Trophic State

Trophic status is used by limnologists to refer to the overall productivity of a lake. Lake productivity is influenced by nutrient supply, light availability, 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, and 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 as hypoxic (low oxygen) or anoxic (no oxygen) and is not conducive to supporting cold water fish (Wetzel 2001).

Total Phosphorus

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 which is, in turn, 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 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 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 off old growth forest have higher phosphorus concentrations than those flowing off 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 of 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).

Quick Interpretation of Total Phosphorus

Total Phosphorus (µg/L): Trophic Status

<10: Oligotrophic

10 - 20: Mesotrophic

>20: Eutrophic

Chlorophyll-a

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).

Quick Interpretation of Chlorophyll-a

Secchi (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 related to lake productivity, such as total phosphorus and chlorophyll-a, 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.

Quick Interpretation of Secchi

Nitrogen (Nitrate-Nitrite, Ammonia, Total Nitrogen)

Nitrogen is present in many forms in the atmosphere, hydrosphere, and biosphere, and 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 water of oxygen. Generally, nitrogen is not a limiting nutrient in aquatic ecosystems (Schindler 1977).

Nitrogen to Phosphorus Ratio

As the two primary nutrients in aquatic ecosystems, the ratio of nitrogen to phosphorus can influence nutrient limitation and which phytoplankton species are dominant. Increasing occurrence of harmful algal blooms has renewed interest in lake nutrient cycling and how that relates to the occurrence of toxic blooms. The importance of TN:TP to cyanobacterial blooms is debated, but there is evidence that low TN:TP mass ratios favor both nitrogen fixing and non-nitrogen fixing cyanobacteria (Smith 1983). A TN:TP mass ratio of 22:1 appears to be a threshold under which lakes are more likely to be dominated by N-fixing cyanobacteria (Smith et al. 1985). Laboratory experiments have shown that the non-nitrogen fixing Microcystis dominates below ratios of 44:1 (Fujimoto & Sudo 1997). While TN:TP ratios may be an important driver of cyanobacterial blooms, it is important to recognize that other factors are important as well, such as temperature, salinity, NO3:NH4 mass ratios, and pH (Liu et al. 2011).

Quick Interpretation of TN:TP Ratio

TN:TP: Status

<22: Higher risk of cyanobacteria blooms

Conductivity

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.

Chloride & Sodium

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. The practice of using rock salt for road deicing purposes began in New Hampshire in 1938 and the use of rock salt has been on an increasing trend nationally since then. 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. The widespread use of rock salt in the Adirondacks has resulted in long-term increases in these ions in Adirondack lakes (Kelting et al. 2012). Emerging research on the impact of elevated chloride concentrations has on lake similar to those in the Adirondacks have identified several important thresholds for the protection of aquatic life (Hintz et al. 2022). In extreme cases, salt runoff can accumulate in the bottom waters of lakes, interrupting the natural mixing regime (Wiltse et al. 2019).

Quick Interpretation of Chloride

pH

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 showed 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).

Quick Interpretation of pH

Alkalinity

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.

Quick Interpretation of Alkalinity

Sulfate

Sulfate is an essential component of lake chemistry as it plays a significant role in various biogeochemical processes that occur within aquatic ecosystems. Sulfate is present in rainwater and enters lakes through atmospheric deposition, and it can also be released from bedrock weathering and human activities such as mining and industrial processes. Sulfate is an electron acceptor in microbial sulfate reduction, which is a critical process in the breakdown of organic matter and the cycling of carbon, sulfur, and nitrogen. Additionally, sulfate can influence the acidity of lakes by forming sulfuric acid through chemical reactions, which can have detrimental effects on aquatic life. Therefore, understanding the sources and dynamics of sulfate in lakes is crucial for the management and conservation of freshwater resources (Wetzel 2001).

Color (Apparent and True)

Color is an optical property of water that results from light scattering after absorption of water molecules, dissolved materials, and suspended materials. Blue-green wavelengths are often scattered in alkaline lakes giving them a turquoise appearance, whereas lakes rich in dissolved organic matter scatter longer wavelengths (red and yellow), making them appear brown in color.

The quantification of apparent color in water is done through comparison with standards of a platinum-cobalt solutions via spectroscopy. True color is the color of water after removal of suspended material and apparent color is the color of water without filtration. Color can be used to provide information about the quantify of dissolved organic mater (DOM) in water. Though, caution should be used when using color as a surrogate of DOM because it can behave differently, making it a crude predictor of DOM (Dillon and Molot 1997).

Dissolved Organic Carbon (DOC) & Colored Dissolved Organic Matter (CDOM)

Dissolved organic carbon (DOC) and Colored dissolved organic matter (CDOM) are closely related in aquatic systems. CDOM is a subset of DOC that absorbs light in the visible range and imparts a yellow or brown color to water. Both CDOM and DOC are derived from the decomposition and breakdown of organic matter in the lake, including dead plants and animals (autochthonous), as well as organic matter delivered to the lake from the surrounding watershed (allochthonous).

CDOM and DOC levels are influenced by a variety of factors, including inputs of organic matter, water residence time, and the presence of microbial and photochemical processes that can break down or transform organic matter. In general, CDOM levels are positively correlated with DOC levels, but the relationship can be complex and varies depending on the source and composition of the organic matter. For example, some types of organic matter, such as algal-derived organic matter, may contribute more to CDOM than to DOC.

CDOM and DOC have important implications for the health and functioning of lake ecosystems. High levels of CDOM can reduce light penetration into the water column, which can limit photosynthesis and primary productivity. CDOM can also absorb ultraviolet radiation, which can be harmful to aquatic organisms. DOC is an important source of energy and nutrients for microbial communities in the lake, and its availability can influence the structure and function of the food web. Understanding the dynamics of CDOM and DOC in lakes is therefore critical for managing and preserving the health of these ecosystems.

Many lakes in the Adirondacks are experiencing increasing DOC, this is thought to be primarily driven by recovery from acid deposition, but may also be a result of climate change (Driscoll et al. 2016). DOC solubility is decreased in soils that are acidic and have a high ionic strength. Therefore, a recovery from acid deposition that increases soil pH will increase DOC solubility. Climate change may also play an important role in increasing DOC. Warmer temperatures accelerate the breakdown of organic material and increased precipitation increases the leaching of DOC from forest soils. Because of the important role DOC plays in attenuating light, increasing DOC in lakes may help cold water fish species by limiting the warming of deeper waters.

Calcium

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 soils, 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).

Quick Interpretation of Calcium

References

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