ALKALINITY AND HARDNESS 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. 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 CO2 --> 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 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.... or minimally...clogged pipes! CONVERSION OF WATER HARDNESS UNITS Water Hardness Internatio Physical America & English oe Unit nal measures states Definition recomme mval/liter PPM nded mmol/liter French of German odH 100mg CaCO3 28 mg CaO or 1 part CaCO3 1 grainCaCO3 10 mg/ CaCO3 10 mg/CaO per 1000 ml 50 mg CaCO3 per million = 1 per gallon= per 1000 ml per 1000 ml water per 1000 ml mg CaCO3 14.3 mg water water water per 1000 ml CaCO3 per water 1000 ml water 1 mmol/litre 1 2 100 7.0 10.00 5.6 1 mval/litre 0.5 1 50 3.5 5.00 2.8 1 PPM 0.01 0.02 1 0.070 0.10 0.056 1 Oe 0.1429 0.285 14.29 1 1.429 0.7999 1 Of 0.10 0.20 10.00 0.700 1 0.5599 1 OdH 0.1786 0.357 17.86 1.250 1.786 1 DEGREE OF HARDNESS EXPRESION SOFT: < 1.6 mmol/l = 160 PPM = 9 odH SLIGHTLY HARD: 1.6-3.2 mmol/l = 160-320 PPM = 9-18 odH HARD: 3.2-4.6 mmol/l = 320-460 PPM = 18-26 odH VERY HARD: ABOVE 4.6 mmol/l = ABOVE 460 PPM = ABOVE 26 odH 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- 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). 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). 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. 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. CO2 Level Affects Hemoglobin Saturation Chemical Variables: dioxide carbon 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? The Effect of pH on Carbonate Buffering Chemical Variables: 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 carbon 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 15-20 mg/L. Diel fluctuation opposite to that of D.O. Higher levels in warmer months of year.