Appendix A. General Concepts in Lake Water Quality. Black Dog WMO Watershed Management Plan P:\23\19\513\plan\Black Dog WMO adopted plan.

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1 Appendix A General Concepts in Lake Water Quality Black Dog WMO Watershed Management Plan P:\23\19\513\plan\Black Dog WMO adopted plan.doc

2 General Concepts in Lake Water Quality There are a number of concepts and terminology that are necessary to describe and evaluate a lake s water quality. This section briefly discusses those concepts, divided into the following topics: Eutrophication Trophic states Limiting nutrients Nutrient recycling and internal loading To learn more about these topics, one can refer to any text on limnology (the science of lakes and streams). This section also presents information about water body classification systems and a glossary of water quality terms. Eutrophication Eutrophication, or lake degradation, is the accumulation of sediments and nutrients in lakes. As a lake naturally becomes more fertile over time, algae and aquatic plant growth increases. The increasing biological production and sediment inflow from the lake s watershed eventually fill the lake s basin. Over a period of many years, the lake successively becomes a pond, a marsh and, ultimately, a terrestrial site. This process of eutrophication is natural and results from the normal environmental forces that influence a lake. Cultural eutrophication, however, is an acceleration of the natural process caused by human activities. Nutrient and sediment inputs (i.e. phosphorus loadings) from wastewater treatment plants, septic tanks, and stormwater runoff can far exceed the natural inputs to the lake. The accelerated rate of water quality degradation caused by these pollutants results in unpleasant consequences. These include profuse and unsightly growths of algae (algal blooms) and/or the proliferation of rooted aquatic plants (macrophytes). P:\23\19\513\plan\Black Dog WMO adopted plan.doc Page A-1

3 Trophic States Not all lakes are at the same stage of eutrophication; therefore, criteria have been established to define the nutrient status of lakes. Trophic state indices (TSIs) are calculated for lakes on the basis of total phosphorus, chlorophyll-a concentrations, and Secchi disc transparencies (Carlson, 1977). A TSI value is obtained from any one of these three parameters. TSI values range upward from 0, describing the condition of the lake in terms of its trophic status (its degree of fertility). Four trophic status designations for lakes are listed below, along with their corresponding TSI value ranges: 1. Oligotrophic [TSI < 37] Clear, low productivity lakes with total phosphorus concentrations less than or equal to 10 µg/l, Secchi disc depths greater than or equal to 4.6 meters, and chlorophyll-a concentrations less than or equal to 2 µg/l. 2. Mesotrophic [38 < TSI < 50] Intermediate productivity lakes with total phosphorus concentrations greater than 10 µg/l, but less than 25 µg/l, Secchi disc depths from 4.6 meters to 2 meters, and chlorophyll-a concentrations between 2 µg/l and 7.5 µg/l. 3. Eutrophic [51 < TSI < 63] High productivity lakes generally having 25 to 60 µg/l total phosphorus, Secchi disc depths from 2 meters to 0.85 meters, and chlorophyll-a concentrations greater than 7.5 µg/l, but less than 26µg/L. The relatively high concentrations of algae and aquatic plants can create aesthetically unpleasant conditions. 4. Hypereutrophic [TSI > 64] Extremely productive, highly eutrophic lakes, with total phosphorus concentrations above 60 µg/l, Secchi disc depths less than or equal to 0.85 meters, and chlorophyll-a concentrations more than 26 µg/l. These lakes are also disturbed and unstable. The water quality of these lakes can fluctuate daily and seasonally, producing gases, off-flavor, and toxic substances, experiencing periodic anoxia and fish kills, etc. Determining the trophic status of a lake is an important step in diagnosing water quality problems. Trophic status indicates the severity of a lake s algal growth problems and the degree of change needed to meet its recreational goals. Additional information is needed to determine the cause of algal growth and a means of reducing it. P:\23\19\513\plan\Black Dog WMO adopted plan.doc Page A-2

