1. Conservative Water Quality Lecture 7

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

3. Chemical Variables: dissolved oxygen
Although O2 is rather abundant in the atm (21%), it is only marginally soluble in water (6 ppm is not much)
What are the 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

32. Part II: Nitrogenous Compounds in Water

33. Evolution of the Nitrogen Cycle
Unlike carbon or oxygen, nitrogen is not very available to life
it’s conversion requires biological activity
nitrogen cycle is required by life, but also driven by it
cycle is rather complex and has evolved as the atmosphere became oxygenated
as we know, Earth’s original atm was oxygen-poor

34. Evolution of the Nitrogen Cycle
Earliest forms of nitrogen-reducing bacteria had to have been anaerobic
other option: NH4+ already existed in some form
today these ancient N-fixers either only exist in anaerobic environments or the N-fixing apparati are carefully guarded from intracellular oxygen

35. Evolution of the Nitrogen Cycle
As Earth’s atmosphere became more O2-rich, more NO3 became available
this created niches occupied by organisms that could reduce NO3 to NH3 (many higher plants can do this)
converting NO3 back to N2 (denitrification) is an arduous process and has evolved more recently

36. Gaseous Nitrogen Nitrogen is the major gas in the atmosphere
after oxygen, second limiting factor
constitutes 78.1% of total gases in air
solubility in water is largely dependent upon two physio-chemical factors: temperature and salinity
at saturation/equillibrium it has higher values than oxygen or CO2

37. Nitrogen Saturation Values

38. Generalized Nitrogen Cycle
Nitrogen dynamics in the environment involves some fairly complex cycling
N is relatively unreactive as an element
cyclic conversions from one form to another are mainly mediated by bacteria
Cycle occurs in both aerobic and anaerobic environments
nitrogen cycle

39. Process 1: fixation
Nitrogen fixation refers to the conversion of N2 to either NO3 or NH4 by bacteria
terrestrial systems: soil bacteria in root nodules of legumes
aquatic systems: blue green algae
biological, meteorological, industrial transformations also occur

40. Nitrogen Fixation Type of Fixation N2 fixed (1012 g per year)
Non-biological
industrial
About 50
combustion
About 20
lightning
About 10
Total
About 80
Biological
Agricultural land
About 90
Forest + nonag land
Sea
About 35
About 175

41. Process 2: nitrification
The term nitrification refers to the conversion of ammonium to nitrate (pathway 3-4 opposite)
Responsible: nitrifying bacteria known as chemoautotrophs
These bacteria gain their energy by oxidizing NH3, while using CO2 as a source of carbon to synthesize organic compounds
The nitrogen cycle, once more!

42. Process 3: denitrification
By this process, NO3 in soil or water is converted into atm N2, nitric oxide or nitrous oxide
this must occur under anaerobic conditions (anaerobic respiration)
presence of O2 can reverse the reaction
again, mediated by bacteria (Pseudomonas sp., Alkaligenes sp. and Bacillus sp.)
Denitrification = step 5, above

43. Aquatic Nitrogen Cycling
For aquaculturists, cycling transforms usually begin with the decomposition of organic matter from either plant or animal sources
major source in aquaculture: feeds
ultimately excreted as amine groups on amino acids or excreted in soluble form primarily as NH3/NH4+, other compounds
amino acid

44. Release of NH3
NH3 separated from organic protein via microbial activity
Process referred to as deaminification or ammonification
NH3 is released to water column (mineralization) and assimilated into primary productivity (NH3 + H+ --> NH4+)
ammonification is heterotrophic, under aerobic or anaerobic conditions
ammonification

45. Aquatic Nitrogen Cycling
NH3 and NH4+ are both either assimilated by aquatic plants for growth or nitrified (oxidized) to NO3- (nitrate)
nitrate can also be used as a growth substrate (Guillard’s F medium)
two step process:
NH O2  NO2- + 2H+ + H2O
NO O2  NO3-
Note: these are oxygen-driven reactions

46. Aquatic Nitrogen Cycling
Conversion of ammonia (NH3) to nitrate (NO3-) is via chemoautotrophic bacteria
first step by Nitrosomonas sp.
second step by Nitrobacter sp.
Both steps/reactions use NH4+ and NO2- as an energy source, CO2 as a carbon source
this is a non-photosynthetic type of growth

