Water Chemistry Parameters
Alkalinity is a measure of buffering capacity of a waterbody, typically expressed as mg/L of calcium carbonate (CaCO3). The amount of calcium carbonate in a waterbody is primarily related to the bedrock geology of its watershed. Lakes with watersheds underlain by granitic bedrock tend to have low alkalinity due to slow rates of weathering of the bedrock and low amounts of calcium carbonate in the rock. Conversely, lakes underlain by sedimentary rocks such as limestone tend to both weather faster and contain more calcium carbonate. Many lakes in the Adirondacks are underlain by granitic bedrock, and therefore have lower alkalinity.
The primary source of calcium in lakes is CaCO3, thus the discussion of calcium is closely tied to that of alkalinity. CaCO3 is not very soluble in water, but in the presence of carbonic acid it is converted to more soluble forms. The primary source of calcium in lakes is from weathering of parent material. Calcium is an important element in biology because it serves a role in the structure and physiology of many organisms. In the Adirondacks, the granitic parent material contains little calcium, and therefore Adirondack lakes tend to be low in calcium. Regionally, lakes are showing calcium declines, in part because of acid deposition. Acid deposition resulted in increased calcium leaching from watershed soil, eventually reducing the pool available for export to lakes (Keller et al. 2001). Concentrations are low enough in some lakes (<2 mg/L) to cause declines in zooplankton that utilize calcium to build their carapace (Jeziorski et al. 2008).
The element chlorine can occur in various forms or states of oxidation, but the chloride form (Cl-) is most common in surface waters. There are several natural sources of sodium and chloride, including various rocks that contain sodium- and chlorine-bearing minerals. The most abundant natural mineral form of sodium and chloride is NaCl or Halite, also known as rock salt. Large halite deposits form when ocean water evaporates and mineral deposits are buried, eventually becoming rock.
Chloride is present in most natural waters at very low concentrations, except where surface or groundwater mixes with ocean water. Minimally impacted Adirondack lakes have average chloride and sodium concentrations of 0.2 mg/L and 0.5 mg/L, respectively (Kelting et al. 2012). Another source of chloride is road runoff, in regions where rock salt is used as a road deicing agent in winter. New York has one of the highest rock salt application rates per lane mile in the United States (Kelting & Laxson 2010). These application rates are mandated on state roads across the state, regardless of proximity to surface waters. Within the Village of Lake Placid, significant amounts of rock salt are applied to the sidewalk around Mirror Lake. The runoff from this area, and the adjacent roadways, goes directly into Mirror Lake.
Chloride toxicity to organisms is complex and not well understood. Toxicity standards (based on LD50 or LC50 values, the dosage or concentration lethal to 50% of the tested population) are set by state and federal agencies as the result of laboratory studies. They do not take into consideration the complex interactions that occur in natural ecosystems—effects of chronic exposure, or regional differences in sensitivity that may result from adaptations to local conditions. EPA chloride guidelines for aquatic life are 230 mg/L for chronic exposure (four-day average) and 860 mg/L for acute exposure (one-hour average) (EPA 1998). The NYS DEC Water Quality Regulation for chloride in surface waters is 250 ppm for class A, AS, and AA-S waterbodies.
Some researchers have observed negative effects from chloride levels much lower than the EPA and NYS DEC guidelines. Certain zooplankton species may be affected at concentrations as low as 5 to 30 ppm (Dalinsky et al., 2014; Palmer and Yan, 2013), while others have observed the ability of these species to evolve a tolerance to high chloride concentrations (Coldsnow et al. 2017). A study by the US Geological Survey showed very low tolerances (3.1 ppm) to chloride for brook trout (Meador and Carlisle, 2007). Research on the impact of road salt on rainbow trout development showed reduced growth rates in trout exposed to sodium chloride (9% reduced length; 27% reduced mass at 3000 mg Cl/L) and calcium chloride (5% reduced length; 16% reduced mass at 860 mg Cl/L), but not effected by magnesium chloride (Hintz & Relyea 2017). Chloride toxicity may also depend upon a variety of biotic and abiotic factors. Eurasian water milfoil, an invasive aquatic plant, has been shown to have higher tolerances to chloride than native milfoil species (Dalinsky et al., 2014). A study of chloride toxicity to zooplankton found that decreases in the quantity of food increase chloride toxicity (Brown & Yan, 2015). This means zooplankton in Mirror Lake may experience toxic effects at lower chloride concentrations than lakes with greater nutrient availability. Finally, while little is known about the impact of road salt additives to freshwater ecosystems, a recent study shows that they can alter aquatic food webs (Schuler et al. 2017).
