Transcript Water Quality - Cloudfront.net

Water Quality
Water Quality Parameters of Interest to
Aquaculture Include:
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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
Water Quality
Salinity
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Aquatic ecosystem classification (by water salt concentration)
freshwater:
< 0.5 mg/L (ppt)
estuarine (brackish) water:
0.5-30 ppt
seawater:
33-37 ppt (average, 35 ppt)
99%
• 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)
Water Quality
Dissolved oxygen (DO)
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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 > 100%) can occur in surface waters
due to high photosynthetic activity during long summer days
Water Quality
Dissolved CO2, pH, alkalinity, hardness
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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+
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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 OHCO32- + H2O = HCO3- + OHHCO3- + H2O = H2CO3 + OH-
Water Quality
Dissolved CO2, pH, alkalinity, hardness (continued)
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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
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< 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
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soft water with low alkalinity has poor buffering capacity and pH tends to
fluctuate quickly and widely – not good for fish culture
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natural freshwaters greater than 40 mg/L total hardness are more productive for
aquaculture
USGS Hardness Definitions
• Soft:
• Moderately hard:
• Hard:
• Very hard:
exception
<60 mg/L
61-120 mg/L
121-180 mg/L
>180 mg/L
Water Quality
Solids (dissolved + particulate; organic + inorganic)
and turbidity
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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 550°C (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 550°C 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.
Water Quality
Dissolved and particulate organic matter
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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 20°C 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.
Water Quality
Nitrogen (N) compounds
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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-
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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)
Units of expression are in mg of elemental N (not nitrogenous
compound) per liter of water.
Water Quality
N-cycle bacteria
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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 NO3Because nitrifying bacteria require oxygen to function, their presence is restricted
to the surface layer of sediment (or artificial biological filters). DO levels > 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 30-36°C.
Denitrification (reverse reaction) can be enhanced in low-oxygen (< 0.2 mg/L) or
anaerobic conditions. Temperature optimum for denitrification bacteria is high,
65-75°C.
Water Quality
Phosphorous (P) compounds
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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.
Water Quality
Sediment quality
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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.
Water Quality
Sediment Quality (continued)
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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 < -200 mV) and methane (at Eh < -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
Water Quality
Water quality criteria
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General optimal ranges.
Patterns of effects.
Water Quality
Temperature effects
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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 10°C.
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
Water Quality
Temperature effects: temperature tolerance
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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.
Water Quality
Salinity effects
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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.
Water Quality
Oxygen effects
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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.
Water Quality
pH effects
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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 HCO3excretion, 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.
Water Quality
CO2 effects
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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.
Water Quality
Nitrogenous waste effects
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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
Water Quality
Ammonia (NH3/NH4+) effects
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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-
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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:”
Water Quality
Nitrite (NO2-) effects
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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)
Water Quality
Nitrate (NO3-) effects
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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.
Water Quality
Hydrogen sulfide (H2S) effects
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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.
Water Quality
Methane effects
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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.