ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - LIMNOLOGY doc

608 LIMNOLOGY INTRODUCTION Limnology is the scientific study of the physical, chemical, and biological factors that affect aquatic productivity and water quality in lakes. Lakes are important resources—much more than places for groundwater, surface water, and pre- cipitation to collect. They control flooding, provide water for domestic and agricultural uses, and provide recreational opportunities such as swimming, fishing, boating, and water- skiing. Lakes also provide habitat for insects, fish, and wild- life such as frogs, turtles, waterfowl, and shorebirds. Lakes’ commercial value in food supply, tourism, and transporta- tion is worth many billions of dollars each year. Lakes also offer opportunities for relaxation and appreciation of natural beauty. According to the North American Lake Management Society, this quality is not a minor asset; over 60 percent of Wisconsin lake property owners who were asked what they valued in lakes rated aesthetics as especially important. (U.S. EPA, 1990). However, a lake cannot be all things to all people. Desirable uses, even obtainable ones, can conflict. For example, swim- mers may want no plants but some plants are needed in order to provide fish habitat. Lakeside property owners and lake associations often want their lake to do everything; they want aesthetic pleasure, great fishing, clean water, sandy shore- lines and bottoms, and a healthy wildlife population—without insects or weeds. No lake can meet all of these demands. This article will provide an overview of the physical, chemical, and biological components of lake ecosystems. An ecosystem is a system of interrelated organisms and their physical–chemical environment. It is impossible to alter one characteristic of a lake ecosystem without affecting some other characteristic of the ecosystem. The article will also explain how lake ecosystems get out of balance, and what can be also done to restore the balance. LAKE WATERSHEDS AND ZONES Lakes are receiving bodies—constantly receiving water, dis- solved materials, and particulates from their watersheds and from the atmosphere, and energy from the sun and wind. A watershed is the area which drains to a lake. Watersheds come in all sizes. For example, the watershed that drains to Beaver Lake in western Washington near Seattle is less than two square miles in area, whereas the Lake Washington watershed is 350 square miles. Lakes are sensitive to existing conditions in the surrounding watershed and atmosphere. Each lake has a unique watershed, size, and shape. The size and shape are often determined by the origin of the lake basin and, in turn, influence the lake’s productivity, water quality, habitat, and lifespan. The most common origin of lake basins in North America has been glacial activity such as the erosion of bedrock and deepening of valleys by expansion and recession of glaciers. Glacial lakes of Canada and the upper midwestern United States were formed about 8,000 to 12,000 years ago. For example, the Finger Lakes of upper New York State were formed when deep depressions left by receding glaciers filled with meltwater (U.S. EPA, 1990). The depressions left by melting ice blocks form kettle or “pothole” lakes. This type of lake is common through- out the upper midwestern United States, the eastern portion of the state of Washington, and large portions of Canada. Kettle lakes and their watersheds are popular home sites and recreational areas. The size and shape of kettle lake basins reflect the size of the original ice block and how deeply it was buried in the glacial debris (U.S. EPA, 1990). Some lakes are formed by volcanic activity; i.e., a volcano erupts creating a huge depression or caldera which then fills with water. Crater Lake in Oregon is an example of a volcanic lake. Movements of large segments of the earth’s crust cre- ated Reelfoot Lake in Tennessee, Lake Tahoe in California and Nevada, and many other lakes (U.S. EPA, 1990). Solution lakes are formed where groundwater has dis- solved limestone; this is the case for many Florida lakes. Other lakes originate from shifting of river channels. For example, oxbow lakes are stranded segments of meander- ing rivers. The persistence of dam-building beavers can also create lakes (U.S. EPA, 1990). A lake has four zones, each with different plants and lake that extends from the shoreline lakeward to the great- est depth occupied by rooted plants. By contrast, the pelagic zone is the open area of a lake from the edge of the littoral zone to the center of the lake. The benthic or profundal zone refers to the deep waters at the bottom of a lake where pho- tosynthesis does not occur because light does not penetrate. The marginal zone refers to the margins of the lake on the lake shoreline (U.S. EPA, 1990). Shallow lakes tend to be more biologically productive than deep lakes because of the large area of bottom sediments © 2006 by Taylor & Francis Group, LLC animals (Figure 1). The littoral zone is the portion of the LIMNOLOGY 609 Marginal zone Littoral zone Littoral zone Pelagic zone Profundal zone Pela g ic zone (open water) Benthic zone FIGURE 1 The location and nature of typical lake communities, habitats, and organisms. In addition to the lake’s watershed, all of these components are part of the lake ecosystem. (U.S. EPA, 1990) © 2006 by Taylor & Francis Group, LLC 610 LIMNOLOGY relative to the volume of