4 Limiting Nutrients The quantity or biomass of algae in a lake is usually limited by the water s concentration of an essential element or nutrient the limiting nutrient. (For rooted aquatic plants, the nutrients are derived from the sediments.) The limiting nutrient concept is a widely applied principle in ecology and in the study of eutrophication. It is based on the idea that plants require many nutrients to grow, but the nutrient with the lowest availability, relative to the amount needed by the plant, will limit plant growth. It follows then, that identifying the limiting nutrient will point the way to controlling algal growth. Nitrogen (N) and phosphorus (P) are generally the two growth-limiting nutrients for algae in most natural waters. Analysis of the nutrient content of lake water and algae provides ratios of N:P. By comparing the N:P ratio in water to the N:P ratio in the algae, one can estimate whether a particular nutrient may be limiting. Algal growth is generally phosphorus-limited in waters with N:P ratios greater than 12. Laboratory experiments (bioassays) can demonstrate which nutrient is limiting by growing the algae in lake water with various concentrations of nutrients added. Bioassays, as well as fertilization of in-situ enclosures and whole-lake experiments, have repeatedly demonstrated that phosphorus is usually the nutrient that limits algal growth in temperate lakes. Reducing phosphorus in a lake, therefore, is required to reduce algal abundance and improve water transparency. Failure to reduce phosphorus concentrations will allow the process of eutrophication to continue at an accelerated rate. Nutrient recycling and internal loading Phosphorus enters a lake from either runoff from the watershed or direct atmospheric deposition. It would seem reasonable to expect phosphorus concentrations in a lake to decline by reducing these external loads of phosphorus to the lake. But all lakes accumulate phosphorus (and other nutrients) in the sediments from the settling of particles and dead organisms. In some lakes this reservoir of phosphorus can be reintroduced in the lake water and become available again for plant uptake. This resuspension or dissolution of nutrients from the sediments to the lake water is known as internal loading. The relative amounts of phosphorus coming from internal and external loading vary with each lake. The amount of phosphorus released from internal loading can be estimated from depth profiles (measurements from surface to bottom) of dissolved oxygen and phosphorus concentrations. Rooted aquatic plants (macrophytes) obtain their nutrients from the sediments and so do not directly benefit from nutrient levels in the water. But rooted plants can contribute to internal phosphorus P:\23\19\513\plan\Black Dog WMO adopted plan.doc Page A-3

5 loading by storing nutrients in their plant tissue and releasing nutrients when the plants die and decompose. Decomposing plants can also contribute indirectly to the internal load by reducing dissolved oxygen concentrations that could lead to increased phosphorus release from the sediments. Most aquatic plants decompose primarily in the fall when the release of nutrients are not likely to lead to increased algae growth because low temperatures and shorter days prevent a rapid response by the algae. In contrast to most aquatic plants, curlyleaf pondweed begins to die and decompose in mid-summer when the increase in available nutrients to algae can be significant. Therefore, curlyleaf pondweed can be a nuisance plant by obstructing recreational use of a lake, and it can lead to reduced water quality in the summer by contributing to increased growth of algae. Water Body Classification Systems This plan references the following existing state and regional agency classification systems: The Trophic State Index (TSI) developed by Carlson (1977) describes the trophic status of water bodies in a quantifiable scale. TSI values range from 0 to 100, with trophic states falling within defined ranges. The quantitative nature of this index allows trends to be determined. Many classification systems have used the trophic state index for their basis or in the establishment of water quality goals. This index is used extensively to describe the lake conditions for all Black Dog WMO lakes presented in this report. The Minnesota DNR has classified lakes throughout the state based on the fish communities they are likely to support. This classification system is based on many factors including surface area, littoral area (i.e. zone where rooted aquatic plants can survive), maximum depth, degree of shoreline development, Secchi disc transparency, and total alkalinity. The MDNR assigns a TSI value to each of these lake classes, but cautions that this value is only a representative approximate value. Therefore, the fishery-related TSI values should not be construed as goals for any water body. This classification system is generally not used for management purposes, but is used to describe the fish communities in lakes. The Minnesota Pollution Control Agency (MPCA) has classified water bodies based on their support of swimming use. This classification is made to comply with the reporting requirements outlined in the Clean Water Act and the issuance of 305(b) reports to the United States Environmental Protection Agency (USEPA). This classification is generally descriptive, as the goal of the Clean Water Act is full support of swimming in all water bodies. P:\23\19\513\plan\Black Dog WMO adopted plan.doc Page A-4