47. Aquatic Nitrogen Cycling
Reaction runs best at pH 7-8 and 25-30oC
however; under low DO, it runs in reverse
NO3- is converted to NO2= and other forms
can go all the way backwards to NH3
occurs in the hypolimnion under eutrophic (stagnant) conditions
REM: nitrogen also fixed by leguminous plants, free living bacteria, blue-green algae
magnitude of this transform not well studied

48. Nitrogen: aqueous forms
Gaseous form of nitrogen (N2) is most prevalent
followed by: nitrite, nitrate, ammonia or ammonium
nitrite is seldom a problem unless DO levels are low (to be discussed later)
ratio of NH3:NH4+ rises with pH
unfertilized ponds: TAN (NH3 +NH4+) = mg/L
fertilized ponds: TAN = 0.5 mg/L, mg NO3-

49. Nitrogen Amendments Nitrogen added as fertilizer to ponds: urea
Immediately upon addition, it starts to decline
only small portion detectable from metabolic processes
plants typically take it up, die, mud deposit
inorganic nitrogen typically denitrified in the hypolimnion
high afternoon pH = increased volatization
urea

50. Nitrogen Equillibria: NH3/NH4+
ammonia (NH3) is toxic to fish/inverts
pH affects proportion of NH3/NH4+
as pH increases, NH3 increases
calculation example TAN = 1.5 mg/L, 26oC, pH = 8.6
answer: mg NH3/L
Affect of pH/temp on NH3/NH4+ equillibria

51. More on Ammonia
As mentioned, initial source: feed, direct source: excretion
can calculate daily dosage/loading if you know: NPU and % protein in feed
NPU is 0.4 (approx.) for most aquaculture feeds
equ.: (1.0 - NPU)(pro/6.25)(1000) = g NH3/kg feed
for 1.0 ha pond receiving 100 kg of 30% protein feed/day, loading is 1,920 g NH3
dilution in 10 x 106 L is mg NH3/L
if NPU stays constant, NH3 production increases with increased feeding

52. Ammonia Toxicity Both NH3 and NH4+ are toxic to fish/inverts:
as medium NH3 increases, ability to excrete internal NH3 decreases (fighting gradient)
blood/tissue NH3 increases causes increase in blood pH
result: imbalance in enzyme activity, reduced membrane stability
increased O2 consumption by tissues, gill damage, reduced O2 transport (Root/Bohr, but other direction)
reduced growth, histological changes in gills/other organs

53. Ammonia Toxicity Short term exposure toxic at 0.7-2.4 mg/L
96 hr LC50 varies from mg/L for most fish
toxicity tolerance varies due to biological variability of different strains of species
eggs are most tolerant (fish)
larvae least tolerant, older = more tolerant
same probably holds true for inverts

54. Ammonia Toxicity

55. Ammonia Toxicity in Ponds
NH3 is more toxic when DO levels are low
however, toxic effect is probably nullified by resultant increase in CO2
thus, increased CO2 = decreased NH3
increased CO2 = decreased pH
in some cases, fish have been shown to acclimate to increases in NH3

56. Nitrite (NO2-) Toxicity
Nitrite reacts with hemoglobin to form methemoglobin
in process, iron converted from ferrous (Fe2+) to ferric (Fe3+) form
ferric form of iron cannot bind with oxygen
blood changes from red to brown, appears anemic
those fish having methemoglobin reductase enzyme can convert iron moeity back to ferrous
maybe same for crustaceans?

57. Nitrite (NO2-) Toxicity
Recovery from nitrite toxicity usually occurs when fish are transferred to better water
complete recovery can occur in 24 h
how does it get into system in first place?
Nitrite is quickly transported across gill membrane by lamellar chloride cells
cells can’t distinguish between NO2- and Cl-
thus: nitrite absorption regulated by nitrite:chloride ratio in medium

58. Nitrite (NO2-) Toxicity
Nitrite is about 55 times more toxic in freshwater vs. 16 ppt seawater
Question: Can you add NaCl to water to reverse nitrite toxicity?
24 hr LC50 values vary tremendously in fish
safe bet: authors say 4.5 mg/L

59. Nitrite (NO3-) Toxicity

60. Nitrate (NO3-) Toxicity
Nitrate builds up in ponds, like nitrite, when ponds are cooler
Nitrobacter does not function well under cool or cold water conditions
however, nitrates are least toxic form of soluble nitrogen
effects are similar to nitrite toxicity, but values of levels are much higher

61. Nitrate Toxicity