pH
Why Is it Important?
The pH of a sample of water is a measure of the concentration of hydrogen ions. The term pH was derived from the manner in which the hydrogen ion concentration is calculated - it is the negative logarithm of the hydrogen ion (H+) concentration. What this means to those of us who are not mathematicians is that at higher pH, there are fewer free hydrogen ions, and that a change of one pH unit reflects a tenfold change in the concentrations of the hydrogen ion. For example, there are 10 times as many hydrogen ions available at a pH of 7 than at a pH of 8. The pH scale ranges from 0 to 14. A pH of 7 is considered to be neutral. Substances with pH of less that 7 are acidic; substances with pH greater than 7 are basic.
The pH of water determines the solubility (amount that can be dissolved in the water) and biological availability (amount that can be utilized by aquatic life) of chemical constituents such as nutrients (phosphorus, nitrogen, and carbon) and heavy metals (lead, copper, cadmium, etc.). For example, in addition to affecting how much and what form of phosphorus is most abundant in the water, pH may also determine whether aquatic life can use it. In the case of heavy metals, the degree to which they are soluble determines their toxicity. Metals tend to be more toxic at lower pH because they are more soluble.
Reasons for Natural Variation
Photosynthesis uses up dissolved carbon dioxide which acts like carbonic acid (H2CO3) in water. CO2 removal, in effect, reduces the acidity of the water and so pH increases. In contrast, respiration of organic matter produces CO2, which dissolves in water as carbonic acid, thereby lowering the pH. For this reason, pH may be higher during daylight hours and during the growing season, when photosynthesis is at a maximum. Respiration and decomposition processes lower pH. Like dissolved oxygen concentrations, pH may change with depth in a lake, due again to changes in photosynthesis and other chemical reactions. There is typically a seasonal decrease in pH in the lower layers of a stratified lake because CO2 accumulates. There is no light for plants to fix CO2 and decomposition releases CO2.
Fortunately, lake water is complex; it is full of chemical "shock absorbers" that prevent major changes in pH. Small or localized changes in pH are quickly modified by various chemical reactions, so little or no change may be measured. This ability to resist change in pH is called buffering capacity. Not only does the buffering capacity control would-be localized changes in pH, it controls the overall range of pH change under natural conditions. The pH scale may go from 0 to 14, but the pH of natural waters hovers between 6.5 and 8.5.
Expected Impact of Pollution
When pollution results in higher algal and plant growth (e.g., from increased temperature or excess nutrients), pH levels may increase, as allowed by the buffering capacity of the lake. Although these small changes in pH are not likely to have a direct impact on aquatic life, they greatly influence the availability and solubility of all chemical forms in the lake and may aggravate nutrient problems. For example, a change in pH may increase the solubility of phosphorus, making it more available for plant growth and resulting in a greater long-term demand for dissolved oxygen.
Values for pH are reported in standard pH units, usually to one or two decimal places depending upon the accuracy of the equipment used.
Since pH represents the negative logarithm of a number, it is not mathematically correct to calculate simple averages or other summary statistics.
Instead, pH should be reported as a median and range of values; alternatively the values could be converted to hydrogen ion concentrations, averaged, and re-converted to pH values.
Generally, during the summer months in the upper portion of a productive or eutrophic lakes, pH will range between 7.5 and 8.5. In the bottom of the lake or in less productive lakes, pH will be lower, 6.5 to 7.5, perhaps. This is a very general statement to provide an example of the differences you might measure.
The Case of Acid Rain
An important exception to the buffering of pH changes in lakes is the case of lakes affected by acid rain. Lakes that have received too much rain with a low pH (acid rain), lose their buffering capacity. At a certain point, it takes only a small bit of rain or snowmelt runoff for the pH to change. After that point, change occurs relatively quickly. According to the EPA, a pH of 5-6 or lower has been found to be directly toxic to fish.
Turbidity
Why Is it Important?
Turbidity refers to how clear the water is.The greater the amount of total suspended solids (TSS) in the water, the murkier it appears and the higher the measured turbidity. The major source of turbidity in the open water zone of most lakes is typically phytoplankton, but closer to shore, particulates may also be clays and silts from shoreline erosion, resuspended bottom sediments (this is what turns the western arm of Lake Superior near Duluth brown on a windy day), and organic detritus from stream and/or wastewater discharges. Dredging operations, channelization, increased flow rates, floods, or even too many bottom-feeding fish (such as carp) may stir up bottom sediments and increase the cloudiness of the water.
High concentrations of particulate matter can modify light penetration, cause shallow lakes and bays to fill in faster, and smother benthic habitats - impacting both organisms and eggs. As particles of silt, clay, and other organic materials settle to the bottom, they can suffocate newly hatched larvae and fill in spaces between rocks which could have been used by aquatic organisms as habitat. Fine particulate material also can clog or damage sensitive gill structures, decrease their resistance to disease, prevent proper egg and larval development, and potentially interfere with particle feeding activities. If light penetration is reduced significantly, macrophyte growth may be decreased which would in turn impact the organisms dependent upon them for food and cover. Reduced photosynthesis can also result in a lower daytime release of oxygen into the water. Effects on phytoplankton growth are complex depending on too many factors to generalize.
Very high levels of turbidity for a short period of time may not be significant and may even be less of a problem than a lower level that persists longer. The figure below shows how aquatic organisms are generally affected.
Reasons for Natural Variation
Algal turbidity varies seasonally and with depth in a complex manner as discussed previously in response to physical, chemical and biological changes in the lake. Inorganic and detrital particles from the watershed vary largely in response to hydrological events such as storms and snowmelt.
Even relatively small amounts of wave action can erode exposed lakeshore sediments, in this case a minepit lake from northeastern Minnesota. Can you guess what mineral was mined here?
The major effect turbidity has on humans might be simply aesthetic - people don't like the look of dirty water. However, turbidity also adds real costs to the treatment of surface water supplies used for drinking water since the turbidity must be virtually eliminated for effective disinfection (usually by chlorine in a variety of forms) to occur. Particulates also provide attachment sites for heavy metals such as cadmium, mercury and lead, and many toxic organic contaminants such as PCBs, PAHs and many pesticides.
Turbidity is reported by RUSS in nephelometric units (NTUs) which refers to the type of instrument (turbidimeter or nephelometer) used for estimating light scattering from suspended particulate material. Turbidity can be measured in several ways. Turbidity is most often used to estimate the TSS (total suspended solids as [mg dry weight]/L) in the lake's tributaries rather than in the lake itself unless it is subject to large influxes of sediments. For the WOW project we will attempt to develop empirical (meaning: based upon direct measurements) relationships between TSS and turbidity for each system since turbidity is easily measured and TSS analyses are not very sensitive at the typically low concentrations found in the middle of most lakes. Also, TSS is a parameter that directly relates to land uses in the watershed and is a key parameter used for modeling efforts and for assessing the success of mitigation and restoration efforts.
What in the world are Nephelometric Turbidity Units (NTU’s)?
They are the units we use when we measure Turbidity. The term Nephelometric refers to the way the instrument estimates how light is scattered by suspended particulate material in the water. The Nephelometer, also called a turbidimeter, attached to the RUSS unit has the photocell (similar to the one on your camera or your bathroom nightlight) set at 90 degrees to the direction of the light beam to estimate scattered rather than absorbed light. This measurement generally provides a very good correlation with the concentration of particles in the water that affect clarity.