Chlorophyll-a is the primary photosynthetic pigment in all photosynthetic organisms including algae and cyanobacteria. The concentration of chlorophyll-a is used as an index for algal biomass, or productivity. Nutrient concentrations, light, and water temperature all control algal productivity. Depending on the time of year, these three variables change and can limit algal production. Therefore, we expect to see variability in chlorophyll-a throughout the year. Major shifts in chlorophyll-a concentration over many years can usually be attributed to changes in nutrients (phosphorus, nitrogen, and silica) (Wetzel, 2001).
The primary sources of dissolved oxygen in lakes occur through diffusion from the atmosphere and primary production. Dissolved oxygen is consumed through respiration and decomposition. The solubility of oxygen in water is directly related to temperature and salinity, and decreases as both increase. Oxygen is vital to aerobic forms of life such as aquatic insects, zooplankton, and fish. Oxygen availability from the atmosphere and primary production vary throughout the year. During periods of lake turnover (spring and fall), oxygen is redistributed throughout the water column. During the summer stratified period, warm water at the surface of the lake prevents the cold bottom waters from mixing and cuts o the source of atmospheric oxygen. Similarly, during the winter months, while the lake is covered with ice, the supply of atmospheric oxygen is cut o and contributions from primary production are low.
Generally, oxygen depletion in the hypolimnion is a problem in eutrophic lakes with high rates of decomposition. It can also be prevalent in lakes that have a shallow hypolimnion and large sediment surface area to volume ratio. Reductions in hypolimnetic dissolved oxygen are of primary concern to native salmonid species. During the summer months, these fish species seek out an optimal zone in the lake, an area which is both cool (<18 °C) and well oxygenated (>7 mg/L). In lakes with low hypolimnetic dissolved oxygen, this often pushes these species into a narrow range of depths as oxygen concentrations decline throughout the summer. Many lakes are experiencing longer periods of stratification due to climate change (De Stasio et al. 1996). This is causing the cold hypolimnetic waters to be cut off from atmospheric sources of oxygen for longer periods of time and threatening the ability of cold water fish species to find both cold and well oxygenated water.
Nitrogen is present in many forms in the atmosphere, hydrosphere, and biosphere. It is the most common gas in the earth's atmosphere. The behavior of nitrogen in surface waters is strongly influenced by its vital importance to plant and animal nutrition. Nitrogen occurs in water as nitrite (NO2-) or nitrate (NO3-) anions, ammonium (NH4+) cations, or organic nitrogen. Excessive, or high levels of nitrite are an indicator of organic waste or sewage. Nitrate or ammonium may also be from a pollutant source, but, generally, are introduced at a site far removed from the sample point. This is because nitrate is stable over a range of conditions, but nitrite rapidly volatilizes in oxygenated water. Ammonium is an important nutrient for primary producers, but, at high concentrations, is a dangerous pollutant in lakes and rivers, because the bacterial conversion of NH4 to NO3 robs the water of oxygen. Generally, nitrogen is not a limiting nutrient in aquatic ecosystems (Schindler 1977).
pH is an index of the hydrogen ion activity in solution, it is defined as the logarithm of the reciprocal of the concentration of free hydrogen ions in solution. Therefore, high pH values represent lower hydrogen ion concentrations than low pH values, and there is a 10-fold difference in hydrogen ion concentration across a single pH unit. The pH scale extends from 0 to 14, with 7 being neutral. pH values below 7 indicate acidic conditions and pH values greater than 7 indicate alkaline conditions.
Acidity in Adirondack surface waters has two sources: acid deposition (rain, snow, and dry deposition) and organic acids from evergreen needles and other plant matter. Long-term monitoring by the Adirondack Lakes Survey Corporation shows that 25% of lakes in the Adirondacks have a pH of 5.0 or lower and another 25% are vulnerable to springtime acidification (ALSC, 1990).
Shifts in pH can have major effects on the dominant biological and chemical process present within a lake. Many organisms have narrow pH tolerances, resulting in significant declines in individual health and population numbers if pH values stray outside of their tolerances. Changes in pH also influence the mobility of ions and heavy metals which can result in issues related to nutrient availability and toxicity (Driscoll 1985; Schindler et al. 1985).