water, more complete wind mixing of the lake water, and the large littoral zone along the lake perimeter that can be colonized by plants. Shallow lakes often have most of their plants in littoral areas and have little pelagic habitat. On the other hand, deep lakes have fewer areas that receive enough light for rooted aquatic plants to grow, and therefore have a high proportion of pelagic habitat and less littoral habitat. HYDROLOGIC CYCLE AND WATER BUDGET Since precipitation and surface water runoff have direct influ- ences on lake ecosystems, understanding the hydrologic cycle and water budget are key concepts in limnology. The hydro- logic (water) cycle refers to the circulation of water between the Earth’s surface and the atmosphere. This is powered by the sun. Water falls to Earth as precipitation. About 75 percent of the precipitation is returned to the atmosphere as vapor through direct evaporation and transpiration from both ter- restrial and aquatic plants during photosynthesis. The remain- ing 25 percent of the precipitation is stored in ice caps, drains directly off the land into lakes, streams, wetlands, rivers, and oceans, or infiltrates the soil and underlying rock layers and enters the groundwater system. Groundwater enters lakes and streams through underwater seeps, springs, or surface channels (Cooke et al. , 1986; U.S. EPA, 1990; Wetzel, 1983). Drainage lakes are formed primarily by inflowing rivers and streams. Therefore, their water levels vary with the sur- face water runoff from their watersheds. On the other hand, seepage lakes form where groundwater intersects with the land surface. Since seepage lakes are maintained primarily by groundwater inflow, their water levels fluctuate with sea- sonal variations in the local water table. For both drainage and seepage lakes, the balance between water inputs and outputs influences the supply of plant nutrients (nitrogen and phosphorus) to the lake and the lake’s hydraulic (water) residence time, thereby influencing the lake’s water quality and biological productivity (U.S. EPA, 1990). The hydraulic (water) residence time is the amount of time that water entering a lake will remain in it or the aver- age amount of time required to completely renew a lake’s water volume. The amount of water entering a lake from its watershed controls the volume of the lake. The hydraulic residence time is calculated by dividing the water volume of a lake by its flow rate, and varies greatly among lakes. For example, if a lake has a volume of 500 acre-feet and the out- flow rate is 10 acre-feet per day, then the hydraulic residence time would be 50 days. If the hydraulic residence time of a lake is 100 days to several years, this means that plant nutri- ents and pollutants remain in the water column long enough to degrade water quality and to allow plants to accumulate (U.S. EPA, 1990; Wetzel, 1983). Each lake has a water balance, in which water input ϭ water output ϩ the change in the amount of water stored in the lake. If inputs are greater than outputs, lake levels rise as water is stored in the lake. When outputs are greater than inputs, lake levels fall. This happens during summer droughts. A related concept is the lake water budget, which is a measure of the sources of water entering and flowing out of a lake over the course of a year. A lake’s water budget is affected by the hydrologic cycle, and the quantity and timing of water entering and leaving the lake. Types of data used in calculating a water budget include precipitation, stream flow into and out of the lake, and lake surface elevation (water level). Sources of water input or inflow include the lake inlet(s), precipitation, surface water runoff, point source discharges, and ground- water. Sources of water output or outflow include the lake outlet(s), evaporation, transpiration from lake plants, ground- water seepage, and water withdrawals for domestic, agricul- tural, and industrial purposes. The change in storage accounts for changes in surface elevation over the year. This change is positive if lake volume increases over the year, negative if lake volume decreases (U.S. EPA, 1990; Wetzel, 1983). Land use and geology of the surrounding watershed affect the water budget. For example, lakes in areas with permeable soils receive inflowing groundwater throughout the year. Lakes in areas with impervious surfaces can receive large volumes of stormwater runoff. PHOSPHORUS BUDGET AND LOADING Another important characteristic of lakes is the phosphorus budget, a measure of the sources of phosphorus entering and leaving the lake over the course of a year. Phosphorus is a nutrient that is essential in plant growth. The amount of phosphorus in a lake directly influences biological produc- tivity. The phosphorus budget will indicate if the phosphorus in the lake is coming from within the lake, from sources in the watershed, or from both internal and external sources. For a given lake, phosphorus inputs (inflow loading) –phosphorus outputs (outflow loading) ϩ net sedimentation ϩ change in storage. This means that phosphorus inputs to the lake equal phosphorus losses from the lake plus or minus the change in the total amount of phosphorus stored in the lake. Change in phosphorus storage within the lake equals the amount of phosphorus entering the lake minus the amount of phosphorus leaving the lake minus the net loss of phosphorus to the lake sediments. Sources of phosphorus inputs