In lakes and streams, there are 3 major types of particles: algae, detritus (dead organic material), and silt (inorganic, or mineral, suspended sediment). The algae grow in the water and the detritus comes from dead algae, higher plants, zooplankton, bacteria, fungi, etc. produced within the water column, and from watershed vegetation washed in to the water. Sediment comes largely from shoreline erosion and from the resuspension of bottom sediments due to wind mixing.
Usually, we measure turbidity to provide a cheap estimate of the total suspended solids or sediments (TSS) concentration (in milligrams dry weight/L). TSS measurement requires you to filter a known volume of water through a pre-weighed filter disc to collect all the suspended material (greater than about 1 micron in size) and then re-weigh it after drying it overnight at ~103°C to remove all water in the residue and filter. This is tedious and difficult to do accurately for low turbidity water - the reason why a turbidimeter is often used. Another even cheaper method is to use an inexpensive devise called a Turbidity Tube. This is a simple adaptation for streams of the Secchi disk technique for lakes. It involves looking down a tube at a black and white disk and recording how much stream water is needed to make the disk disappear.
This device yields data for streams that is similar to a secchi depth measurement in lakes. As for secchi measurements are made in the shade with the sun to your back to make an accurate and reproducible reading - the shadow of the observer should be adequate.
Why Is it Important?
The secchi disk depth provides an even lower "tech" method for assessing the clarity of a lake. A Secchi disk is a circular plate divided into quarters painted alternately black and white. The disk is attached to a rope and lowered into the water until it is no longer visible. Secchi disk depth, then, is a measure of water clarity. Higher Secchi readings mean more rope was let out before the disk disappeared from sight and indicates clearer water. Lower readings indicate turbid or colored water. Clear water lets light penetrate more deeply into the lake than does murky water. This light allows photosynthesis to occur and oxygen to be produced. The rule of thumb is that light can penetrate to a depth of about 2 - 3 times the Secchi disk depth.
Clarity is affected by algae, soil particles, and other materials suspended in the water. However, Secchi disk depth is primarily used as an indicator of algal abundance and general lake productivity. Although it is only an indicator, Secchi disk depth is the simplest and one of the most effective tools for estimating a lake's productivity.
Reasons for Natural Variation
Secchi disk readings vary seasonally with changes in photosynthesis and therefore, algal growth. In most lakes, Secchi disk readings begin to decrease in the spring, with warmer temperature and increased growth, and continue decreasing until algal growth peaks in the summer. As cooler weather sets in and growth decreases, Secchi disk readings increase again. (However, cooler weather often means more wind. In a shallow lake, the improved clarity from decreased algal growth may be partly offset by an increase in concentration of sediments mixed into the water column by wind.) In lakes that thermally stratify, Secchi disk readings may decrease again with fall turnover. As the surface water cools, the thermal stratification created in summer weakens and the lake mixes. The nutrients thus released from the bottom layer of water may cause a fall algae bloom and the resultant decrease in Secchi disk reading.
Rainstorms also may affect readings. Erosion from rainfall, runoff, and high stream velocities may result in higher concentrations of suspended particles in inflowing streams and therefore decreases in Secchi disk readings. On the other hand, temperature and volume of the incoming water may be sufficient to dilute the lake with cooler, clearer water and reduce algal growth rates. Both clearer water and lower growth rates would result in increased Secchi disk readings.
The natural color of the water also affects the readings. In most lakes, the impact of color may be insignificant. But some lakes are highly colored. Lakes strongly influenced by bogs, for example, are often a very dark brown and have low Secchi readings even though they may have few algae.
Expected Impact of Pollution
Pollution tends to reduce water clarity. Watershed development and poor land use practices cause increases in erosion, organic matter, and nutrients, all of which cause increases in suspended particulates and algae growth.
Secchi disk depth is usually reported in feet to the nearest tenth of a foot, or meters to the nearest tenth of a meter. Secchi disk readings can be used to determine a lake's trophic status. Though trophic status is not related to any water quality standard, it is a mechanism for "rating" a lake's productive state since unproductive lakes are usually much clearer than productive lakes.
Dissolved Oxygen
Why Is It Important?
Like terrestrial animals, fish and other aquatic organisms need oxygen to live. As water moves past their gills (or other breathing apparatus), microscopic bubbles of oxygen gas in the water, called dissolved oxygen (DO), are transferred from the water to their blood. Like any other gas diffusion process, the transfer is efficient only above certain concentrations. In other words, oxygen can be present in the water, but at too low a concentration to sustain aquatic life. Oxygen also is needed by virtually all algae and all macrophytes, and for many chemical reactions that are important to lake functioning.
Reasons for Natural Variation
Oxygen is produced during photosynthesis and consumed during respiration and decomposition.Because it requires light, photosynthesis occurs only during daylight hours. Respiration and decomposition, on the other hand, occur 24 hours a day. This difference alone can account for large daily variations in DO concentrations. During the night, when photosynthesis cannot counterbalance the loss of oxygen through respiration and decomposition, DO concentration may steadily decline. It is lowest just before dawn, when photosynthesis resumes.
Other sources of oxygen include the air and inflowing streams. Oxygen concentrations are much higher in air, which is about 21% oxygen, than in water, which is a tiny fraction of 1 percent oxygen. Where the air and water meet, this tremendous difference in concentration causes oxygen molecules in the air to dissolve into the water. More oxygen dissolves into water when wind stirs the water; as the waves create more surface area, more diffusion can occur. A similar process happens when you add sugar to a cup of coffee - the sugar dissolves. It dissolves more quickly, however, when you stir the coffee.
Another physical process that affects DO concentrations is the relationship between water temperature and gas saturation. Cold water can hold more of any gas, in this case oxygen, than warmer water. Warmer water becomes "saturated" more easily with oxygen. As water becomes warmer it can hold less and less DO. So, during the summer months in the warmer top portion of a lake, the total amount of oxygen present may be limited by temperature. If the water becomes too warm, even if 100% saturated, O2 levels may be suboptimal for many species of trout.
Mid-summer, when strong thermal stratification develops in a lake, may be a very hard time for fish. Water near the surface of the lake - the epilimnion - is too warm for them, while the water near the bottom - the hypolimnion - has too little oxygen. Anoxia forces the fish to spend more time higher in the water column where the warmer water is suboptimal for them. This may also expose them to higher predation, particularly when they are younger and smaller.
Seasonal changes also affect dissolved oxygen concentrations. Warmer temperatures during summer speed up the rates of photosynthesis and decomposition. When all the plants die at the end of the growing season, their decomposition results in heavy oxygen consumption. Other seasonal events, such as changes in lake water levels, volume of inflows and outflows, and presence of ice cover, also cause natural variation in DO concentrations.
Expected Impact of Pollution
To the degree that pollution contributes oxygen-demanding organic matter (like sewage, lawn clippings, soils from streambank and lakeshore erosion, and from agricultural runoff) or nutrients that stimulate growth of organic matter, pollution causes a decrease in average DO concentrations. If the organic matter is formed in the lake, for example by algal growth, at least some oxygen is produced during growth to offset the eventual loss of oxygen during decomposition. However, in lakes where a large portion of the organic matter is brought in from outside the lake, oxygen production and oxygen consumption are not balanced and low DO may become even more of a problem.
