Water Quality

1
Water Quality (Chapter 3)

• Water Quality Parameters of Interest to
  Aquaculture Include
• Salinity
• Dissolved oxygen
• CO2, pH, alkalinity, hardness
• Dissolved and particulate organic matter
• Total solids, suspended inorganic particles, and
  turbidity
• Nitrogen
• Phosphorous
• Sediment quality (especially Redox Potential)
• Temperature

2
Water Quality

• Salinity
• Aquatic ecosystem classification (by water salt
  concentration)
• freshwater lt 0.5 mg/L (ppt)
• estuarine (brackish) water 0.5-30 ppt
• seawater 33-37 ppt (average, 35 ppt)

• There are about 61 elements in SW (for more
  information see Table 3.2 in textbook)
• Phosphorous and nitrogen are important elements
  that vary considerably in concentration due to
  their association with biological processes (more
  about this later)

3
Water Quality

• Dissolved oxygen (DO)
• DO derives from atmosphere or oxygen-producing
  biological processes (e.g., photosynthesis)
• DO level in water reflects balance between oxygen
  available and oxygen consumed (e.g., by aerobic
  respiration)
• DO is inversely related to temperature and
  salinity, and directly related to partial
  pressure across the water surface
• Percent DO saturation (DO) is independent of
  temperature and salinity
• DO levels range 0-14 mg/L in water and 210,000
  mg/L in air
• DO levels typically are higher on the surface and
  decrease with depth (mixing, wind action,
  diffusion serve to provide DO below water
  surface)
• High nutrient concentrations in eutrophic waters
  promote algal growth, which consume oxygen at
  night causing low DO levels
• Low DO levels also occur in winter at high
  latitudes due to decay of organic matter under
  ice cover
• Oxygen super-saturation (DO gt 100) can occur in
  surface waters due to high photosynthetic
  activity during long summer days

4
Water Quality

• Dissolved CO2, pH, alkalinity, hardness
• All four parameters are interrelated
• Air is source of CO2 (180-300 ppm by volume
  before industrial revolution 380 ppm at present)
• Aerobic plant and animal respiration also
  produces CO2
• CO2 is more soluble in water than O2. In
  seawater, dissolved CO2 levels range from 67 to
  111 mg/L
• CO2 influences the carbonate system in water as
  follows
• Carbon dioxide dissolves in water and produces
  carbonic acid
• CO2 H2O H2CO3
• Carbonic acid dissociates producing H
• H2CO3 HCO3- H HCO3- CO32- H
• Increased H can lower the pH of water (normally
  7.5-8.4 in seawater and 6.0-8.5 in freshwater)
• The ability of water to absorb H ions (anions)
  without a change in pH is known as its
  alkalinity.
• In freshwater, alkalinity typically is due to the
  presence of excess carbonate anion (from the
  weathering of silicate or carbonate rocks) that
  when hydrolyzed produces OH- (and neutralizes H)
  as follows
• Hydrolysis of carbonate and carbonate produces
  OH-
• CO32- H2O HCO3- OH-
• HCO3- H2O H2CO3 OH-

5
Water Quality

• Dissolved CO2, pH, alkalinity, hardness
  (continued)
• Alkalinity and hardness are generally associated,
  but not always (Table 3.4)
• Total hardness is primarily the total
  concentration of metal ions (cations) in water
  (mg/L), which includes mainly Ca2 and Mg2
• Anions of alkalinity (CO3-) and cations of
  hardness (e.g., Ca2) are normally derived from
  the same carbonate minerals and this is the
  reason for the observed general association
  between alkalinity and hardness
• CaCO3 concentrations in water generally increase
  with salinity
• lt 20 mg/L total hardness is generally not good
  for fish or shellfish culture (Ca is needed for
  skeletal and exoskeletal growth). SW 6600 mg/L
• soft water with low alkalinity has poor buffering
  capacity and pH tends to fluctuate quickly and
  widely not good for fish culture
• natural freshwaters greater than 40 mg/L total
  hardness are more productive for aquaculture

• USGS Hardness Definitions
• Soft lt60 mg/L
• Moderately hard 61-120 mg/L
• Hard 121-180 mg/L
• Very hard gt180 mg/L

