Due to the dissolved humic substances from runoff and uptake, “blackwater” sources can have a pH as low as 4.43. The acid lakes at Dallol in Ethiopia are the result of acid leaching from nearby volcanoes. pH stands for “potential for Hydrogen”. Photosynthesis, respiration and decomposition all contribute to pH fluctuations due to their influences on CO2 levels. While ideal pH levels for fish are 7-8 (fish blood has a pH of 7.4) ²⁰, most fish can adapt to the pH level of their environment (6.0-9.0) as long as there are no dramatic fluctuations. This fish thrives in the acidic waters of Lake Osorezan, resting comfortably at a pH of 3.5, and swims into neutral pH waters only to spawn ³⁴. In non-volcanic areas, acid lakes can also develop after acidic deposition from events such as acid rain, pollution or acid runoff from mining operations ³³. This is done by taking the negative logarithm of the H+ concentration (-log(H+)). In other words, alkalinity is a measurement of water’s ability to resist changes in pH. These chemicals can come from agricultural runoff, wastewater discharge or industrial runoff. However, because the pH scale is logarithmic, attempting to average two pH values would be mathematically incorrect. The recommended pH range for most fish is between 6.0 and 9.0 with a minimum alkalinity of 20 mg/L, with ideal CaCO3 levels between 75 and 200 mg/L ²⁰. Low pH can reduce how many fish eggs successfully hatch, and make life difficult for fish and macroinvertebrates (the small creatures that are the backbone of our water ecosystems). These minerals often form columns of mineral deposits, known as tufa columns. Freshwater lakes, ponds and streams usually have a pH of 6-8 depending on the surrounding soil and bedrock ²¹. Most natural changes occur due to interactions with surrounding rock (particularly carbonate forms) and other materials. Amphibians are very sensitive to low pH (their skin is sensitive). pH can also affect the solubility and toxicity of chemicals and heavy metals in the water ¹². pH sets up the conditions for how easy it is for nutrients to be available and how easily things like heavy metals (toxicity for aquatic life) can dissolve in the water. The acidity of the surrounding environment can also affect the pH of water. The above equations also explain why rain has a pH of approximately 5.65 ¹⁵. These emissions usually come from mining and smelting operations or fossil fuel combustion (coal burning and automobiles) ¹⁸. This significant drop comes from the saturated CO2 that is stored up in the lower strata of the lake. In eutrophic lakes, pH-tolerant algae can dominate, driving the pH levels to diurnal high and low extremes, forming algae blooms that can kill the lake ¹⁶. pH can also fluctuate with precipitation (especially acid rain) and wastewater or mining discharges ¹³. Likewise, respiration and decomposition can lower pH levels. Ocean acidification is caused by an influx of dissolved carbon dioxide. In general, fish reproduction is affected at pH levels below 5.0 and many species (such as saltwater fish or sensitive freshwater fish like smallmouth bass) will leave the area ²¹. Uncertainty in the indicator apparent dissociation constant limits the accuracy of the pH measurement to ∼0.05 pH units. If an average value is required, it can be reported as a median or a range, not as a simple calculation ¹⁰. With more accessible nutrients, aquatic plants and algae thrive, increasing the demand for dissolved oxygen. The numerical value of pH is determined by the molar concentration of hydrogen ions (H+) ³. Even minor pH changes can have long-term effects. Some states, such as Alaska, are attempting to maintain a pH standard for water quality. In addition, CO2 concentrations can influence pH levels. However, insoluble bases (such as copper oxide) should only be described as basic, not alkaline. However, there are some substances that fit the operational definition (altering pH), without fitting the Arrhenius definition (releasing an ion). As the concentrations of heavy metals increase, their toxicity also increases. In general, the higher the alkalinity, the higher the pH; the lower the alkalinity, the lower the pH ⁶. Chemoclines can be based on oxygen, salinity, or other chemical factors that do not cross the cline, such as carbon dioxide. 1 mg/L alkalinity as CaCO3 = 0.01998 meg/L alkalinity 1 mg/L alkalinity as CaCO3 = 0.5995 mg/L alkalinity as CO32- 1 mg/L alkalinity as CaCO3 = 1.2192 mg/L alkalinity as HCO3–. For example if the pH falls below 5 or over 10 (this can be caused by algae blooms) you will start to see fish life spans and reproduction affected. Intense photosynthesis increases the pH of water as it removes CO2, though this change is usually diurnal ²⁰. IMPORTANT! Fondriest Environmental, Inc. “pH of Water.” Fundamentals of Environmental Measurements.
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