6 Another descriptive water quality index is the Recreational Suitability Index. This index is calculated from Secchi disc transparency and is based on empirical relationships between chlorophyll-a and Secchi disc depth in the Twin Cities Metropolitan area (Osgood, 1989). Recreational Suitability Index values are calculated using the following equation: RSI = X log 10 (Secchi disc depth) The RSI scale is as follows: RSI Value Description 1 Beautiful, could not be better 2 Very minor aesthetic problems: Excellent for swimming, boating, etc. 3 Swimming and aesthetic slightly impaired because of algal levels 4 Desire to swim and level of enjoyment substantially reduced 5 Swimming and aesthetic enjoyment nearly impossible because of algae This index is useful because it describes the relative recreational status of the lakes, and most individuals can relate to the index descriptions for each value. The lake quality report card classification system is used by the Metropolitan Council to describe the water quality of lakes and ponds in the Metropolitan area. This system is useful in that it is easy to understand and gives a relative scale with which to compare many lakes in the metro area. The grading curve represents percentile ranges for the summer (May-September) average of total phosphorus, chlorophyll-a, and Secchi disc transparency. The percentiles use ranked data from 119 lakes sampled from The grades are as follows: Grade Percentile TP(µg/L) chla(µg/l) SD(m) A <10 <23 <10 >3.0 B C D F >90 >152 >77 <0.7 These water quality grades only characterize open-water quality of lakes. Other nuisances, such as the abundance of aquatic macrophytes, are not included in this system (Anhorn, 1999). Grades will generally correspond to descriptive rankings and recreational-use impairment of lakes. Lakes receiving an A (<10 percentile) are exceptional as compared to other area lakes, and have no recreational use impairments. Lakes receiving the grade B, are considered to have very good water quality and some recreational use impairment, while lakes receiving a C grade are considered to P:\23\19\513\plan\Black Dog WMO adopted plan.doc Page A-5

7 have average water quality and are recreationally impaired. A D -grade lake indicates very poor water quality (i.e. severely impaired), and a lake receiving an F would mean extremely poor water quality compared to other area lakes and indicates no recreational use (Anhorn, 1999). Trends can be indicated within this system by adding a plus (+) for an improving trend, or a minus (-) for a decline in water quality. Some other classification systems have been used to determine current conditions, as well as in determining water quality goals for lakes. Some of these systems are purely descriptive in nature, and others imply certain management strategies appropriate for the classification. City of Lakeville Water Body Classification System The City of Lakeville classified lakes and wetlands into one of five categories, based on function and measured parameters (Lakeville Stormwater Management Plan, Barr, 1995). The categories used by the City of Lakeville are described below: Category I Direct Contact Recreational Activities Lakes in this category should be managed to provide water quality capable of supporting full body contact activities. These would include swimming, scuba diving, and snorkeling and water skiing. A reasonable goal for lakes of this classification is to maintain a minimum summer Secchi disc measurement of at least 1.0 meters and an average of at least 1.4 meters. A lake with a Carlson TSI of as high as 55, total phosphorus concentrations less than 45 µg/l, and chlorophyll-a concentrations less than 20 µg/l would be included in this category. Category II Non-Contact Recreational Activities Lakes and ponds in this category should be managed to provide water quality capable of supporting sail boating, motor boating, canoeing, and fishing. These activities bring people in close proximity to the lake and involve incidental contact with lake water. A Category II lake does not require clarity comparable to direct contact recreational waters. Swimming and water skiing would likely be limited in a Category II lake by the probable algae blooms in mid-to latesummer. A reasonable goal to for these activities is a mean summer Secchi disc depth of 0.9 meters. Total phosphorus concentrations of about 75 µg/l, and chlorophyll-a concentrations of about 40 µg/l P:\23\19\513\plan\Black Dog WMO adopted plan.doc Page A-6