to a lake are the lake inlet(s), point sources discharging directly to the lake, precipitation, surface water runoff, leachate from mal- functioning shoreline septic tanks, other groundwater inputs, and migrant waterfowl wastes. Sources of phosphorus out- puts from a lake are the lake outlet(s), groundwater seep- age, and water withdrawals for domestic, agricultural, and industrial purposes (Cooke et al. , 1993b; U.S. EPA, 1990; Cottage Lake, near Seattle, Washington. As indicated in the figure, most of the phosphorus in the lake comes from the Daniels Creek inlet and from the lake sediments. Most of the phosphorus that leaves the lake does so via the lake outlet (KCM, 1994; Solomon et al. , 1996). Net sedimentation refers to the amount of phosphorous accumulated in lake bottom sediments, i.e., the difference in the amount of phosphorus that binds to the sediments and © 2006 by Taylor & Francis Group, LLC Wetzel, 1983). Figure 2 illustrates the phosphorus budget for LIMNOLOGY 611 the amount of phosphorus released from the sediments to the water column. In general, lake water quality will improve as the magnitude of sedimentation increases because higher sedi- mentation means there is less phosphorus in the water column to stimulate overgrowth of aquatic plants (for more details, see changes in the total amount of phosphorus in the lake water column between the beginning and end of the year. Phosphorus concentration differs from phosphorus load- ing. Phosphorus loading to a lake is calculated on the basis of the water budget for the lake and measured phosphorus concentrations in the lake, its inlets, its outlets, precipita- tion, surface water runoff, and groundwater. Loadings are based on concentrations and flow rates and most accurately express the relative impacts of various watershed sources on lake water quality. For example, a stream that is an inlet to a lake may have a high concentration of phosphorus. This does not necessarily mean that the stream is a major contributor to the lake phosphorus budget. If the stream has a low flow, it will contribute a relatively low annual phosphorus loading. The concept of phosphorus loading can be illustrated with Cottage Lake Creek 11% Internal 29% Daniels Creek 51% Basin C5- Subsurface 5% Basin C5-Surface 2% Precipitation 2% Inputs Outputs Sedimentation 36% Cottage Lake Cree k 64% FIGURE 2 Cottage Lake total phosphorus inputs and outputs. © 2006 by Taylor & Francis Group, LLC an analogy to a grocery bill as shown in Table 1. For each section on eutrophication). The change in storage accounts for 612 LIMNOLOGY grocery item, the cost is determined by multiplying the unit cost and the number of items purchased. The total grocery bill is the sum of the costs of all items. Likewise, for each source of phosphorus input to a lake, the phosphorus load- ing is determined by multiplying the flow rate for the source (lake inlet, groundwater, etc.) and its phosphorus concentra- tion over annual and seasonal periods. The total “phosphorus bill” (total phosphorus loading from all sources) is the sum of the loadings from each source. Phosphorus loadings change in response to season, storm events, upstream point sources, and land use changes. For example, converting an acre of forest into residential or commercial land typically increases the phosphorus loading to a lake in that watershed fivefold to twentyfold. This is because there will be increases in both water flow (runoff from the newly created impervious surfaces) and phosphorus concentration (deposition of phosphorus on impervious surfaces). An evaluation of phosphorus load- ings provides a basis for predicting lake responses to changes in land use. STRATIFICATION Many swimmers notice that when they dive into a lake during the summer, the deeper waters of the lake are much colder than the surface waters. This is due to stratification, an interesting temperature-related characteristic of most temperate climate lakes. TABLE 1 Phosphorus Loading Concept (U.S. EPA, 1990) Grocery Bill Phosphorus Loading Item Source Quantity Flow Unit Cost Concentration Cost of Item Loading from Source Total Cost of All Items Total Loading from All Sources EPILIMNION or mixed layer-warm (light) water HYPOLIMNION cool (heavy) water Dissolved Oxygen Temperature Profile Low High Low High METALIMNION THERMOCLINE FIGURE 3 Thermal stratification. © 2006 by Taylor & Francis Group, LLC LIMNOLOGY 613 Temperature-related characteristics of water have a large effect on the water quality and ecology of lakes. Water is at its densest at 4ЊC or 39ЊF, then expands (becomes less dense) until it freezes at 0ЊC (32ЊF). This anomalous expan- sion of water allows ice to float and form at the surface of lakes at 0ЊC (32ЊF) or less, and thermal stratification to occur during the warmer, summer weather. During spring and early summer, energy from the sun heats the upper water layer. The warmer, less-dense surface waters float on top of the cooler, denser bottom waters. This results in the upper layer, or epilimnion, becoming isolated from the separated by the middle layer, or metalimnion, where large temperature changes occur with changes in depth. The thermocline, which is located within the metalimnion, is a horizontal plane of water across the lake through the point of the greatest temperature change. The metalimnion pres- ents a physical barrier to the mixing of the epilimnion and hypolimnion. Since there is little or no exchange