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)
solubility decreases with increased temp/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. Size for Channel Catfish (26oC)
O2 cons. Rate Increase in
(mg/kg/hr) oxygen consumption
Fish size (g) Nonfed Fed from feeding (%)
,230 40 1,
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/ mg/L
Litopenaeus stylirostris: adults/ 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 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 mg/L
diel fluctuation opposite to that of D.O.
higher levels in warmer months of year