6
Water Quality

• Dissolved CO2, pH, alkalinity, hardness
  (continued)
• Non-graded Quiz 2

7
Answers to Quiz
Salinity in the upper Brazos River gt 1 ppt
(brackish water)
8
Water Quality

• Solids (dissolved particulate organic
  inorganic) and turbidity
• Total solids include organic and inorganic
  matter. Total solid concentration is the weight
  of the residue left after water is evaporated to
  dryness (mg/L), and includes dissolved and
  particulate matter (with the exception of gases).
• If residue is ignited at 550C (usually for 2
  hours) and reweighed
• the weight loss (Loss-on-Ignition, LOI)
  represents total volatile solids, a measure of
  dissolved and particulate organic matter and
• the weight left represents dissolved and
  particulate inorganic particles.
• Dissolved and particulate solids can be separated
  and measured by filtration using 0.5-1 micron
  filters, evaporating the filtrate (dissolved
  solids) and drying the filters (particulate
  solids), and weighing the fractions and LOI at
  550C allows estimation of organic and inorganic
  matter in each fraction.
• High levels of particulate (suspended) solids are
  associated with increased turbidity and can be
  also estimated with the use of Secchi disks or
  spectrophotometers.

9
Water Quality

• Dissolved and particulate organic matter
• Organic substances derive from animal and plant
  metabolic waste, dead biota, natural seepage, and
  human waste.
• Photosynthetic activity incorporates carbon into
  plants, which is released during plant growth,
  during periods of stress, and after plant death.
• Although materials other than carbon-based
  substances are also released into the environment
  by living organisms, dissolved organic carbon
  (DOC) can be used as estimate of dissolved
  organic matter (DOM).
• DOC can be converted into particulate organic
  carbon (POC).
• POC includes living particles (phytoplankton,
  bacteria), non-living matter (detritus), and
  suspended carbon-based particles larger than
  0.5-1 micron in diameter.
• Detrital POC often exceeds living POC, but
  overall POC generally is only a fraction of DOC.
• Chemical oxygen demand (COD amount of oxidizing
  agent that can be reduced) or biological oxygen
  demand (BOD e.g., oxygen depletion over 5 days
  at 20C in the dark) can be used to estimate the
  amount of DOC. UV absorbance at 254 nm can also
  be used. DOC estimates based on BOD and UV
  absorbance are both used in aquaculture.

10
Water Quality

• Nitrogen (N) compounds
• Nitrogen forms found in water
• dissolved gaseous N2
• dissolved free (unionized) ammonia (NH3)
• ionized ammonia (NH4)
• nitrite ion (NO2-)
• nitrate ion (NO3-)
• variety of organic nitrogen in living and
  non-living materials
• Total ammonia is the combined amount of free
  (NH3) and ionized (NH4) ammonia. Sources of
  ammonia in aquaculture include mineralization of
  organic nitrogen (more in a minute) and fish
  metabolic waste derived from protein degradation
• In water, free and ionized ammonia are in
  equilibrium according to the following equation
• NH3 H2O NH4 OH-
• Increasing water temperature or pH, or decreasing
  salinity will shift the equilibrium to higher
  levels of the highly toxic form of ammonia,
  unionized ammonia (see Table 3.5)

11
Units of expression are in mg of elemental N (not
nitrogenous compound) per liter of water.
12
Water Quality

• N-cycle bacteria
• N-cycle bacteria metabolize N compounds and are
  endemic to water and surfaces that come in
  contact with water, especially sediment.
• Heterotrophic N-cycle bacteria (e.g., Bacillus
  pasteurii) mineralize organic N (e.g., urea) into
  inorganic N (e.g., ammonia) these bacteria are
  typically facultative anaerobes.
• Autotrophic N-cycle bacteria (nitrifying
  bacteria) are strictly aerobic and oxidize
  inorganic N in a two step process
• Nitrosomonas species (or Nitrosocystis oceanus,
  marine bacterium) oxidize NH4 to NO2-
• Nitrobacter species oxidize NO2- to NO3-
• Because nitrifying bacteria require oxygen to
  function, their presence is restricted to the
  surface layer of sediment (or artificial
  biological filters). DO levels gt 0.6 mg/L are
  typically required for proper bacterial function.
• Nitrobacter are sensitive to high ammonia or
  nitrate concentrations under these conditions,
  nitrite is not metabolized and will accumulate.
• The optimal pH range for both types of nitrifying
  bacteria is 8.5-8.8, but they can also adapt to
  lower pH values. Optimum temperature is 30-36C.
• Denitrification (reverse reaction) can be
  enhanced in low-oxygen (lt 0.2 mg/L) or anaerobic
  conditions. Temperature optimum for
  denitrification bacteria is high, 65-75C.