of water between the epilimnion and the hypolimnion, water quality can be quite different in each layer. In the temperate regions of the world where there are not strong contrasts in seasonal conditions (e.g., mild winters and summers), this type of thermal stratification is common during the summer and early fall. After the summer, the epilimnion tends to cool, and by late fall or early winter the temperature difference between the two water layers is small enough that the winds will mix the water throughout the lake, which will then remain fully mixed until the onset of stratification in late spring. Lakes that undergo this type of seasonal pattern (i.e., they stratify once and re-mix or turn over once each year) are called monomictic lakes. These include lakes in mountainous regions of the temperate zones, warm regions of the tem- perate zones, many coastal regions of North America and Europe, and mountainous areas of subtropical latitudes (U.S. EPA, 1990; Wetzel, 1983). By contrast, in the temperate regions of the world with strong contrasts in seasonal conditions (e.g., very cold win- ters and very hot summers), lakes undergo complete turn- over in the spring and fall separated by thermal stratification in the summer (i.e., warmer surface waters float on top of cooler bottom waters) and inverse thermal stratification in the winter. Ice cover forms and floats on the surface of such lakes under clam, cold conditions. Inverse stratification of water temperatures occurs under the ice, in which colder, less-dense water overlies warmer, more-dense water near the temperature of maximum density at 4ЊC. Some gradual heat- ing of the water occurs during the winter under ice cover. When the ice cover melts in the spring, the water column is nearly uniform in temperature. If the lake receives sufficient wind energy, as is usually the case, then the lake circulates completely and undergoes spring turnover. Stratification occurs during the warmer days of summer, with another complete turnover in fall. Lakes that undergo this type of seasonal pattern (i.e., they stratify twice and re-mix twice each year) are called dimictic lakes and include most lakes of the cool temperate regions of the world (U.S. EPA, 1990; Wetzel, 1983). LAKE BIOTA The types of organisms found in a lake may include phyto- plankton (algae), zooplankton, benthic infauna, fish, amphibia (such as tadpoles, frogs, and salamanders), reptiles (such as turtles and water snakes), and birds (waterfowl and shorebirds). Lake plants and animals are interrelated via a food chain. Algae are microscopic plants found in the lake water column. Algal species may occur in many different forms including filamentous, colonial, and single-celled. Algae are easily carried by wind-generated currents and will often accumulate in windward areas of the lake, forming surface scums. When algae populations increase rapidly, the algae can become a nuisance by forming high concentrations in the water column, or even surface accumulations, called algal blooms. Several different algal species can usually be found in a lake at any time of the year. A variety of environmental factors including light, temperature, and nutrient levels, affect phytoplankton production and the occurrence of algal blooms. Diatoms are algae that are golden in color and con- tain silica. They predominate in the spring and autumn due to their ability to reproduce and grow in cooler temperatures and less light. During the summer, increased water tempera- tures and available light create conditions that favor green algae or blue-green algae. Blue-green algae can form nui- sance blooms; they are particularly problematic because they will float to the surface, forming scums that affect the recreational uses and aesthetic qualities of a lake. In some lakes with high biological productivity, blue-greens domi- nate in spring, summer, and fall. In addition to algae, large vascular plants (plants with roots, stems, and leaves) or macrophytes are found in lakes. Macrophytes are classified as emergent, floating, or sub- mersed. Emergent plants grow on the shoreline and include cattails, irises, and purple loosestrife. Floating plants are plants that float on the surface of the lake. They can be rooted in the lake bottom such as water lilies or watershield or free- floating such as duckweed. Submersed plants are rooted plants that live below the lake surface and include pondweed types and common examples of plants associated with each type (Washington State Department of Ecology, 1994). Some macrophytes are native to the particular lake and geographic region; others, called exotics, have been imported or are transported to the lake from other lakes. For example, native macrophytes in lakes of the Pacific Northwest region of the United States include cattails, yellow water lilies, and pondweed. Exotic or non-native plants in Pacific Northwest lakes include purple loosestrife, white water lilies, and Eurasian watermilfoil. Some non-native plants are invasive, crowding out native plants and not providing useful habitat for fish and wildlife. © 2006 by Taylor & Francis Group, LLC lower layer, or hypolimnion (Figure 3). The two layers are and water weed ( Elodea ). Figure 4 illustrates the community 614 LIMNOLOGY Aquatic plants provide many benefits, including sedi- ment and shoreline stabilization; food source and habitat for benthic invertebrates, fish, and wildlife; oxygenation of the