Water Qualtiy: Dissolved Oxygen, pH, Alkalinty

1. Water Qualtiy: Dissolved Oxygen, pH, Alkalinty From Lawson, Boyd

2. Chemical Properties: dissolved oxygen • along with temperature, dissolved oxygen (DO), is important in metabolic regulation • dissolved oxygen concentration and temp both determine the environmental niche aquaculture organisms occupy • occupation of niches is controlled by a complex set of behavioral and physiological (acclimatorial) activities • acclimation is slow wrt D.O. (hours, weeks)

3. Chemical Variables: dissolved oxygen • although oxygen is rather abundant in the atm (21%), it is only slightly soluble in water (6 mg/L is not much) • implications to fish/invertebrates? • Even metabolic rates of aqua-communities can effect rapid changes in [D.O.] • this effect increases with temp (interaction) • solubilitydecreases withincreasedtemp/sal • other factors: BP (direct), altitude (indirect), impurities (indirect)

4. Oxygen Solubility Curve

5. Chemical Variables: dissolved oxygen • factors affecting D.O. consumption: • water temperature (2-3x for every 10oC) • environmental (medium) D.O. concentration (determines lower limit) • fish size (Rc greater for small vs. large) • level of activity (resting vs. forced) • post-feeding period, etc. (2x, 1-6 hrs post feeding)

6. Oxygen Consumption vs. Sizefor Channel Catfish (26oC) O2 cons. Rate Increase in (mg/kg/hr) oxygen consumption Fish size (g) Nonfed Fed from feeding (%) 2.5 880 1,230 40 100 400 620 55 500 320 440 38 1,000 250 400 60 From Lovell (1989)

7. Chemical Variables: dissolved oxygen • What might be considered minimal levels of maintenance of D.O.? • hard to determine due to compounding effects (can’t standardize conditions) • major factor: exposure time • for most species: • long-term: 1.5 mg/L • medium term: 1.0 mg/L • short-term: 0.3 mg/L

8. Chemical Variables: dissolved oxygen • In general warm-water species are more tolerant of low D.O. concentrations • Ictalurus punctatus: adults/1.0 mg/L, fingerlings 0.5 mg/L • Procamberus clarkii: adults/2.0 mg/L, juveniles/1.0 mg/L • Litopenaeus vannamei: adults/0.5-0.8 mg/L • Litopenaeus stylirostris: adults/1.2-1.4 mg/L

9. Chemical Variables: dissolved oxygen • Many practical aquaculturists will recommend that D.O. concentrations do not drop below 6.0 mg/L • this is an impractical guideline in that this level can seldom be achieved at night • a more practical guideline might be to maintain D.O. levels around 90% saturation • no lower than 25% saturation for extended periods

10. Chemical Variables: dissolved oxygen/behavior • if D.O. levels in the medium are adequate, fish meet increased demands due to locomotion or post-feeding by increased rate of ventilation or large “gulps” of water • declining D.O.: seek zones of higher D.O., reduce activity (reduced MR), stop consumption of feed • compensatory point: when D.O. demand cannot be met by behavioral or physiological responses

11. Chemical Variables: dissolved oxygen/behavior • upon reaching compensatory point: gaping at surface, removal of oxygen from surface • shown in both fish and invertebrates • small aquatic animals are more efficient • some oxygen provided by glycolysis or anaerobic metabolism, but blood pH drops • pH drop in blood reduces carrying capacity of hemoglobin (hemocyanin?)--> death

12. Oxygen/Temperature Interaction • Oxygen consumption increases with temperature until a maximum is achieved • peak consumption rate is maintained over a small temp range • consumption rate decreases rapidly as temp increases • lethal temperature finally achieved

13. Chemical Variables: dissolved oxygen/sources • major producer of D.O. in ponds is primary productivity (up to 80%), diffusion is low (<3%) • incoming water can often be deficient depending upon source water conditions • major consumers: primary productivity, aquatic species (density dependent), COD • diel fluctuation • indirect relationships (algae, secchi)

14. Oxygen Budget

15. Diel Oxygen Fluctuation • Typical pattern = oxygen max during late afternoon • difference in surface vs. benthic for stratified ponds • dry season = faster heating at surface and less variation

