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

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)

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) solubility decreases with increased temp/sal other factors: BP (direct), altitude (indirect), impurities (indirect)

Oxygen Solubility Curve

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)

Oxygen Consumption vs. Size for 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)

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

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

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

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

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

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

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)

Oxygen Budget

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

Influence of Sunlight on Photosynthesis/O2 Production

Photorespiration: predictable

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

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

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

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

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-

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)

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)

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

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

CO2 Level Affects Hemoglobin Saturation

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?

The Effect of pH on Carbonate Buffering

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

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