The development of anoxia in lakes is most pronounced in thermally stratified systems in summer and under the ice in winter when the water mass is cut-off from the atmosphere. Besides the direct effects on aerobic organisms, anoxia can lead to increased release of phosphorus from sediments that can fuel algal blooms when mixed into the upper euphotic (sunlit) zone. It also leads to the buildup of chemically reduced compounds such as ammonium and hydrogen sulfide (H2S, rotten egg gas) which can be toxic to bottom dwelling organisms. In extreme cases, sudden mixing of H2S into the upper water column can cause fish kills.
Dissolved oxygen concentrations are most often reported in units of milligrams of gas per liter of water - mg/L. (The unit mg/L is equivalent to parts per million = ppm).
DO - % saturation
Oxygen saturation is calculated as the percentage of dissolved O2 concentration relative to that when completely saturated at the temperature of the measurement depth. Recall that as temperature increases, the concentration at 100% saturation decreases. The elevation of the lake, the barometric pressure, and the salinity of the water also affect this saturation value but to a lesser extent. In most lakes, the effect of dissolved solutes (salinity) is negligible; but the elevation effect due to decreased partial pressure of oxygen in the atmosphere as you go up (recall the breathing difficulties faced by Mt. Everest climbers) is about 4% per 300 meters (1000 feet). The DO concentration for 100% air saturated water at sea level is 8.6 mg O2/L at 25°C (77°F) and increases to 14.6 mg O2/L at 0°C.
Electrical Conductivity
Why is it important?
Electrical conductivity (EC) estimates the amount of total dissolved salts (TDS), or the total amount of dissolved ions in the water. EC is controlled by:
1. geology (rock types) - The rock composition determines the chemistry of the watershed soil and ultimately the lake. For example, limestone leads to higher EC because of the dissolution of carbonate minerals in the basin.
2. The size of the watershed (lake basin) relative to the area of the lake (Aw : Ao ratio) - A bigger watershed to lake surface area means relatively more water draining into the lake because of a bigger catchment area, and more contact with soil before reaching the lake.
3. "other" sources of ions to lakes - There are a number of sources of pollutants which may be signaled by increased EC:
a. wastewater from sewage treatment plants (point source pollutants; see: links)
b. wastewater from septic systems and drainfield on-site wastewater treatment and disposal systems (nonpoint source pollutants; see: links )
c. urban runoff from roads (especially road salt; see: links). This source has a particularly episodic nature with pulsed inputs when it rains or during more prolonged snowmelt periods. It may "shock" organisms with intermittent extreme concentrations of pollutants which seem low when averaged over a week or month (see: Measures of Variability Lesson and other links)
d. agricultural runoff of water draining agricultural fields typically has extremely high levels of dissolved salts (another major nonpoint source of pollutants; see: links). Although a minor fraction of the total dissolved solids, nutrients (ammonium-nitrogen, nitrate-nitrogen and phosphate from fertilizers) and pesticides (insecticides and herbicides mostly) typically have significant negative impacts on streams and lakes receiving agricultural drainage water. If soils are also washed into receiving waters, the organic matter in the soil is decomposed by natural aquatic bacteria which can severely deplete dissolved oxygen concentrations (see above).
e. atmospheric inputs of ions are typically relatively minor except in ocean coastal zones where ocean water increases the salt load ( "salinity" ) of dry aerosols and wet (precipitation) deposition. This oceanic effect can extend inland about 50-100 kilometers and be predicted with reasonable accuracy.
4. evaporation of water from the surface of a lake concentrates the dissolved solids in the remaining water - and so it has a higher EC. This is a very noticeable effect in reservoirs in the southwestern US (the major type of lake in arid climates), and is, of course, the reason why the Great Salt Lake in Utah and Mono Lake, California and Pyramid Lake, Nevada are so salty.
5. bacterial metabolism in the hypolimnion when lakes are thermally stratified for long periods of time (in Minnesota this might be May - November depending on the basin shape, lake depth and weather). During this period, there is a steady "rain" of detritus (dead stuff, mostly algae and washed in particulate material from the watershed) down to the bottom. This material is decomposed by bacteria in the water column and after it reaches the sediments. Their metabolism releases the potential energy stored in the chemical bonds of the organic carbon compounds, consumes oxygen in oxidizing these compounds, and releases carbon dioxide (CO2) after the energy has been liberated (burned). This CO2 rapidly dissolves in water to form carbonic acid (H2CO3), bicarbonate ions (HCO3- ) and carbonate ions (CO3-) the relative amounts depending on the pH of the water. This newly created acid gradually decreases the pH of the water and the "new" ions increase the TDS, and therefore the EC, of the hypolimnion. Essentially, they are "eating" organic matter much as we do and releasing CO2. We oxidize organic carbon using O2 that we breathe out of the air as an oxidant. We use the energy to drive our metabolism and exhale the oxidized carbon as CO2. The oxygen is simultaneously chemically reduced and exhaled as water vapor (H2O; the oxidation state of gaseous molecular oxygen is reduced from 0 to -2 in the process). Other higher aquatic organisms that have aerobic metabolisms, such as zooplankton, insects and fish also consume oxygen dissolved in the water while releasing carbon dioxide as they metabolize organic carbon (food items).
What in the world are microSiemens per centimeter (µS/cm)?
These are the units for electrical conductivity (EC). The sensor simply consists of two metal electrodes that are exactly 1.0 cm apart and protrude into the water. A constant voltage (V) is applied across the electrodes. An electrical current (I) flows through the water due to this voltage and is proportional to the concentration of dissolved ions in the water - the more ions, the more conductive the water resulting in a higher electrical current which is measured electronically. Distilled or deionized water has very few dissolved ions and so there is almost no current flow across the gap (low EC). As an aside, fisheries biologists who electroshock know that if the water is too soft (low EC) it is difficult to electroshock to stun fish for monitoring their abundance and distribution.
Up until about the late 1970's the units of EC were micromhos per centimeter (µmhos/cm) after which they were changed to microSiemens/cm (1 µS/cm = 1 µmho/cm). You will find both sets of units in the published scientific literature although their numerical values are identical. Interestingly, the unit "mhos" derives from the standard name for electrical resistance reflecting the inverse relationship between resistance and conductivity - the higher the resistance of the water, the lower its conductivity. This also follows from Ohm’s Law, V = I x R where R is the resistance of the centimeter of water. Since the electrical current flow (I) increases with increasing temperature, the EC values are automatically corrected to a standard value of 25°C and the values are then technically referred to as specific electrical conductivity.
All WOW conductivity data are temperature compensated to 25°C (usually called specific EC). We do this because the ability of the water to conduct a current is very temperature dependent. We reference all EC readings to 25°C to eliminate temperature differences associated with seasons and depth. Therefore EC 25°C data reflect the dissolved ion content of the water (also routinely called the TDS or total dissolved salt concentration).
How much salt is there in lakewater?
The image below was developed to give you an idea of how much salt (dissolved solids and ions) is present in some of the WOW lakes and to compare them to a range of other aquatic systems. TDS, in milligrams per liter (mg/L) stands for total dissolved salts or solids and is the weight of material left behind were you to filter a liter of water to remove all the suspended particulates and then evaporate the water from the container (usually done in a drying oven in the lab unless you work on Lake Mead in southern Nevada where you can just set it outside for a few minutes in the summer). Each of the piles represents the amount of salt present in a liter of water. We used sodium bicarbonate (baking soda) for the lakes and sodium chloride (table salt) for the ocean.