Phosphorus is relatively common in igneous rocks such as those found in the Adirondacks and is also abundant in sediments. The concentration of phosphorus in natural waters is low however, because of the low solubility of these inorganic forms (Wetzel 2001). Phosphorus is also a component of wastewater and this is a primary source of phosphorus in many waters. Typical concentrations of phosphorus in surface water are a few micrograms per liter. Additions of phosphorus to the aquatic environment enhance algal growth and accelerate eutrophication of waterbodies that leads to depletion of dissolved oxygen (Schindler 1977; Wetzel 2001).
Phosphorus is also added to surface waters from non-point sources such as eroding soils, stormwater runoff, runoff from fertilized fields, lawns, and gardens, and runoff from livestock areas or poorly managed manure pits. Poorly maintained or sited septic systems can also add phosphorus to surface waters. In addition, analyses of water chemistry in Adirondack upland streams shows that streams coming o old growth forest have higher phosphorus concentrations than those flowing o managed forests (Myers et. al, 2007).
Phosphorus plays an important role in biology and is an important nutrient in aquatic ecosystems. Phosphorus is often a limiting nutrient in lakes, meaning that it is a lack of phosphorus that limits aquatic primary production (Schindler 1977). Phosphorus normally enters a lake bound to soil and sediment through overland flow. In developed or urban areas, excess phosphorus can enter a lake due to application to the land as fertilizer or through poor wastewater management. This increase in phosphorus may lead to increased primary production, resulting in aesthetic changes to the lake. If the increase in primary production is large enough, there may be subsequent problems with oxygen depletion because of decomposition. The reduction in oxygen can lead to fish kills and other negative impacts (Carpenter et al. 1998).
Secchi Disk (Transparency)
Water column transparency is a simple measure of lake productivity. Generally, secchi depth is lower in highly productive eutrophic lakes and higher in less productive oligotrophic lakes. Secchi depth can also be influenced by other water quality parameters that impact clarity, such as dissolved organic carbon, total suspended solids, colloidal minerals, and water color. Therefore, it is valuable to keep other water quality parameters, such as total phosphorus and chlorophyll-a, related to lake productivity in mind when looking at changes in transparency. Changes in watershed characteristics, such as the amount of runoff from precipitation or the export of organic matter, can also influence transparency.
Conductivity—the ability of water to pass an electrical current because of the presence of dissolved ions—is often called the "watchdog" environmental test since it is informative and easy to perform. Calculations of specific conductance standardize conductivity measurements to the temperature of 25 °C for the purposes of comparison. Rain, erosion, snow melt, runoff carrying livestock waste, failing septic systems, and road salt raise conductivity because of the presence of ions such as chloride, phosphate, nitrite etc. Oil spills lower water conductivity. Temperature, shade, sunlight, and sampling depth all affect conductivity. A conductivity probe does not identify the specific ions in a water sample— it simply measures the level of total dissolved solids (TDS) in the water body.
Trophic status is used by limnologists to refer to the overall productivity of a lake. Lake productivity is influenced by the nutrient supply, regional climate, watershed characteristics, and lake morphology. The term cultural eutrophication is often used to describe the process whereby human activities increase lake productivity through an increase in the nutrient supply. This process usually results in unwanted outcomes such as declines in lake aesthetics, increase chance of harmful algal blooms, and fish kills due to elevated bacterial decomposition utilizing all the available oxygen in the water column.
Lakes can be assigned to three main classification categories based on their overall productivity; oligotrophic, mesotrophic, eutrophic. Oligotrophic lakes have the lowest productivity due to low nutrient content. These lakes are often characterized by clear, highly transparent water, with low phytoplankton biomass. The entire water column is often well oxygenated, making these lakes capable of supporting cold water fish species such as lake trout. Mesotrophic lakes are an intermediate state between oligotrophy and eutrophy. Eutrophic lakes are characterized by high productivity and high nutrient content. As a result, the water column is less clear due to increased phytoplankton production. The greater production of organic matter leads to higher rates of bacterial decomposition at the bottom of the lake. Bacteria utilize oxygen, resulting in a decrease in oxygen availability in the bottom waters during the summer stratified period. This reduction in oxygen is referred to hypoxic (low oxygen) or anoxic (no oxygen) conditions and is not conducive to supporting cold water fish.
Carlon's Trophic Status Index (TSI) can be used to identify which of these trophic classifications a lake belongs to. TSI uses phytoplankton biomass as a measure for the trophic state of the lake. It utilizes independent estimates of phytoplankton biomass using chlorophyll-a pigment concentrations, secchi depth, and total phosphorus concentrations (Carlson 1997). Values of less than 40 are common among oligotrophic lakes, 40-50 is common for mesotrophic lakes, and greater than 50 is common for eutrophic lakes.