythiosulfate, the solution clears. The volume ofthiosulfate used to clear the solution is then used tocalculate the concentration of DO in the sample (1mL of 0.025M Na 2S 2O 3= 1 mg-DO/L for a 200 mLsample). The DO concentration can also bedetermined directly using an absorptionspectrophotometer.Advantages1. Can be very accurate and precise.2. Relatively inexpensive (only requiring titrationburets, sample bottles, and chemicals)3. Available in kits from several manufacturers.Disadvantages1. Cannot monitor DO instantaneously orcontinuously.2. More time consuming than membrane electrodemethods.3. Nitrite, iron (ferrous and ferric), suspendedsolids, and color in the sample can interfere withaccurate measurement. Modifications to themethod can be made to remove theseinterferences.Membrane Electrode MethodsThe membrane electrode method uses asubmersible, membrane-covered electrode that isattached to a meter that gives direct readings in mg/L(parts-per-million, PPM) or percent saturation. Thereare a variety of membrane electrodes that arecommercially available and that provide an excellentmethod for DO measurement in highly pollutedand/or highly colored waters, as well as in strongwaste effluents. They are especially useful underconditions that are not favorable for use of theiodometric methods and for in situ measurements inthe field. Two basic types of membrane electrodesexist, polarographic and galvanic. The followingdescription is for the more common polaragraphicmodels.The electrode itself is composed of a physicallyseparated metal anode and cathode (Figure 3). Anelectrolyte solution provides connectivity betweenthe two. A thin, oxygen-permeable membrane isstretched over the sensor isolating the anode, cathode,and electrolyte solution from the environment, whileallowing oxygen to enter. This membrane is typicallycomposed of polyethylene or fluorocarbon materialsbecause of their relative ruggedness and permeabilityto molecular oxygen.When this type of meter is switched ON, abackground electrical current is established throughthe anode-electrolyte-cathode-meter circuit. Oxygenpasses through the membrane at a rate proportional toits concentration in the water. Oxygen, which haspassed through the membrane, reacts at the cathode
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