Chlorophyll - A Measure of Algae
An in-depth microscopic enumeration of the dozens of species of algae present in a water column each time a lake is sampled is prohibitively costly and technically impossible for most monitoring programs. Further, in many lakes a large portion of the algal biomass may be unidentifiable by most experts (these are appropriately called LRGTs or LRBGTs -- little round green things and little round blue-green things). However, measuring the concentration of chlorophyll-a is much easier and provides a reasonable estimate of algal biomass. Chlorophyll-a is the green pigment that is responsible for a plant's ability to convert sunlight into the chemical energy needed to fix CO2 into carbohydrates. To measure chlorophyll-a, a volume of water from a particular depth is filtered through a fine glass-fiber filter to collect all of the particulate material greater than about 1 micron (1/1000th of a millimeter) in size. The chlorophyll-a in this material is then extracted with a solvent (acetone or alcohol) and quantified using a spectrophotometer or a fluorometer.
Both chlorophyll-a and secchi depth are long-accepted methods for estimating the amount of algae in lakes. Secchi depth is much easier and less expensive to determine. However, care must be used in interpreting secchi data because of the potential influence of non-algal particulate material, such as silt from stream discharge or re-suspended bottom sediment. Also, the tea color of some lakes that's due to dissolved organic matter from bogs, can have an effect on secchi depth readings as well. Even if chlorophyll-a is measured, it may be important to also examine the algal community microscopically on occasion, since the mix of species may influence lake management decisions.
Temperature
Why Is it Important?
Most aquatic organisms are poikilothermic - i.e., "cold-blooded" - which means they are unable to internally regulate their core body temperature. Therefore, temperature exerts a major influence on the biological activity and growth of aquatic organisms. To a point, the higher the water temperature, the greater the biological activity. Fish, insects, zooplankton, phytoplankton, and other aquatic species all have preferred temperature ranges. As temperatures get too far above or below this preferred range, the number of individuals of the species decreases until finally there are few, or none. For example, we would generally not expect to find a thriving trout fishery in ponds or shallow lakes because the water is too warm throughout the ice-free season.
The Q10 Rule
Changes in the growth rates of cold-blooded aquatic organisms and many biochemical reaction rates can often be approximated by this rule which predicts that growth rate will double if temperature increases by 10°C (18°F) within their "preferred" range.
Q10 rule
Temperature is also important because of its influence on water chemistry. The rate of chemical reactions generally increases at higher temperature, which in turn affects biological activity. An important example of the effects of temperature on water chemistry is its impact on oxygen. Warm water holds less oxygen that cool water, so it may be saturated with oxygen but still not contain enough for survival of aquatic life. Some compounds are also more toxic to aquatic life at higher temperatures. Temperature is reported in degrees on the Celsius temperature scale(C).
Reasons for Natural Variation
The most obvious reason for temperature change in lakes is the change in seasonal air temperature. Daily variation also may occur, especially in the surface layers, which are warm during the day and cool at night. In deeper lakes (typically greater than 5 m for small lakes and 10 m for larger ones) during summer, the water separates into layers of distinctly different density caused by differences in temperature. Unlike all other fluids, however, as water approaches its freezing point and cools below 4°C, the opposite effect occurs and its density then begins to decrease until it freezes at 0°C (32°F). This is why ice floats. This process is called thermal stratification. The surface water is warmed by the sun, but the bottom of the lake remains cold. You can feel this difference when diving into a lake. Once the stratification develops, it tends to persist until the air temperature cools again in fall. Because the layers don't mix, they develop different physical and chemical characteristics. For example, dissolved oxygen concentration, pH, nutrient concentrations, and species of aquatic life in the upper layer can be quite different from those in the lower layer. It is almost like having two separate lakes. The most profound difference is usually seen in the oxygen profile since the bottom layer is now isolated from the major source of oxygen to the lake - the atmosphere.
When the surface water cools again in the fall to about the same temperature as the lower water, the stratification is lost and the wind can turbulently mix the two water masses together because their densities are so similar (fall turnover). A similar process also may occur during the spring as colder surface waters warm to the temperature of bottom waters and the lake mixes (spring turnover). The lake mixing associated with a turnover often corresponds with changes in many other chemical parameters that in turn affect biological communities. Watch for these changes in your lake this fall and spring.
Because light deceases exponentially with depth in the water column, the sun can heat a greater proportion of the water in a shallow lake than in a deep lake and so a shallow lake can warm up faster and to a higher temperature. Lake temperature also is affected by the size and temperature of inflows (e.g., a stream during snowmelt, or springs or a lowland creek) and by how quickly water flushes through the lake. Even a shallow lake may remain cool if fed by a comparatively large, cold stream.
Expected Impact of Pollution
Thermal pollution (i.e., artificially high temperatures) almost always occurs as a result of discharge of municipal or industrial effluents. Except in very large lakes, it is rare to have an effluent discharge. In urban areas, runoff that flows over hot asphalt and concrete pavement before entering a lake will be artificially heated and could cause lake warming, although in most cases this impact is too small to be measured. Consequently, direct, measurable thermal pollution is not common. In running waters, particularly small urban streams, elevated temperatures from road and parking lot runoff can be a serious problem for populations of cool or cold-water fish already stressed from the other contaminants in urban runoff. During summer, temperatures may approach their upper tolerance limit. Higher temperatures also decrease the maximum amount of oxygen that can be dissolved in the water, leading to oxygen stress if the water is receiving high loads of organic matter. Water temperature fluctuations in streams may be further worsened by cutting down trees which provide shade and by absorbing more heat from sunlight due to increased water turbidity.
REFERENCES
Michaud, J.P. 1991. A citizen's guide to understanding and monitoring lakes and streams. Publ. #94-149. Washington State Dept. of Ecology, Publications Office, Olympia, WA, USA (360) 407-7472.
Moore, M.L. 1989. NALMS management guide for lakes and reservoirs. North American Lake Management Society, P.O. Box 5443, Madison, WI, 53705-5443, USA (http://www.nalms.org).
Why Is it Important?
The pH of a sample of water is a measure of the concentration of hydrogen ions. The term pH was derived from the manner in which the hydrogen ion concentration is calculated - it is the negative logarithm of the hydrogen ion (H+) concentration. What this means to those of us who are not mathematicians is that at higher pH, there are fewer free hydrogen ions, and that a change of one pH unit reflects a tenfold change in the concentrations of the hydrogen ion. For example, there are 10 times as many hydrogen ions available at a pH of 7 than at a pH of 8. The pH scale ranges from 0 to 14. A pH of 7 is considered to be neutral. Substances with pH of less that 7 are acidic; substances with pH greater than 7 are basic.
The pH of water determines the solubility (amount that can be dissolved in the water) and biological availability (amount that can be utilized by aquatic life) of chemical constituents such as nutrients (phosphorus, nitrogen, and carbon) and heavy metals (lead, copper, cadmium, etc.). For example, in addition to affecting how much and what form of phosphorus is most abundant in the water, pH may also determine whether aquatic life can use it. In the case of heavy metals, the degree to which they are soluble determines their toxicity. Metals tend to be more toxic at lower pH because they are more soluble.