water column; and aesthetics. Most rooted macrophytes obtain their nutrients from lake sediments rather than the water column, and take up phosphorus that would otherwise have been available for algal growth, thus preventing the overgrowth of algae. However, when there are too many aquatic plants, par- ticularly non-native plants, the advantages turn into disad- vantages. When a lake is shallow and nutrient-enriched, then there can be too many macrophytes. Too many aquatic plants can decrease the quality of fish and wildlife habitat, interfere with beneficial uses of a lake such as swimming and boating, and even create safety problems, i.e., swimmers can become entangled in milfoil and other plants. When the plants decay, they deplete the lake waters of oxygen and release nutrients into the water column which can promote algal growth. Too many macrophytes can make a lake look unsightly. The advantages and disadvantages of aquatic plants are opposite sides of the same coin; it’s a matter of degree and balance. Many types of animals are found in lakes. Zooplankton are microscopic animals found in the lake water column. Examples are rotifers and water fleas, e.g., Daphnia. They are visible to the naked eye on close inspection of a glass of lake water. Zooplankton are important in the food web of a lake because they eat algae and, in turn, are eaten by plank- tivorous fish. The types and number of zooplankton pres- ent are also indicative of lake water quality. Generally, large grazing species improve water quality by eating algae. On the other hand, a general decrease in the size of zooplank- ton species, with their reduced capacity to graze the phyto- plankton, is a response to the greater availability of bacterial detritus resulting from the relatively ungrazed algae (Welch, 1992). Therefore, the presence of larger zooplankton in a lake usually indicates good water quality, while the presence of smaller zooplankton generally indicates more nutrient- rich waters. Benthic infauna are small invertebrate animals such as molluscs, worms, and midges that live in the bottom sedi- ments of lakes. They feed on detritus in the sediments and recycle nutrients to the water. The species of benthic animals found in a given area are usually indicative of the surround- ing water quality. Some invertebrates, such as mayflies, are intolerant of low dissolved oxygen conditions; their presence in large numbers in lake ecosystems indicates good water quality. Other invertebrates, such as oligochaetes and chi- ronomids, are more tolerant of low dissolved oxygen condi- tions; their presence in large numbers in a lake may indicate the presence of pollutants or degraded water quality. The greatest density and diversity of benthic inverte- brates is usually found in the littoral zone of a lake, where ample vegetation and oxygen are present. The benthic com- munities, in turn, provide food for larger invertebrates, fish, amphibians, and birds. The types of fish found in a lake are influenced by water temperature and dissolved oxygen levels. Fish such as perch, bass, and smelt are warmwater fish and thrive in lakes where the summer water temperature exceeds 65ЊF. The dissolved oxygen level needs to be at least five parts per million (ppm) in order for the fish to remain healthy. Coldwater fish such as salmon and trout are found in lakes where the summer water temperature is less than 65ЊF. The dissolved oxygen level needs to be at least seven ppm for these fish. If the summer water temperature is too high, the dissolved oxygen level is often too low to support healthy fish populations. Each organism in a lake is dependent on other organisms for its food. Each lake has a natural food chain. Algae are eaten by zooplankton. In some lakes, the efficient grazing of zooplankton by algae can help to maintain water clarity. Zooplankton are eaten by planktivorous fish such as long- fin smelt and perch. Planktivorous fish are eaten by larger, piscivorous fish such as northern squawfish and largemouth bass. The larger fish are eaten by birds and by mammals, found in many lakes. Free-floating Emergent Planktonic algae Submergents Rooted, floating-leaved FIGURE 4 Macrophyte community types. © 2006 by Taylor & Francis Group, LLC including humans. Figure 5 illustrates the aquatic food chain LIMNOLOGY 615 The food chain concept involves the flow of energy among the lake organisms and the recycling of nutrients. Each trophic level (food chain level) transfers only 10 to 20 percent of the energy received up the chain to the next trophic level (Kozlovsky, 1968; Gulland, 1970). This means that a few large piscivorous fish depend on a large supply of smaller planktivorous fish which depend on a very large supply of zooplankton which depend on a successively much larger base of photosynthetic production by phyto- plankton and other aquatic plants. By constantly producing wastes and eventually dying, all of these organisms provide nourishment to detritus-eating organisms in the sediments, which obtain their energy by decomposing organic matter. Organic matter decomposition results in the recycling of nutrients that are required for further plant production (U.S. EPA, 1990). PISCIVOROUS FISH EAT EAT EAT USE NUTRIENTS RECYCLE NUTRIENTS ALGAE PLANKTIVOROUS FISH ZOOPLANKTON BENTHIC ORGANISMS MICROSCOPIC 1/10 IN 6"-1 FT 1–2 FT FIGURE 5 Aquatic food chain (U.S. EPA, 1990). © 2006 by Taylor & Francis Group, LLC 616 LIMNOLOGY When one