13
Water Quality

• Phosphorous (P) compounds
• Primary productivity of most surface freshwaters
  is typically limited by P, not N. Common N/P
  ratios in water are 10/1.
• The rate of P supply is considered more important
  to determine primary productivity than is its
  actual concentration.
• P is mainly found in water as soluble mineral
  phosphate (H2PO4-, HPO42-, PO43-), but in fish
  ponds it may also be found as soluble organic P
  and particulate P. Organic P can be mineralized
  into soluble mineral phosphate by bacteria.
• N and P compounds in water are important in the
  extensive culture of herbivores such as mullet
  and milkfish, because they support the growth of
  phytoplankton and blue-green algae.

14
Water Quality

• Sediment quality
• Levels of ammonia nitrogen, Redox potential, pH,
  hydrogen sulfide potential are commonly used
  indices of sediment quality.
• Sediment quality is an important consideration
  for aquaculture, as follows
• In intensive and semi-intensive aquaculture
  operations, organic matter (uneaten food, waste,
  other debris) accumulates on the bottom of ponds
  creating a nutrient-rich sediment.
• Most aquatic bacteria are heterotrophs (they
  mineralize organic N into ammonia) and their
  numbers are determined by the amount of organic
  matter, so that enrichment of sediment with
  organic matter selectively promotes the growth of
  heterotrophic bacteria and the production of
  inorganic N (ammonia).
• Under aerobic conditions, ammonia is oxidized by
  the nitrifying (autotrophic) bacteria into
  nitrite and then nitrate.
• However, under anaerobic conditions ammonia as
  well as other compounds such as hydrogen sulfide
  and methane cannot be oxidized consequently,
  these compounds accumulate in sediment and will
  diffuse into the overlying water. These
  conditions are suboptimal for aquaculture (more
  about this later).
• Poor sediment quality often precedes poor water
  quality and it is thus important to monitor
  sediment quality in aquaculture operations.

15
Water Quality

• Sediment Quality (continued)
• The large microbial populations found in
  organically enriched sediments have a high demand
  for O2, which can create anaerobic ( reducing)
  conditions.
• Highly reducing sediment is indicated by a
  negative Redox potential (Eh value).
• In addition to ammonia, sulfide (at Eh lt -200 mV)
  and methane (at Eh lt -250 mV) are produced under
  anaerobic conditions.
• As example, sediment Eh has been determined in
  shrimp ponds (Fig 3.4) down to 20 cm depth
• (a) well-oxidized (aerobic) sediment
• (b) good sediment surface
• oxidation with reduced
• conditions below 5 cm
• (c) poorly oxidized sediment

16
Water Quality

• Water quality criteria
• General optimal ranges.
• Patterns of effects.

17
Water Quality

• Temperature effects
• Most cultured aquatic organisms are ectothermic
  and are unable to control body temperature other
  than by behavior (by temperature selection).
• Metabolic rate increases 2- or 3-fold for every
  increase of 10C.
• Increased metabolic rate leads to higher oxygen
  consumption and waste production (CO2, ammonia).
  
• Aquaculture considerations
• feeding regime must be appropriately adjusted to
  the water temperature
• know that grow-out period will be affected by
  environmental temperatures
• need to avoid abrupt temperature changes
• to minimize stress while transporting fish, it
  may be advisable to reduce the water temperature
  thus reducing fish activity and toxic waste
  accumulation
• cultured species must be carefully selected to
  match their temperature requirements to the
  regional environmental temperatures

18
Water Quality

• Temperature effects temperature tolerance
• Temperature tolerance is influenced by past
  thermal history.
• Acclimation to higher temperatures usually occurs
  faster than acclimation to lower temperatures.
• For most species, the preferred temperature is
  several degrees higher than the optimum
  temperature for growth rate in the presence of
  excess feed.