Reasons for Natural Variation
Photosynthesis uses up dissolved carbon dioxide which acts like carbonic acid (H2CO3) in water. CO2 removal, in effect, reduces the acidity of the water and so pH increases. In contrast, respiration of organic matter produces CO2, which dissolves in water as carbonic acid, thereby lowering the pH. For this reason, pH may be higher during daylight hours and during the growing season, when photosynthesis is at a maximum. Respiration and decomposition processes lower pH. Like dissolved oxygen concentrations, pH may change with depth in a lake, due again to changes in photosynthesis and other chemical reactions. There is typically a seasonal decrease in pH in the lower layers of a stratified lake because CO2 accumulates. There is no light for plants to fix CO2 and decomposition releases CO2.
Fortunately, lake water is complex; it is full of chemical "shock absorbers" that prevent major changes in pH. Small or localized changes in pH are quickly modified by various chemical reactions, so little or no change may be measured. This ability to resist change in pH is called buffering capacity. Not only does the buffering capacity control would-be localized changes in pH, it controls the overall range of pH change under natural conditions. The pH scale may go from 0 to 14, but the pH of natural waters hovers between 6.5 and 8.5.
Expected Impact of Pollution
When pollution results in higher algal and plant growth (e.g., from increased temperature or excess nutrients), pH levels may increase, as allowed by the buffering capacity of the lake. Although these small changes in pH are not likely to have a direct impact on aquatic life, they greatly influence the availability and solubility of all chemical forms in the lake and may aggravate nutrient problems. For example, a change in pH may increase the solubility of phosphorus, making it more available for plant growth and resulting in a greater long-term demand for dissolved oxygen.
Values for pH are reported in standard pH units, usually to one or two decimal places depending upon the accuracy of the equipment used.
Since pH represents the negative logarithm of a number, it is not mathematically correct to calculate simple averages or other summary statistics.
Instead, pH should be reported as a median and range of values; alternatively the values could be converted to hydrogen ion concentrations, averaged, and re-converted to pH values.
Generally, during the summer months in the upper portion of a productive or eutrophic lakes, pH will range between 7.5 and 8.5. In the bottom of the lake or in less productive lakes, pH will be lower, 6.5 to 7.5, perhaps. This is a very general statement to provide an example of the differences you might measure.
The Case of Acid Rain
An important exception to the buffering of pH changes in lakes is the case of lakes affected by acid rain. Lakes that have received too much rain with a low pH (acid rain), lose their buffering capacity. At a certain point, it takes only a small bit of rain or snowmelt runoff for the pH to change. After that point, change occurs relatively quickly. According to the EPA, a pH of 5-6 or lower has been found to be directly toxic to fish.
Turbidity
Why Is it Important?
Turbidity refers to how clear the water is.The greater the amount of total suspended solids (TSS) in the water, the murkier it appears and the higher the measured turbidity. The major source of turbidity in the open water zone of most lakes is typically phytoplankton, but closer to shore, particulates may also be clays and silts from shoreline erosion, resuspended bottom sediments (this is what turns the western arm of Lake Superior near Duluth brown on a windy day), and organic detritus from stream and/or wastewater discharges. Dredging operations, channelization, increased flow rates, floods, or even too many bottom-feeding fish (such as carp) may stir up bottom sediments and increase the cloudiness of the water.
High concentrations of particulate matter can modify light penetration, cause shallow lakes and bays to fill in faster, and smother benthic habitats - impacting both organisms and eggs. As particles of silt, clay, and other organic materials settle to the bottom, they can suffocate newly hatched larvae and fill in spaces between rocks which could have been used by aquatic organisms as habitat. Fine particulate material also can clog or damage sensitive gill structures, decrease their resistance to disease, prevent proper egg and larval development, and potentially interfere with particle feeding activities. If light penetration is reduced significantly, macrophyte growth may be decreased which would in turn impact the organisms dependent upon them for food and cover. Reduced photosynthesis can also result in a lower daytime release of oxygen into the water. Effects on phytoplankton growth are complex depending on too many factors to generalize.
Very high levels of turbidity for a short period of time may not be significant and may even be less of a problem than a lower level that persists longer. The figure below shows how aquatic organisms are generally affected.
Reasons for Natural Variation
Algal turbidity varies seasonally and with depth in a complex manner as discussed previously in response to physical, chemical and biological changes in the lake. Inorganic and detrital particles from the watershed vary largely in response to hydrological events such as storms and snowmelt.
Even relatively small amounts of wave action can erode exposed lakeshore sediments, in this case a minepit lake from northeastern Minnesota. Can you guess what mineral was mined here?
Impacts
The major effect turbidity has on humans might be simply aesthetic - people don't like the look of dirty water. However, turbidity also adds real costs to the treatment of surface water supplies used for drinking water since the turbidity must be virtually eliminated for effective disinfection (usually by chlorine in a variety of forms) to occur. Particulates also provide attachment sites for heavy metals such as cadmium, mercury and lead, and many toxic organic contaminants such as PCBs, PAHs and many pesticides.
Turbidity is reported by RUSS in nephelometric units (NTUs) which refers to the type of instrument (turbidimeter or nephelometer) used for estimating light scattering from suspended particulate material. Turbidity can be measured in several ways. Turbidity is most often used to estimate the TSS (total suspended solids as [mg dry weight]/L) in the lake's tributaries rather than in the lake itself unless it is subject to large influxes of sediments. For the WOW project we will attempt to develop empirical (meaning: based upon direct measurements) relationships between TSS and turbidity for each system since turbidity is easily measured and TSS analyses are not very sensitive at the typically low concentrations found in the middle of most lakes. Also, TSS is a parameter that directly relates to land uses in the watershed and is a key parameter used for modeling efforts and for assessing the success of mitigation and restoration efforts.
What in the world are Nephelometric Turbidity Units (NTU’s)?
They are the units we use when we measure Turbidity. The term Nephelometric refers to the way the instrument estimates how light is scattered by suspended particulate material in the water. The Nephelometer, also called a turbidimeter, attached to the RUSS unit has the photocell (similar to the one on your camera or your bathroom nightlight) set at 90 degrees to the direction of the light beam to estimate scattered rather than absorbed light. This measurement generally provides a very good correlation with the concentration of particles in the water that affect clarity.
In lakes and streams, there are 3 major types of particles: algae, detritus (dead organic material), and silt (inorganic, or mineral, suspended sediment). The algae grow in the water and the detritus comes from dead algae, higher plants, zooplankton, bacteria, fungi, etc. produced within the water column, and from watershed vegetation washed in to the water. Sediment comes largely from shoreline erosion and from the resuspension of bottom sediments due to wind mixing.
Usually, we measure turbidity to provide a cheap estimate of the total suspended solids or sediments (TSS) concentration (in milligrams dry weight/L). TSS measurement requires you to filter a known volume of water through a pre-weighed filter disc to collect all the suspended material (greater than about 1 micron in size) and then re-weigh it after drying it overnight at ~103°C to remove all water in the residue and filter. This is tedious and difficult to do accurately for low turbidity water - the reason why a turbidimeter is often used. Another even cheaper method is to use an inexpensive devise called a Turbidity Tube. This is a simple adaptation for streams of the Secchi disk technique for lakes. It involves looking down a tube at a black and white disk and recording how much stream water is needed to make the disk disappear.
This device yields data for streams that is similar to a secchi depth measurement in lakes. As for secchi measurements are made in the shade with the sun to your back to make an accurate and reproducible reading - the shadow of the observer should be adequate.