level of the food chain of a lake is altered, it affects all other levels, sometimes positively and sometimes adversely. For example, in Lake Washington, Daphnia popu- lations increased in the 1970s.Why? The longfin smelt popu- lation increased in the 1960s when flood control activities in the main inlet stopped and spawning beds were no longer damaged. Longfin smelt feed on a large crustacean called Neomysis which feeds on Daphnia. Predation on Daphnia was thereby reduced. This had a positive effect on Lake Washington because Daphnia grazed on the algae, resulting in improved water clarity in the lake. Another example of food chain manipulation is stock- ing a lake with piscivorous fish. When the fish are removed by anglers, there will be more planktivorous fish which will result in a decreased zooplankton population. Fewer zoo- plankton will mean more algae in the lake, which could have adverse effects on water clarity in the lake. In sum, altering one part of a lake’s ecosystem has repercussions throughout the ecosystem. TROPHIC STATUS/EUTROPHICATION Lakes are characterized according to their level of biological productivity, or trophic status. The trophic status of a lake depends on the concentration of chlorophyll a (the pigment found in green plants that traps energy from the sun to enable the plants to produce their own food by the process of photo- synthesis), frequency of algal blooms, the concentrations of nutrients, particularly phosphorus, and water clarity (trans- parency). The phosphorus concentration determines how many algae and other plants will grow in the lake. The clar- ity of the water is influenced by a variety of factors including algae, turbidity from sediments or other suspended particles, and the natural color of the water in the lake. Water clarity is measured with the use of a Secchi disk, a 20-centimeter plastic or metal disk that is divided into alternating black and white quadrants. The disk is lowered into the water until the observer can no longer see it. The distance between the lake surface and the point at which the disk disappears from view is called the “Secchi transparency” or “Secchi depth” of the lake. Three trophic classifications are commonly used for lakes. An oligotrophic lake is one in which there is clear water, low levels of chlorophyll a and nutrients, and hence little aquatic life. Oligotrophic lakes tend to be found in alpine and other wilderness areas. The lakes are beautiful to look at and are fine swimming and boating lakes, but are not good fishing lakes unless they are stocked with fish. There are few naturally occurring fish in oligotrophic lakes because there are few plants or insects for fish to eat. At the other end of the scale are eutrophic lakes. A eutro- phic lake has murky water, high levels of chlorophyll a and nutrients, and is full of aquatic life. Many lakes in urban and suburban areas are eutrophic, as evidenced by algal blooms. A mesotrophic lake is in between, i.e., is moderately trans- parent, with moderate levels of chlorophyll a and nutrients, and some aquatic life. Transparency, chlorophyll a and total phosphorus (both organic and inorganic forms of phosphorus) are most fre- quently used to assign trophic status to lakes. The general relationship between these lake water quality parameters and trophic status index (TSI) is summarized in Table 2. A lake’s natural level of productivity is determined by a combination of factors, including the geology and size of the watershed, depth of the lake, climate, and water sources entering and leaving the lake. Some lakes are naturally eutro- phic based on their inherent physical attributes and watershed characteristics. Increases in a lake’s natural productivity over time, a pro- cess called eutrophication, occurs naturally in some lakes, and may be accelerated in others by human activities. For many small lakes, natural eutrophication typically occurs over hun- dreds or thousands of years, and is hence not observable in a single lifetime. What is observable in a single lifetime is the human-induced, or cultural eutrophication of lakes. Our land-based activities, including home-building, agriculture, forestry, resource extraction, landscaping, gardening, and animal husbandry, all contribute nutrients and sediments to surface waters, which in turn contribute to increasing a lake’s biological productivity. Land erosion and forest clearcutting contribute sediments to lakes. Surface water runoff from impervious surfaces such as construction sites, parking lots, and pavement contributes nutrients and pollutants to lakes. Agricultural practices such as horses grazing near lakes, cows wandering in streams, and extensive pesticide use contribute nutrients and toxic pollutants to lakes. If oil or other toxic chemicals are poured down storm drains, these end up in the nearby lake, stream, or bay. Gardening chemicals such as fer- tilizers and household toxic chemicals can end up in storm TABLE 2 Trophic status and associated values (Carlson, 1977; Cooke et al., 1993b; Porecella et al., 1980) Trophic Status Transparency (meters) Chl. a (mg/L) Total Phosphorus (mg/L) TSI (average) Oligotrophic Ͼ4 Ͻ3 Ͻ4 Ͻ40 Mesotrophic 2–4 3–9 14–25 40–50 Eutrophic Ͻ2 Ͼ9 Ͼ25 Ͼ50 mg /L ϭ micrograms per liter (parts per billion). © 2006 by Taylor & Francis Group, LLC LIMNOLOGY 617 drains and thus in lakes. Failing septic systems can discharge nutrients from raw sewage to lakes. The result of