16. Influence of Sunlight on Photosynthesis/O2 Production

17. Photorespiration: predictable

18. Chemical Variables: total alkalinity • total alkalinity: the total amount of titratable bases in water expressed as mg/L of equivalent CaCO3 • “alkalinity” is primarily composed of the following ions: CO3-, HCO3-, hydroxides, ammonium, borates, silicates, phosphates • alkalinity in ponds is determined by both the quality of the water and bottom muds • calcium is often added to water to increase its alkalinity, buffer against pH changes

19. Chemical Variables: total alkalinity • thus, a total alkalinity determination of 200 mg/L would indicate good buffering capacity of a water source • natural freshwater alkalinity varies between 5 mg/L (soft water) to over 500 mg/L (hard water) • natural seawater is around 115-120 mg/L • seldom see pH problems in natural seawater • water having alkalinity reading of less than 30 mg/L are problematic

20. Chemical Variables: total alkalinity • total alkalinity level can be associated with several potential problems in aquaculture: • < 50 mg/L: copper compounds are more toxic, avoid their use as algicides • natural waters with less than 40 mg/L alkalinity as CaCO3 have limited biofiltration capacity, pH independent • low alkalinity = low CO2 --> low nat prod • low alkalinity = high pH

21. Chemical Variables: total hardness • total hardness: total concentration of metal ions expressed in terms of mg/L of equiva- lent CaCO3 • primary ions are Ca2+ and Mg2+, also iron and manganese • total hardness approximates total alkalinity • calcium is used for bone and exoskeleton formation and absorbed across gills • soft water = molt problems, bone deformities

22. Chemical Variables: pH • pH: the level or intensity of a substance’s acidic or basic character • pH: the negative logarithm of the hydrogen ion concentration (activity) of a substance • pH = -log(1/[H+]) • ionization of water is low (1x10-7 moles of H+/L and 1x10-7 moles OH-/L) • neutral pH = similar levels of H+ and OH-

23. Chemical Variables: pH • at acidic pH levels, the quantity of H+ predominates • acidic pH = pH < 7, basic = pH >7 • most natural waters: pH of 5-10, usually 6.5-9; however, there are exceptions • acid rain, pollution • can change due to atm CO2, fish respiration • pH of ocean water is stable (carbonate buffering system, later)

24. Chemical Variables: pH • Other sources of change: • decay of organic matter • oxidation of compounds in bottom sediments • depletion of CO2 by phytoplankton on diel basis • oxidation of sulfide containing minerals in bottom soils (e.g., oxidation of iron pyrite by sulfide oxidizing bacteria under anaerobic conditions)

25. Chemical Variables: carbon dioxide • normal component of all natural waters • sources: atmospheric diffusion, respiration of cultured species, biological oxidation of organic compounds • usually transported in the blood as HCO3- • converted to CO2 at the gill interface, diffusion into medium • as the level of CO2 in the medium increases, the gradient allowing diffusion is less

26. Chemical Variables: carbon dioxide • this causes blood CO2 levels to increase, lowering blood pH • with lower blood pH, carrying capacity of hemoglobin decreases, also binding affinity for oxygen to hemoglobin • this phenomenon is known as the Bohr-Root effect • CO2 also interferes with oxygen uptake by eggs and larvae

27. CO2 Level Affects Hemoglobin Saturation

28. Chemical Variables: carbon dioxide • in the marine environment, excesses of CO2 are mitigated by the carbonate buffering system • CO2 reacts with water to produce H2CO3, carbonic acid • H2CO3 reacts with CaCO3 to form HCO3- (bicarbonate) and CO32- (carbonate) • as CO2 is used for photosynthesis, the reaction shifts to the left, converting bicarbonates back to CO2 • what large-scale implications does this have?

29. The Effect of pH on Carbonate Buffering

30. Chemical Variables: carbon dioxide • Concentrations of CO2 are small, even though it is highly soluble in water • inverse relationship between [CO2] and temperature/salinity • thus, CO2 solubility depends upon many factors

31. Chemical Variable: carbon dioxide • CO2 is not particularly toxic to fish or invertebrates, given sufficient D.O. is available • maximum tolerance level appears to be around 50 mg/L for most species • good working level of around 15-20 mg/L • diel fluctuation opposite to that of D.O. • higher levels in warmer months of year