Turbidity is a standard measurement in stream sampling programs where suspended sediment is an extremely important parameter to monitor. It may also be useful for estimating TSS in lakes, particularly reservoirs, since their useful lifetime depends upon how fast the main basin behind the dam fills with inflowing sediments from mainstem and tributary streams and from shoreline erosion. In the WOW lakes, direct inputs of sediments from tributaries are probably too low to significantly affect the turbidity of the water column out in the main lake. However, algal densities, particularly in the more eutrophic lakes in the Minneapolis Metro area represent enough particulate material to be easily measureable by the RUSS turbidity sensors. Although chlorophyll sensors (fluorometers) would be the best way for us to estimate algal abundance (we lack the funding at present), in these lakes the turbidity sensors provide an alternate estimate of algae.
- Pour sample water into the tube until the image at the bottom of the tube is no longer visible when looking directly through the water column at the image. Rotate the tube while looking down at the image to see if the black and white areas of the decal are distinguishable.
- Record this depth of water on your data sheet to the nearest 1 cm. Different individuals will get different values and all should be recorded, not just the average. It is a good idea to have the initials of the observer next to the value to be able identify systematic errors.
- If you see the image on the bottom of the tube after filling it, simply record the depth as > the depth of the tube. Then construct a longer tube, more appropriate for your stream.
Why Is it Important?
The secchi disk depth provides an even lower "tech" method for assessing the clarity of a lake. A Secchi disk is a circular plate divided into quarters painted alternately black and white. The disk is attached to a rope and lowered into the water until it is no longer visible. Secchi disk depth, then, is a measure of water clarity. Higher Secchi readings mean more rope was let out before the disk disappeared from sight and indicates clearer water. Lower readings indicate turbid or colored water. Clear water lets light penetrate more deeply into the lake than does murky water. This light allows photosynthesis to occur and oxygen to be produced. The rule of thumb is that light can penetrate to a depth of about 2 - 3 times the Secchi disk depth.
Clarity is affected by algae, soil particles, and other materials suspended in the water. However, Secchi disk depth is primarily used as an indicator of algal abundance and general lake productivity. Although it is only an indicator, Secchi disk depth is the simplest and one of the most effective tools for estimating a lake's productivity.
Reasons for Natural Variation
Secchi disk readings vary seasonally with changes in photosynthesis and therefore, algal growth. In most lakes, Secchi disk readings begin to decrease in the spring, with warmer temperature and increased growth, and continue decreasing until algal growth peaks in the summer. As cooler weather sets in and growth decreases, Secchi disk readings increase again. (However, cooler weather often means more wind. In a shallow lake, the improved clarity from decreased algal growth may be partly offset by an increase in concentration of sediments mixed into the water column by wind.) In lakes that thermally stratify, Secchi disk readings may decrease again with fall turnover. As the surface water cools, the thermal stratification created in summer weakens and the lake mixes. The nutrients thus released from the bottom layer of water may cause a fall algae bloom and the resultant decrease in Secchi disk reading.
Rainstorms also may affect readings. Erosion from rainfall, runoff, and high stream velocities may result in higher concentrations of suspended particles in inflowing streams and therefore decreases in Secchi disk readings. On the other hand, temperature and volume of the incoming water may be sufficient to dilute the lake with cooler, clearer water and reduce algal growth rates. Both clearer water and lower growth rates would result in increased Secchi disk readings.
The natural color of the water also affects the readings. In most lakes, the impact of color may be insignificant. But some lakes are highly colored. Lakes strongly influenced by bogs, for example, are often a very dark brown and have low Secchi readings even though they may have few algae.
Expected Impact of Pollution
Pollution tends to reduce water clarity. Watershed development and poor land use practices cause increases in erosion, organic matter, and nutrients, all of which cause increases in suspended particulates and algae growth.
Secchi disk depth is usually reported in feet to the nearest tenth of a foot, or meters to the nearest tenth of a meter. Secchi disk readings can be used to determine a lake's trophic status. Though trophic status is not related to any water quality standard, it is a mechanism for "rating" a lake's productive state since unproductive lakes are usually much clearer than productive lakes.
Dissolved Oxygen
Why Is It Important?
Like terrestrial animals, fish and other aquatic organisms need oxygen to live. As water moves past their gills (or other breathing apparatus), microscopic bubbles of oxygen gas in the water, called dissolved oxygen (DO), are transferred from the water to their blood. Like any other gas diffusion process, the transfer is efficient only above certain concentrations. In other words, oxygen can be present in the water, but at too low a concentration to sustain aquatic life. Oxygen also is needed by virtually all algae and all macrophytes, and for many chemical reactions that are important to lake functioning.
Reasons for Natural Variation
Oxygen is produced during photosynthesis and consumed during respiration and decomposition.Because it requires light, photosynthesis occurs only during daylight hours. Respiration and decomposition, on the other hand, occur 24 hours a day. This difference alone can account for large daily variations in DO concentrations. During the night, when photosynthesis cannot counterbalance the loss of oxygen through respiration and decomposition, DO concentration may steadily decline. It is lowest just before dawn, when photosynthesis resumes.
Other sources of oxygen include the air and inflowing streams. Oxygen concentrations are much higher in air, which is about 21% oxygen, than in water, which is a tiny fraction of 1 percent oxygen. Where the air and water meet, this tremendous difference in concentration causes oxygen molecules in the air to dissolve into the water. More oxygen dissolves into water when wind stirs the water; as the waves create more surface area, more diffusion can occur. A similar process happens when you add sugar to a cup of coffee - the sugar dissolves. It dissolves more quickly, however, when you stir the coffee.
Another physical process that affects DO concentrations is the relationship between water temperature and gas saturation. Cold water can hold more of any gas, in this case oxygen, than warmer water. Warmer water becomes "saturated" more easily with oxygen. As water becomes warmer it can hold less and less DO. So, during the summer months in the warmer top portion of a lake, the total amount of oxygen present may be limited by temperature. If the water becomes too warm, even if 100% saturated, O2 levels may be suboptimal for many species of trout.
Mid-summer, when strong thermal stratification develops in a lake, may be a very hard time for fish. Water near the surface of the lake - the epilimnion - is too warm for them, while the water near the bottom - the hypolimnion - has too little oxygen. Anoxia forces the fish to spend more time higher in the water column where the warmer water is suboptimal for them. This may also expose them to higher predation, particularly when they are younger and smaller.
Eutrophication exacerbates this condition by adding organic matter to the system which accelerates the rate of oxygen depletion in the hypolimnion. Urban, and other forms of runoff, can also add to this problem very suddenly and dramatically by causing fish kills after excess soils and road hydrocarbons are washed in from intense rainstorms. Conditions may become especially serious during a stretch of hot, calm weather, resulting in the loss of many fish. You may have heard about summertime fish kills in local lakes that likely results from this problem.
In eutrophic and hypereutrophic lakes, summertime fish kills can happen most easily during periods with high temperatures, little wind and high cloud cover. The clouds reduce daytime photosynthesis with its oxygen production and so the DO in the mixed layer. Or even throughout the water column of a shallow unstratified lake, can become critical for fish and other aquatic organisms.
The same basic phenomenon can occur in winter (winterkill) when ice cover removes re-aeration from the atmosphere and snowcover can light-limit algal and macrophyte photosynthesis under the ice. Many lakes in the upper midwest are mechanically re-aerated or injected with air, oxygen or even liquid oxygen to keep ice off of some of the lake and to add oxygen directly to prevent winterkills.