all these inputs of phosphorus, nitrogen, sediment, and organic matter in large algal blooms which are unsightly and can severely restrict lake beneficial uses including swimming, fishing, boating, and aesthetic appre- ciation. Beneficial uses of a lake may also be degraded by other water quality problems related to eutrophic conditions, including low dissolved oxygen levels, fish kills, algal toxic- ity, and excessive aquatic macrophyte growth. The level of dissolved oxygen in lakes is one determi- nant of the habitat available to aquatic organisms. Oxygen is added to a lake from exposure to the air, and by the contri- bution or aquatic plants through photosynthesis. Oxygen is removed from a lake by the respiration of aquatic organisms and plants, and the bacterial decomposition of organic matter in the water and sediments. Eutrophic lakes with large algal blooms are characterized by high phosphorus concentrations and low dissolved oxygen concentrations in the lake hypo- limnion in the summer. This happens because decaying algae and other plants fall to the bottom of the lake where they contribute phosphorus and remove oxygen. Photosynthesis does not take place in the hypolimnion of a eutrophic lake because light does not penetrate to that depth; hence, the oxygen that is being depleted is not replaced. Anoxic (lack of oxygen) conditions at the water—sediment interface on the lake bottom usually increase the potential for nutrient release by converting iron phosphate in the sediments from a water-insoluble to a water-soluble form. The very low dissolved oxygen levels in the hypolim- nion of eutrophic lakes during the summer months may be too low to support coldwater fish such as salmon and trout. The salmon and trout would then move to the lake epilim- nion, but the water temperatures may be too high for them in the surface waters. Eutrophic conditions in lakes often lead to decreased quantity and quality of fish habitat and stressed fish populations. RESTORING BALANCE TO LAKE ECOSYSTEMS Management of Eutrophic Lakes When a lake is eutrophic with unsightly algal blooms, water quality problems, and impaired beneficial uses, its ecosys- tem is out of balance. Lake restoration involves reducing the impact of human activities on lake water quality, with the goal of decreasing biological productivity and improv- ing water quality and associated beneficial uses of the lake. Several methods are available to accomplish this goal. Each method has its advantages and drawbacks. In order to determine the most effective method(s) to use in a given lake, it is first necessary to be knowledgeable about the physical, chemical, and biological components of the lake’s ecosystem. This can be accomplished through one or two years of monitoring parameters such as transparency, lake temperature, acidity, alkalinity, dissolved oxygen, lake level, amount of precipitation, nutrient levels, chlorophyll a, fecal coliform bacteria (a group of bacteria associated with human, other mammal, and bird wastes), algae, zooplankton, benthic infauna, an fish. Once monitoring data are obtained, they need to be summarized and pollution sources priori- tized for control. Most lake water quality problems are associated with an overabundance of nutrients, which results in excessive plant growth. In managing such water quality problems, it is impor- tant to assess what nutrient limits plant growth. In eutrophic lakes, phosphorus is often the limiting nutrient; this means that the amount of phosphorus in the lake will determine the amount of plant growth. Therefore, most lake management strategies focus on reducing phosphorus loading. If the lake’s phosphorus budget shows that most of the phosphorus is coming from within the lake, in-lake restora- tion techniques should help to reduce phosphorus levels and make the lake less eutrophic. On the other hand, if most of the phosphorus is coming from the watershed (this is often true of small lakes with very large watersheds), then the focus should be on watershed best management practices (BMPs) to control sources of nutrients. In some lakes, phosphorus comes from within the lake and the watershed, so both types of actions are needed. Watershed Best Management Practices Implementation of watershed best management practices (BMPs) improves water quality by reducing the quantity of pollutants enter- ing the lake. Most pollutants within a watershed result from human activities. Pollutants originating on each parcel of land within a watershed can collectively become a serious threat to the receiving water quality. BMPs are structural and nonstructural methods, including common sense “house- keeping measures,” used to prevent or reduce pollution by controlling erosion, surface water runoff, sources of nutri- ents, and sources of toxic chemicals. Watershed BMPs can be basin-wide or can target management of developed prop- erty. These measures can include native plant revegetation of lake shorelines, retention/detention ponds and biofiltration swales for stormwater treatment, and homeowner/business owner BMPs to enhance water quality through better land- scaping methods, alternative household and gardening prac- tices, better animal-keeping practices, drainage controls, and septic system maintenance and repairs. Local and state agencies can work in partnership with lake associations and other citizen groups in a watershed to educate residents and business owners about BMPs that are inexpensive, easy to implement, and make a difference in protecting lake water quality and aquatic biota. Ideally, this environmental educa- tion should include hands-on water quality activities (e.g., storm drain stenciling, lakeshore revegetation) and habitat monitoring activities for community volunteers including schoolchildren because these activities impart a sense of lake stewardship to people who live on or upstream of a lake. Following is a discussion of each type of BMP. Many lakes have no native plants growing on the shore- line; houses may have manicured lawns leading to the water’s edge. Where shoreline vegetation is absent, surface water runoff enters the lake directly, degrading lake water quality. © 2006 by Taylor & Francis Group, LLC [...]