8 would be commonly associated with the above transparency. The Carlson TSI index should be no greater than 60 (moderately eutrophic) to meet this management category. Category III Aesthetic Viewing and Wildlife Habitat Many small lakes and ponds with shallow depth and large watershed area/lake size ratios will likely fall into this category because they cannot meet Category I or II water quality standards. These water bodies serve important functions however, including wildlife habitat and warm water fishing (provided winterkill does not occur). A mean summer Secchi disc depth of 0.7 meters appears to be a reasonable goal for Category III ponds. Total phosphorus concentrations less than 105 µg/l and chlorophyll-a concentrations less than 60 µg/l are reasonable goals for Category III ponds. These water bodies would have a Carlson s TSI index no greater than 65 (highly eutrophic) and management would be aimed at preventing the lakes or ponds from moving to hypereutrophy which would begin at a TSI of about 70. Category IV Nutrient Traps Water bodies in Category IV are intended to reduce the downstream loading of phosphorus and other nutrients that contribute to algae or other forms of water pollution. These ponds are generally artificially modified to improve their nutrient trapping capacity. These ponds will be allowed to become hyper-eutrophic and frequent summer algal blooms will be considered normal. The ponds should be maintained at a shallow depth to mitigate anaerobic conditions and prevent the odors associated with algal blooms. Category V Sediment Traps Water bodies in Category V are similar to Category IV basins (nutrient traps), but are too small to effectively remove a significant fraction of nutrients. These water bodies should be managed similarly to Category IV ponds to minimize anaerobic conditions. Management strategies to achieve or maintain these category conditions are described in the City of Lakeville Stormwater Management Plan (Barr, 1995). P:\23\19\513\plan\Black Dog WMO adopted plan.doc Page A-7

9 City of Burnsville Water Body Classification System The City of Burnsville categorizes water bodies into four groups, depending on the water body s intended use (Comprehensive Stormwater Management Plan for the City of Burnsville, Minnesota, Orr, Schelen, Mayeron, and Assoc., 1994). The current Burnsville system does not include any quantifiable component (i.e., Trophic State Index values, Secchi disc standards, etc.). Descriptions of the Burnsville classification categories follow: Recreational Water bodies classified as recreational should be suitable for most recreational activities and for the propagation and maintenance of fish. Stormwater runoff that is directed to these water bodies should be treated to Nation-Wide Urban Runoff Program standards (whenever reasonable opportunities exist to construct and maintain these systems). Lakes within the BDWMO that are included in this classification are Crystal Lake and Lac Lavon. Aesthetic These water bodies should be maintained in such a way that they are suitable for use as aesthetic resources. Management should attempt to maintain visual quality and enhance property values. Pre-treatment of stormwater runoff discharging to these basins generally includes removal of fine sands and sediments, and the skimming of oil and floatable materials (if possible). These water bodies may be used to provide secondary treatment of stormwater runoff including nutrient removal. These waters may also be used to provide for temporary storage for peak flows of stormwater runoff, but outflow rates and elevations must be controlled to avoid water elevations that may affect the character of the resource. All protected waters and wetlands as identified by the Minnesota DNR that are not Recreational are included in this classification. Runoff Management These water bodies are managed for the express purpose of providing stormwater storage and treatment to reduce downstream peak flow rates and/or improve water quality. Management of P:\23\19\513\plan\Black Dog WMO adopted plan.doc Page A-8