19
Water Quality

• Salinity effects
• In fishes, ion concentrations in body fluid are
  not the same as those in water animals in FW are
  hypertonic to their environment and those in SW
  are hypotonic.
• In seawater organisms such as molluscs, their
  body fluid osmotic pressure conforms to the
  environment in the high salinity range
• Terms to remember
• Osmoconformers/osmoregulators
• Ionoconformers/ionoregulators
• Stenohaline/euryhaline
• Anadromous/catadromous/diadromous
• General aquaculture considerations
• Water salinity influences metabolic rates. Thus,
  feeding must be adjusted according to salinity.
• Salinity requirements may vary with development.
• Because of their greater tolerance to salinity
  variations, most aquacultural species are
  euryhaline.

20
Water Quality

• Oxygen effects
• Aquatic organisms have very efficient respiratory
  systems oxygen concentration in water (by
  volume) is 0.005 of its concentration in air.
• Some species can switch to anaerobic metabolism
  when DO levels are low (some bivalve molluscs).
• But generally, growth and activity can be
  considerably influenced by DO levels.
• General aquaculture considerations
• DO supply is an important water quality parameter
  to be considered in the selection of farm sites
  (availability of electricity to run mechanical
  aerators).
• A useful rule of thumb to keep in mind is that a
  DO level of 5 mg/L is adequate for most fish
  species provided that other water quality
  conditions are favorable. Shellfish generally
  can do with lower levels (e.g., 3 mg/L).
• Some fish species (e.g., gar) are able to breathe
  air using, for example, a modified swim bladder
  and thus can tolerate near anoxic aquatic
  environments.

21
Water Quality

• pH effects
• As previously mentioned, pH is a measure of the
  H concentration in fluids
• high H low pH
• low H high pH
• Alterations in the blood pH of fishes can be
  corrected by the exchange of ions between their
  internal (blood) and external (water)
  environments. The most important site of ion
  transfer are the gills.
• This ion exchange requires external Cl- for
  internal HCO3-, and external Na for internal H.
• Blood acidosis (low pH) is corrected by reducing
  the uptake of Cl- by the gills and to some extent
  increasing uptake of Na. The reduction in Cl-
  uptake thus reduces HCO3- excretion, and the
  increase in Na uptake increases the excretion of
  H. The net effect is a compensatory increase an
  return to normal blood pH.
• However, the ionic content of water can affect
  ionic transfers across the gills. Of most
  importance is the availability of the appropriate
  counter-ions for exchange Cl- and Na.
• Also, high water H content (low pH) limits the
  ability of the organism to excrete H and thus
  maintain adequate internal pH levels.
• Water pH of 6-9 is adequate for most freshwater
  fishes and 6.5-8.5 for marine fishes. Levels of 4
  and lower or 9.5 and higher are typically lethal.
• Water pH also affects the toxicity of ammonia and
  other toxic compounds.
• The presence of certain metals (e.g., iron) can
  decrease tolerance for low pH waters.

22
Water Quality

• CO2 effects
• In poorly buffered waters (soft water), small
  amounts of CO2 released from phytoplankton
  metabolism at night can cause considerable
  changes in water pH.
• In addition, discharge of acidic compounds into
  water with high carbonate alkalinity will cause
  the production of high levels of dissolved CO2
  without significant changes in pH. These high
  levels of CO2 can have direct toxic effects in
  fishes. For example, a correlation between high
  water levels of CO2 and nephrocalcinosis
  (calcium-based kidney stones) has been shown in
  trout farms.
• The degree to which CO2 will affect organisms
  depends on its concentration and the length of
  exposure.

23
Water Quality

• Nitrogenous waste effects
• Inorganic nitrogenous compounds of most relevance
  to aquaculture include
• Ammonia
• Nitrite
• Nitrate
• The main nitrogenous waste generated by teleost
  fishes and shellfish is ammonia. This is an
  important source of inorganic N in intensive
  aquacultural operations the other source is
  mineralization of organic N (waste) by
  heterotrophic bacteria discussed earlier.
  Ammonia is excreted primarily via the gills.
• Ammonia production is directly proportional to
  water temperature and feeding rate, and inversely
  proportional to fish size, stocking density and
  water flow.
• Ammonia is converted to nitrite and nitrate by
  nitrifying bacteria

24
Water Quality

• Ammonia (NH3/NH4) effects
• Accumulation of ammonia in water is a major cause
  of physiological impairments in aquatic animals.
• Total ammonia is the sum of unionized and ionized
  ammonia
• NH3 H2O NH4 OH-
• The relative proportions of unionized and ionized
  ammonia in water are affected by pH, temperature,
  salinity, etc (discussed earlier, see Table 3.5).
  
