ALKALINITY AND HARDNESS

Chemical Variables: Total Alkalinity

   

Total Alkalinity

CaCO 3.

: the total amount of titratable bases in water expressed as mg/L of equivalent “Alkalinity” is primarily composed of the following ions: CO 3 . HCO 3 . 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.

Total Alkalinity

 Total alkalinity = 200 mg/L. Good buffering capacity of a water source.

 Freshwater 5 mg/L (soft water) to 500 mg/L (hard water).

 Seawater ~ 115-120 mg/L.

 Seldom see pH problems in natural seawater.

 Alkalinity < 30 mg/L? Problem?

Total Alkalinity

Total Alkalinity (TA) level can be associated with several potential problems in aquaculture:  If TA< 50 mg/L: copper compounds are more toxic. avoid their use as algicides (copper sulfate)  Natural waters with less than 40 mg/L alkalinity as CaCO3 have limited biofiltration capacity. pH independent (What does this mean?)   Low alkalinity = low CO 2 --> low natural productivity Low alkalinity equals low pH.

Total Hardness

    

Total Hardness

: total concentration of metal ions expressed in terms of mg/L of equiva- lent CaCO 3.

Primary ions are Ca manganese.

2+ and Mg 2+ . also iron and Total Hardness approximates total alkalinity.

Calcium is used for bone and exoskeleton formation and absorbed across gills.

Soft water = molt problems. bone deformities....

or minimally...clogged pipes!

CONVERSION OF WATER HARDNESS UNITS Water Hardness Unit Definition Internatio nal recomme nded mmol/liter Physical measures mval/liter America & states PPM

1 mmol/litre

English

o

e French

o

f

100mg CaCO 3 per 1000 ml water 1 28 mg CaO or 50 mg CaCO 3 per 1000 ml water 1 part CaCO mg CaCO 3 per 1000 ml 3 per million = 1 water 1 grainCaCO 3 per gallon= 14.3 mg CaCO 3 per 1000 ml water 10 mg/ CaCO 3 per 1000 ml water 2 100 7.0

10.00

German

o

dH

10 mg/CaO per 1000 ml water 5.6

1 mval/litre 1 PPM 1 O e 1 O f 1 O dH 0.5

0.01

0.1429

0.10

0.1786

1 0.02

0.285

0.20

0.357

50 1 14.29

10.00

17.86

3.5

0.070

1 0.700

1.250

5.00

0.10

1.429

1 1.786

2.8

0.056

0.7999

0.5599

1

DEGREE OF HARDNESS EXPRESION SOFT: < 1.6 mmol/l = 160 PPM = 9 o dH SLIGHTLY HARD: 1.6-3.2 mmol/l = 160-320 PPM = 9-18 o dH HARD: 3.2-4.6 mmol/l = 320-460 PPM = 18-26 o dH VERY HARD: ABOVE 4.6 mmol/l = ABOVE 460 PPM = ABOVE 26 o dH

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 and 1x10 -7 moles OH /L).

moles of H + /L Neutral pH = similar levels of H + and OH -

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. CO 2 , fish respiration.

pH of ocean water is stable (carbonate buffering system. later).

Chemical Variables: pH

 Other sources of change:  Decay of organic matter.

 Oxidation of compounds in bottom sediments.

  Depletion of CO 2 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).

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 HCO 3 Converted to CO into medium.

2 at the gill interface. diffusion As the level of CO 2 in the medium increases. the gradient allowing diffusion is less.

Chemical Variables: Carbon Dioxide

    This causes blood CO 2 blood pH.

levels to increase. lowering 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.

CO 2 also interferes with oxygen uptake by eggs and larvae.

CO

2

Level Affects Hemoglobin Saturation

   

Chemical Variables: carbon



dioxide

In the marine environment. excesses of CO 2 are mitigated by the

carbonate buffering system.

CO 2 acid.

reacts with water to produce H 2 CO 3 . carbonic H 2 CO 3 reacts with CaCO 3 to form HCO 3 (bicarbonate) and CO 3 2 (carbonate). As CO 2 is used for photosynthesis. the reaction shifts to the left. converting bicarbonates back to CO 2.

What large-scale implications does this have?

The Effect of pH on Carbonate Buffering

Chemical Variables: carbon dioxide

   Concentrations of CO 2 small. even though it is are highly soluble in water inverse relationship between [CO 2 ] and temperature/salinity thus. CO 2 solubility depends upon many factors

Chemical Variable: Carbon Dioxide

 CO 2 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 15-20 mg/L.

 Diel fluctuation opposite to that of D.O.

 Higher levels in warmer months of year.