lec 2-4_water quality

7/28/2019 Lec 2-4_Water Quality

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Water Quality(Chapter 3)

Water Quality Parameters of Interest toAquaculture 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


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Water Quality

Salinity 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 (formore 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 aboutthis later)


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Water QualityDissolved oxygen (DO)

DO derives from atmosphere or oxygen-producing biologicalprocesses (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 andsalinity

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 algalgrowth, 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


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Water QualityDissolved 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- = CO3

2- + 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

-


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Water QualityDissolved CO2, pH, alkalinity, hardness (continued)

Alkalinity and hardness are generally associated, but not always (Table 3.4) Total hardness is primarily the total concentration ofmetal 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

< 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

exception

USGS Hardness Definitions

Soft: 180 mg/L


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Water QualityDissolved CO2, pH, alkalinity, hardness (continued)

Non-graded Quiz #2


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Salinity in the upper Brazos River: > 1 ppt (brackish water)

very hard

Answers to Quiz


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Water QualitySolids (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 orspectrophotometers.


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Water QualityDissolved 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.


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Water QualityNitrogen (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)


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Units of expression are in mg of elemental N (not nitrogenous

compound) per liter of water.


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Water QualityN-cycle bacteria

N-cycle bacteria metabolize N compounds and are endemic to water andsurfaces 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 (orNitrosocystis oceanus, marine bacterium)oxidize NH4

+ to NO2-

Nitrobacterspecies 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 > 0.6

mg/L are typically required for proper bacterial function.

Nitrobacterare 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 (< 0.2 mg/L) or

anaerobic conditions. Temperature optimum for denitrification bacteria is high,

65-75C.


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Water QualityPhosphorous (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-, HPO4

2-, 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 herbivoressuch as mullet and milkfish, because they support the growth of phytoplankton

and blue-green algae.


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Water QualitySediment quality

Levels of ammonia nitrogen, Redox potential, pH, hydrogen sulfide potential arecommonly 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 organicmatter, 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.


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Water QualitySediment Quality (continued)

The large microbial populations found in organically enriched sediments have a highdemand 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


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Water QualityWater quality criteria

General optimal ranges. Patterns of effects.


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Water QualityTemperature effects

Most cultured aquatic organisms are ectothermic and are unable to control bodytemperature 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


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Water QualityTemperature 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.


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Water QualitySalinity effects

In fishes, ion concentrations in body fluid are not the same as those in water; animalsin 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.


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Oxygen effects

Aquatic organisms have very efficient respiratory systems oxygen concentration inwater (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 mechanicalaerators).

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

Rio Playa Ejido Tropical Gar Farm, Mexico


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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.

Water Quality


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Water QualityCO2 effects In poorly buffered waters (soft water), small amounts of CO

2released 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 thelength of exposure.


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Water QualityNitrogenous 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


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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:

Water Quality

Reason for new

tank syndrome


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Water QualityNitrite (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)


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Water QualityNitrate (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 bedenitrified 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.


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Water QualityHydrogen 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.


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Water QualityMethane 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.

lec 2-4_water quality

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