Water Qualtiy: Dissolved Oxygen, pH, Alkalinty

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Water Qualtiy Dissolved Oxygen, pH, Alkalinty

• From Lawson, Boyd

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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)

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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)

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Oxygen Solubility Curve
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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)

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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)
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Chemical Variables dissolved oxygen

• What might be considered minimal levels of
  maintenance of D.O.?
• hard to determine due to compounding effects
  (cant 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

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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

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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

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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

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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?)--gt death

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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

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Chemical Variables dissolved oxygen/sources

• major producer of D.O. in ponds is primary
  productivity (up to 80), diffusion is low (lt3)
• 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)

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Oxygen Budget
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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

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Influence of Sunlight on Photosynthesis/O2
Production
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Photorespiration predictable
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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

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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

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Chemical Variables total alkalinity

• total alkalinity level can be associated with
  several potential problems in aquaculture
• lt 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 --gt low nat prod
• low alkalinity high pH

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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

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Chemical Variables pH

• pH the level or intensity of a substances
  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-

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Chemical Variables pH

• at acidic pH levels, the quantity of H
  predominates
• acidic pH pH lt 7, basic pH gt7
• 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)

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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)

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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

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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

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CO2 Level Affects Hemoglobin Saturation
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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?

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The Effect of pH on Carbonate Buffering
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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

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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