• Effects of ammonia in fishes
• Unionized ammonia (NH3) is the toxic form of
  ammonia in fishes.
• Causes external irritations of gills, eyes, fins
  (ammonia burns).
• Unionized ammonia can also diffuse across the
  gill and cell membranes causing internal damage
  to the fish. High levels of unionized ammonia
  impairs osmoregulation, affect the oxygen
  carrying capacity of blood, and have other direct
  toxic effects on internal organs such as the
  liver.
• Recommended unionized ammonia limit for intensive
  fish culture systems is less than 0.02 mg/L
  (Wedemeyer 1996).
• Cycling of a fish aquarium and the new tank
  syndrome

25
Water Quality

• Nitrite (NO2-) effects
• Nitrite is actively taken up by the gills (by the
  chloride cells) its uptake mechanism is so
  effective that blood concentrations 10-70 times
  higher than in water have been recorded.
• Nitrite is considered highly toxic to fishes. It
  combines with hemoglobin to form methemoglobin,
  which is unable to bind oxygen. Fish blood
  normally contains some methemoglobin (up to 10),
  but nitrite can increase the levels to the point
  that respiratory impairments occur.
• Water temperature influences nitrite toxicity
  (higher temperatures higher toxicity).
• Water salinity influences nitrite toxicity
  (higher salinities lower toxicity).
• Water hardness influences nitrite toxicity
  (higher hardness lower toxicity).
• Rule of thumb keep levels below 0.02 mg/L for
  most freshwater fish (0.01 mg/L for salmonids)
  although higher levels can be tolerated by marine
  fish (up to 1 mg/L)

26
Water Quality

• Nitrate (NO3-) effects
• Nitrate is the end product of the nitrification
  process.
• In recirculating culture systems nitrate will
  accumulate with time unless a denitrification or
  plant filter is installed or water is
  periodically replaced (the latter can be labor
  and cost prohibitive).
• Nitrate is not considered acutely toxic to
  fishes for example, catfish and largemouth bass
  appear to tolerate levels as high as 400 mg/L.
• However, the chronic effects of nitrate have not
  well characterized and long term effects on
  performance cannot be ruled out. In particular,
  nitrate can potentially be denitrified by the
  intestinal flora into toxic nitrite or ammonia.
  (The European standard for nitrate levels in
  drinking water is 50 mg/L. The World Health
  Organization guideline for drinking water is 10
  mg/L.)
• In any case, nitrate accumulation in fish tanks
  or ponds can lead to algal blooms as well as
  inhibition of the second step of nitrification
  and consequent accumulation of nitrite, which is
  toxic. Thus, management of nitrate levels is
  also important for aquacultural operations.

27
Water Quality

• Hydrogen sulfide (H2S) effects
• Sulfide is water soluble and is toxic to marine
  and freshwater organisms. It is produced in
  sediment under anaerobic conditions (negative
  redox potential values). Its effects include
  damage to the gills and even mortality.
• Sulfide production is typically 10-fold lower in
  freshwater than in seawater. Under aerobic
  conditions H2S is readily transformed into
  non-toxic SO42- ions.

28
Water Quality

• Methane effects
• We have already mentioned that methane gas is
  normally produced by sediment microbes under
  reducing conditions in conjunction with sulfide
  production. Natural seepage can also occur from
  shallow oil and gas-bearing structures.
• Natural processes of production and distribution
  of methane are under the increasing influence of
  anthropogenic activities. Salmon net-pen farming
  in coastal waters has been associated with
  significant production of methane by sediment.
• There is little or no information about the
  toxicity of methane to fishes. The primary
  concern is rather with its usual partner in
  production, hydrogen sulfide, which is toxic.