Dissolved oxygen concentrations may change dramatically with lake depth. Oxygen production occurs in the top portion of a lake, where sunlight drives the engines of photosynthesis. Oxygen consumption is greatest near the bottom of a lake, where sunken organic matter accumulates and decomposes. In deeper, stratified, lakes, this difference may be dramatic - plenty of oxygen near the top but practically none near the bottom. If the lake is shallow and easily mixed by wind, the DO concentration may be fairly consistent throughout the water column as long as it is windy. When calm, a pronounced decline with depth may be observed.Seasonal changes also affect dissolved oxygen concentrations. Warmer temperatures during summer speed up the rates of photosynthesis and decomposition. When all the plants die at the end of the growing season, their decomposition results in heavy oxygen consumption. Other seasonal events, such as changes in lake water levels, volume of inflows and outflows, and presence of ice cover, also cause natural variation in DO concentrations.
Expected Impact of Pollution
To the degree that pollution contributes oxygen-demanding organic matter (like sewage, lawn clippings, soils from streambank and lakeshore erosion, and from agricultural runoff) or nutrients that stimulate growth of organic matter, pollution causes a decrease in average DO concentrations. If the organic matter is formed in the lake, for example by algal growth, at least some oxygen is produced during growth to offset the eventual loss of oxygen during decomposition. However, in lakes where a large portion of the organic matter is brought in from outside the lake, oxygen production and oxygen consumption are not balanced and low DO may become even more of a problem.
The development of anoxia in lakes is most pronounced in thermally stratified systems in summer and under the ice in winter when the water mass is cut-off from the atmosphere. Besides the direct effects on aerobic organisms, anoxia can lead to increased release of phosphorus from sediments that can fuel algal blooms when mixed into the upper euphotic (sunlit) zone. It also leads to the buildup of chemically reduced compounds such as ammonium and hydrogen sulfide (H2S, rotten egg gas) which can be toxic to bottom dwelling organisms. In extreme cases, sudden mixing of H2S into the upper water column can cause fish kills.
Dissolved oxygen concentrations are most often reported in units of milligrams of gas per liter of water - mg/L. (The unit mg/L is equivalent to parts per million = ppm).
DO - % saturation
Oxygen saturation is calculated as the percentage of dissolved O2 concentration relative to that when completely saturated at the temperature of the measurement depth. Recall that as temperature increases, the concentration at 100% saturation decreases. The elevation of the lake, the barometric pressure, and the salinity of the water also affect this saturation value but to a lesser extent. In most lakes, the effect of dissolved solutes (salinity) is negligible; but the elevation effect due to decreased partial pressure of oxygen in the atmosphere as you go up (recall the breathing difficulties faced by Mt. Everest climbers) is about 4% per 300 meters (1000 feet). The DO concentration for 100% air saturated water at sea level is 8.6 mg O2/L at 25°C (77°F) and increases to 14.6 mg O2/L at 0°C.
Electrical Conductivity
Why is it important?
Electrical conductivity (EC) estimates the amount of total dissolved salts (TDS), or the total amount of dissolved ions in the water. EC is controlled by:
1. geology (rock types) - The rock composition determines the chemistry of the watershed soil and ultimately the lake. For example, limestone leads to higher EC because of the dissolution of carbonate minerals in the basin.
2. The size of the watershed (lake basin) relative to the area of the lake (Aw : Ao ratio) - A bigger watershed to lake surface area means relatively more water draining into the lake because of a bigger catchment area, and more contact with soil before reaching the lake.
3. "other" sources of ions to lakes - There are a number of sources of pollutants which may be signaled by increased EC:
a. wastewater from sewage treatment plants (point source pollutants; see: links)
b. wastewater from septic systems and drainfield on-site wastewater treatment and disposal systems (nonpoint source pollutants; see: links )
c. urban runoff from roads (especially road salt; see: links). This source has a particularly episodic nature with pulsed inputs when it rains or during more prolonged snowmelt periods. It may "shock" organisms with intermittent extreme concentrations of pollutants which seem low when averaged over a week or month (see: Measures of Variability Lesson and other links)
d. agricultural runoff of water draining agricultural fields typically has extremely high levels of dissolved salts (another major nonpoint source of pollutants; see: links). Although a minor fraction of the total dissolved solids, nutrients (ammonium-nitrogen, nitrate-nitrogen and phosphate from fertilizers) and pesticides (insecticides and herbicides mostly) typically have significant negative impacts on streams and lakes receiving agricultural drainage water. If soils are also washed into receiving waters, the organic matter in the soil is decomposed by natural aquatic bacteria which can severely deplete dissolved oxygen concentrations (see above).
e. atmospheric inputs of ions are typically relatively minor except in ocean coastal zones where ocean water increases the salt load ( "salinity" ) of dry aerosols and wet (precipitation) deposition. This oceanic effect can extend inland about 50-100 kilometers and be predicted with reasonable accuracy.
4. evaporation of water from the surface of a lake concentrates the dissolved solids in the remaining water - and so it has a higher EC. This is a very noticeable effect in reservoirs in the southwestern US (the major type of lake in arid climates), and is, of course, the reason why the Great Salt Lake in Utah and Mono Lake, California and Pyramid Lake, Nevada are so salty.
5. bacterial metabolism in the hypolimnion when lakes are thermally stratified for long periods of time (in Minnesota this might be May - November depending on the basin shape, lake depth and weather). During this period, there is a steady "rain" of detritus (dead stuff, mostly algae and washed in particulate material from the watershed) down to the bottom. This material is decomposed by bacteria in the water column and after it reaches the sediments. Their metabolism releases the potential energy stored in the chemical bonds of the organic carbon compounds, consumes oxygen in oxidizing these compounds, and releases carbon dioxide (CO2) after the energy has been liberated (burned). This CO2 rapidly dissolves in water to form carbonic acid (H2CO3), bicarbonate ions (HCO3- ) and carbonate ions (CO3-) the relative amounts depending on the pH of the water. This newly created acid gradually decreases the pH of the water and the "new" ions increase the TDS, and therefore the EC, of the hypolimnion. Essentially, they are "eating" organic matter much as we do and releasing CO2. We oxidize organic carbon using O2 that we breathe out of the air as an oxidant. We use the energy to drive our metabolism and exhale the oxidized carbon as CO2. The oxygen is simultaneously chemically reduced and exhaled as water vapor (H2O; the oxidation state of gaseous molecular oxygen is reduced from 0 to -2 in the process). Other higher aquatic organisms that have aerobic metabolisms, such as zooplankton, insects and fish also consume oxygen dissolved in the water while releasing carbon dioxide as they metabolize organic carbon (food items).
What in the world are microSiemens per centimeter (µS/cm)?
These are the units for electrical conductivity (EC). The sensor simply consists of two metal electrodes that are exactly 1.0 cm apart and protrude into the water. A constant voltage (V) is applied across the electrodes. An electrical current (I) flows through the water due to this voltage and is proportional to the concentration of dissolved ions in the water - the more ions, the more conductive the water resulting in a higher electrical current which is measured electronically. Distilled or deionized water has very few dissolved ions and so there is almost no current flow across the gap (low EC). As an aside, fisheries biologists who electroshock know that if the water is too soft (low EC) it is difficult to electroshock to stun fish for monitoring their abundance and distribution.