... hand-held, portable and mounted on boats, or specialized underwater cutters using a sickle to cut LIMNOLOGY weeds in water as shallow as ten inches and as deep as five feet Cutting results in immediate removal of nuisance submerged plants Hand-pulling, hand-cutting, and mechanical cutting are inexpensive, easy to implement around docks and swimming areas, environmentally safe, and allow removal of undesirable... (Gibbons, Gibbons, and Systma, 1994), and Crary WeedRoller Pilot Project Report (Cooke, 1996) Physical Methods Physical methods of reducing the amount of aquatic plants in a lake include hand-pulling, hand and mechanical cutting, mechanical harvesting, bottom barriers (sediment covers), water level drawdown, water column dyes, rotovating, diver-operated suction dredging, and weed rolling Hand-pulling aquatic... thereby preventing erosion; and moderating impacts of surface water runoff by filtering out suspended solids, nutrients, and toxic chemicals Alternatives to standard lawn maintenance and landscaping practices include minimal use of fertilizers, reduction in lawn size, regular thatching and aeration, incorporation of native plants in new landscaping, soil enhancement through mulching and composting rather... particularly important in the case of invasive, non-native plants such as Eurasian watermilfoil Environmental impacts of hand-pulling and cutting include short-term, localized increases in water turbidity and some disruption of benthic infauna Mechanical harvesting is a short-term technique to temporarily remove plants that interfere with recreational uses or aesthetic enjoyment of a lake Mechanical harvesters... then suctioned up and carried back to a barge through hoses operated by the diver Plants parts are sieved out on the barge and retained for later off-site disposal The water-sediment slurry can be discharged back to the water or piped off-site for upland disposal Efficiency of plant removal is dependent on sediment condition, density of aquatic plants, and underwater visibility Diver-operated suction... fisheries department of the particular state in which the lake is located and will depend on the amount and type of plants in the lake as well as spring and summer water temperatures Survival rates of the fish will vary depending on fish disease and presence of predators such as ospreys and otters Grass carp are inexpensive compared to some other aquatic plant control methods and offer long-term control However,... feeding of waterfowl by lakeside residents should be discouraged Pet and domestic animal waste should be properly disposed of away from a lake and surface water pathways that reach a lake Business owners should be educated about BMPs, such as proper storage of toxic chemicals and proper maintenance and repair of oil-water separators in order to prevent the discharge of petroleum hydrocarbons, metals, and. .. more specialized equipment and materials are required and the process is much more costly Short-term environmental impacts include localized increases in turbidity and release of nutrients and other contaminants from the sediments Some sediments, benthic LIMNOLOGY infauna, and non-target plants may also be inadvertently removed during this process A new mechanical method of controlling aquatic plant... with glyphosate, long-term plant control can be achieved with the use of fluridone Fluridone has a very low order of toxicity of aquatic animals and humans Disadvantages of its use are that it is very slow-acting and therefore not effective in flowing water Fluridone can drift out of the treatment zone, thereby affecting non-target plants Consequently, it is most suitable for whole-lake treatments, not... around docks and beaches Hand and mechanical cutting differ from hand-pulling in that plants are cut below the water surface (roots are usually not removed) with scythes, rakes or other specialized devices that can be pulled through the weed beds by boat or people Rakes can be equipped with floats to allow easier plant and fragment collection Mechanical cutters can be battery-operated and hand-held, . P.J. and D.R. Knauer. 1984. “Long-Term Evaluation of Three Alum-Treated Lakes.” Lake and Reservoir Management, pp. 513–517. EPA-440/ 5-8 4-0 01, U.S. Environmental Protection Agency, Washing- ton,. docks and swim- ming areas, environmentally safe, and allow removal of undesirable aquatic plants while leaving desirable plants. On the other hand, these methods are labor-intensive, time- consuming,. the case of invasive, non-native plants such as Eurasian watermilfoil. Environmental impacts of hand-pulling and cutting include short-term, localized increases in water turbidity and some