10 these water bodies is restricted to maintenance or enhancement of these storage and treatment functions. Wetlands, streams and other conveyance and storage components not identified as protected waters or wetlands by the MDNR are classified as runoff management water bodies. Special Purpose Water bodies that have special features including unique qualities or benefits relating to biological, physical, or geographic characters, could be included in this classification. Management of these water bodies will be determined on a case-by-case basis as is appropriate to maintain the characteristics necessary to support the special purpose. Burnsville has no water bodies with this classification at this time. City of Apple Valley Water Body Classification System The City of Apple Valley has a water body classification system based on potential use and associated water quality parameters (Report: Stormwater Management Plan, Bonestroo, Rosene, Anderlik, and Assoc., 1997). The following five classes are used by the City of Apple Valley: Class 1: Direct Contact Recreational These water bodies have a high potential for swimming, water skiing, and high quality residential development. As such, they have a surface area greater than 20 acres. The outflow phosphorus concentration should be less than or equal to 66 µg/l. Class 2: Indirect Contact Recreational These water bodies can support indirect contact activities such as fishing and boating. Their surface area is greater than 20 acres, and their average depth is greater than or equal to 3 feet. The outflow phosphorus concentration should be less than or equal to 100 µg/l. Class 3: Aesthetic/Wildlife Habitat These water bodies provide primarily wildlife habitat. They are generally accessible to the public for educational, interpretation, and nature appreciation. Such water bodies should not bounce more than 3 feet for a 100-year event. The outflow phosphorus concentration should be less than or equal to 150 µg/l. P:\23\19\513\plan\Black Dog WMO adopted plan.doc Page A-9

11 Class 4: Nutrient Trap Nutrient traps are wetlands or manmade water quality ponds that have phosphorus removal efficiencies of at least 50 percent, and an average depth of at least 3 feet. Class 5: Stormwater Basin These basins are large manmade ponds with large storage capacities and phosphorus removal efficiencies of less than 50 percent. Glossary of Water Quality Terms Chlorophyll-a: The major pigment found in algae and used in the conversion of sunlight into plant biomass (i.e., photosynthesis). The concentration of this pigment is used as a surrogate for algal biomass in freshwater systems. Littoral zone: The zone of shallow (<15 feet deep) where enough light penetrates to the sediments to support rooted aquatic plant growth. Macrophyte: Rooted aquatic plants located in the littoral zone of lakes. Some macrophyte growth is healthy, but excessive growth may limit recreational use of a water body. Phytoplankton: also called algae. These are small aquatic plants naturally present in all lakes. They derive energy from sunlight (through photosynthesis) and from dissolved nutrients found in lake water. They provide food for several types of animals, including zooplankton, which in turn are eaten by fish. The following phytoplankton types are typically found in fresh water: Blue -Green Algae: A highly specialized group of algae some of which are able to fix or take nitrogen from the atmosphere. These algae are often found in highly productive (i.e., eutrophic) systems and can reproduce rapidly. Blue-green algae can cause unsightly scums on the surface of the water and can shade out more healthy green algae. Cryptomonads: These algae are small, and not much is known about their ecology. Dense populations of this algae often develop during cold periods of the year under lower light conditions. Diatoms: An algae that is sensitive to water quality, and generally only found when water quality is good. P:\23\19\513\plan\Black Dog WMO adopted plan.doc Page A-10

12 Green Algae: Algae typically found in healthy ecosystems. However, extremely high levels may also indicate eutrophic conditions. Secchi Disc: A white disc, usually 20 cm in diameter used to measure water clarity. The disc is lowered into the lake until it disappears from sight and is then raised slowly until it is just visible again. The distance halfway between the points of disappearance and reappearance of this disc is taken as the Secchi depth. Total Phosphorus: The sum of dissolved and particulate phosphorus. Phosphorus is an essential nutrient for plant (phytoplankton and macrophyte) growth. Zooplankton: Small invertebrates moving through the water column both by limited locomotion and turbulence in the water column. Zooplankton eat both phytoplankton and other zooplankton. Generally small zooplankton eat algae, and larger zooplankton eat other zooplankton. Zooplankton are a significant food source for some fish. P:\23\19\513\plan\Black Dog WMO adopted plan.doc Page A-11