Up until about the late 1970's the units of EC were micromhos per centimeter (µmhos/cm) after which they were changed to microSiemens/cm (1 µS/cm = 1 µmho/cm). You will find both sets of units in the published scientific literature although their numerical values are identical. Interestingly, the unit "mhos" derives from the standard name for electrical resistance reflecting the inverse relationship between resistance and conductivity - the higher the resistance of the water, the lower its conductivity. This also follows from Ohm’s Law, V = I x R where R is the resistance of the centimeter of water. Since the electrical current flow (I) increases with increasing temperature, the EC values are automatically corrected to a standard value of 25°C and the values are then technically referred to as specific electrical conductivity.
All WOW conductivity data are temperature compensated to 25°C (usually called specific EC). We do this because the ability of the water to conduct a current is very temperature dependent. We reference all EC readings to 25°C to eliminate temperature differences associated with seasons and depth. Therefore EC 25°C data reflect the dissolved ion content of the water (also routinely called the TDS or total dissolved salt concentration).
How much salt is there in lakewater?
The image below was developed to give you an idea of how much salt (dissolved solids and ions) is present in some of the WOW lakes and to compare them to a range of other aquatic systems. TDS, in milligrams per liter (mg/L) stands for total dissolved salts or solids and is the weight of material left behind were you to filter a liter of water to remove all the suspended particulates and then evaporate the water from the container (usually done in a drying oven in the lab unless you work on Lake Mead in southern Nevada where you can just set it outside for a few minutes in the summer). Each of the piles represents the amount of salt present in a liter of water. We used sodium bicarbonate (baking soda) for the lakes and sodium chloride (table salt) for the ocean.
Chlorophyll - A Measure of Algae
An in-depth microscopic enumeration of the dozens of species of algae present in a water column each time a lake is sampled is prohibitively costly and technically impossible for most monitoring programs. Further, in many lakes a large portion of the algal biomass may be unidentifiable by most experts (these are appropriately called LRGTs or LRBGTs -- little round green things and little round blue-green things). However, measuring the concentration of chlorophyll-a is much easier and provides a reasonable estimate of algal biomass. Chlorophyll-a is the green pigment that is responsible for a plant's ability to convert sunlight into the chemical energy needed to fix CO2 into carbohydrates. To measure chlorophyll-a, a volume of water from a particular depth is filtered through a fine glass-fiber filter to collect all of the particulate material greater than about 1 micron (1/1000th of a millimeter) in size. The chlorophyll-a in this material is then extracted with a solvent (acetone or alcohol) and quantified using a spectrophotometer or a fluorometer.
Both chlorophyll-a and secchi depth are long-accepted methods for estimating the amount of algae in lakes. Secchi depth is much easier and less expensive to determine. However, care must be used in interpreting secchi data because of the potential influence of non-algal particulate material, such as silt from stream discharge or re-suspended bottom sediment. Also, the tea color of some lakes that's due to dissolved organic matter from bogs, can have an effect on secchi depth readings as well. Even if chlorophyll-a is measured, it may be important to also examine the algal community microscopically on occasion, since the mix of species may influence lake management decisions.
Temperature
Why Is it Important?
Most aquatic organisms are poikilothermic - i.e., "cold-blooded" - which means they are unable to internally regulate their core body temperature. Therefore, temperature exerts a major influence on the biological activity and growth of aquatic organisms. To a point, the higher the water temperature, the greater the biological activity. Fish, insects, zooplankton, phytoplankton, and other aquatic species all have preferred temperature ranges. As temperatures get too far above or below this preferred range, the number of individuals of the species decreases until finally there are few, or none. For example, we would generally not expect to find a thriving trout fishery in ponds or shallow lakes because the water is too warm throughout the ice-free season.
The Q10 Rule
Changes in the growth rates of cold-blooded aquatic organisms and many biochemical reaction rates can often be approximated by this rule which predicts that growth rate will double if temperature increases by 10°C (18°F) within their "preferred" range.
Q10 rule
Temperature is also important because of its influence on water chemistry. The rate of chemical reactions generally increases at higher temperature, which in turn affects biological activity. An important example of the effects of temperature on water chemistry is its impact on oxygen. Warm water holds less oxygen that cool water, so it may be saturated with oxygen but still not contain enough for survival of aquatic life. Some compounds are also more toxic to aquatic life at higher temperatures. Temperature is reported in degrees on the Celsius temperature scale(C).
Reasons for Natural Variation
The most obvious reason for temperature change in lakes is the change in seasonal air temperature. Daily variation also may occur, especially in the surface layers, which are warm during the day and cool at night. In deeper lakes (typically greater than 5 m for small lakes and 10 m for larger ones) during summer, the water separates into layers of distinctly different density caused by differences in temperature. Unlike all other fluids, however, as water approaches its freezing point and cools below 4°C, the opposite effect occurs and its density then begins to decrease until it freezes at 0°C (32°F). This is why ice floats. This process is called thermal stratification. The surface water is warmed by the sun, but the bottom of the lake remains cold. You can feel this difference when diving into a lake. Once the stratification develops, it tends to persist until the air temperature cools again in fall. Because the layers don't mix, they develop different physical and chemical characteristics. For example, dissolved oxygen concentration, pH, nutrient concentrations, and species of aquatic life in the upper layer can be quite different from those in the lower layer. It is almost like having two separate lakes. The most profound difference is usually seen in the oxygen profile since the bottom layer is now isolated from the major source of oxygen to the lake - the atmosphere.
When the surface water cools again in the fall to about the same temperature as the lower water, the stratification is lost and the wind can turbulently mix the two water masses together because their densities are so similar (fall turnover). A similar process also may occur during the spring as colder surface waters warm to the temperature of bottom waters and the lake mixes (spring turnover). The lake mixing associated with a turnover often corresponds with changes in many other chemical parameters that in turn affect biological communities. Watch for these changes in your lake this fall and spring.
Because light deceases exponentially with depth in the water column, the sun can heat a greater proportion of the water in a shallow lake than in a deep lake and so a shallow lake can warm up faster and to a higher temperature. Lake temperature also is affected by the size and temperature of inflows (e.g., a stream during snowmelt, or springs or a lowland creek) and by how quickly water flushes through the lake. Even a shallow lake may remain cool if fed by a comparatively large, cold stream.
Expected Impact of Pollution
Thermal pollution (i.e., artificially high temperatures) almost always occurs as a result of discharge of municipal or industrial effluents. Except in very large lakes, it is rare to have an effluent discharge. In urban areas, runoff that flows over hot asphalt and concrete pavement before entering a lake will be artificially heated and could cause lake warming, although in most cases this impact is too small to be measured. Consequently, direct, measurable thermal pollution is not common. In running waters, particularly small urban streams, elevated temperatures from road and parking lot runoff can be a serious problem for populations of cool or cold-water fish already stressed from the other contaminants in urban runoff. During summer, temperatures may approach their upper tolerance limit. Higher temperatures also decrease the maximum amount of oxygen that can be dissolved in the water, leading to oxygen stress if the water is receiving high loads of organic matter. Water temperature fluctuations in streams may be further worsened by cutting down trees which provide shade and by absorbing more heat from sunlight due to increased water turbidity.
REFERENCES
Michaud, J.P. 1991. A citizen's guide to understanding and monitoring lakes and streams. Publ. #94-149. Washington State Dept. of Ecology, Publications Office, Olympia, WA, USA (360) 407-7472.
Moore, M.L. 1989. NALMS management guide for lakes and reservoirs. North American Lake Management Society, P.O. Box 5443, Madison, WI, 53705-5443, USA (http://www.nalms.org).
This post first appeared on MAHSEER BREEDING And GOLD BULLION NEWS, please read the originial post: here
