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It is beyond the scope of any monitoring program to sample for every condition that can be found in a lake. Therefore, an initial task is to decide where to focus sampling efforts.
Of all the water quality issues facing lakes and reservoirs nationwide, it is those conditions associated with a phenomenon known as eutrophication that cause the greatest concern among lake users. Eutrophication is a term used to describe the aging of a lake. This aging process results from the accumulation of nutrients, sediments, silt, and organic matter in the lake from the surrounding watershed.
Eutrophication can be accelerated when human activity occurs in the watershed. If proper controls are not in place, pollutants from agricultural, urban, and residential developments can easily be carried into lakes and their tributaries.
Symptoms of human-induced (or cultural) eutrophication are:
Although related, each condition nevertheless has a unique set of parameters that characterize its attributes. It is important to remember that sampling for one condition will not necessarily yield information about another. If, for example, a lake has an aquatic plant problem, a monitoring program that focuses on algae will not provide the necessary answers to solve that problem.
The reader should be aware that there are several other lake conditions that could be a potential focus for a monitoring program. Four notable candidates are:
Algae are photosynthetic plants that contain chlorophyll and have a simple reproductive structure but do not have tissues that differentiate into true roots, stems, or leaves. They do, however, grow in many forms. Some species are microscopic single cells; others grow as mass aggregates of cells (colonies) or in strands (filaments). Some even resemble plants growing on the lake bottom.
The algae are an important living component of lakes. They:
There are a number of environmental factors that influence algal growth. The major factors include:
These environmental changes may or may not present optimal habitats for growth or even survival for any particular species of algae. Consequently, there is usually a succession of different species in a lake over the course of a year and from year to year.
The Overgrowth of Algae
Excessive growth of one or more species of algae is termed a bloom. Algal blooms, usually occurring in response to an increased supply of nutrients, are often a disturbing symptom of cultural eutrophication.
Blooms of algae can give the water an unpleasant taste or odor, reduce clarity, and color the lake a vivid green, brown, yellow, or even red, depending on the species. Filamentous and colonial algae are especially troublesome because they can mass together to form scums or mats on the lake surface. These mats can drift and clog water intakes, foul beaches, and ruin many recreational opportunities.
Citizen programs designed to monitor the algal condition of a lake usually require to measure:
Aquatic plants have true roots, stems, and leaves. They, too, are a vital part of the biological community of a lake. Unfortunately, like algae, they can overpopulate and interfere with lake uses.
Aquatic plants can be grouped into four categories:
Aquatic plants are effective in breaking the force of waves and thus reduce shoreline erosion. Emergents serve to trap sediments, silt, and organic matter flowing off the watershed. Nutrients are also captured and utilized by aquatic plants, thus preventing them from reaching algae in the open portion of a lake.
Factors that Affect Aquatic Plant Growth
There are many factors that affect aquatic plant growth including:
Excessive growth of aquatic plants is unsightly and can severely limit recreation. Submergents and rooted floating-leaf plants hinder swimmers, tangle fishing lines, and wrap around boat propellers. Fragments of these plants can break off and wash up on beaches and clog water intakes.
For many species, fragmentation is also a form of reproduction. An overgrowth problem can quickly spread throughout a lake if boat propellers, harvesting operations, or other mechanical actions fragment the plants, allowing them to drift and settle in new areas of the lake.
Free-floating plants can collect in great numbers in bays and coves due to prevailing winds. Emergent plants can also be troublesome if they ruin lake views and make access to open water difficult. In addition, they create areas of quiet water where mosquitoes can reproduce.
Citizen monitoring programs designed to characterize the aquatic plant condition usually:
The amount of oxygen in the water is an important indicator of overall lake health. In fact, much information can be learned about a lake by examining just this parameter.
Oxygen plays a crucial role in determining the type of organisms that live in a lake. Some species, such as trout, need consistently high oxygen concentrations to survive. Other aquatic species are more tolerant of low or fluctuating concentrations of oxygen.
Oxygen is supplied naturally to a lake by:
Vertical mixing of the water, aided by winds, distributes the oxygen within the lake. In this manner, it becomes available to the lake's community of oxygen-breathing organisms.
Water temperature affects the capacity of water to retain dissolved oxygen. Cold water can hold more oxygen than warm water. Therefore, a lake will typically have a higher concentration of dissolved oxygen during the winter than the summer.
Factors that Determine Dissolved Oxygen Concentration
There are a number of factors that determine the amount of oxygen found in a lake including:
Oxygen is essential for aquatic life. Without oxygen, a lake would be an aquatic desert devoid of fish, plants, and insects. For this reason, many experts consider dissolved oxygen to be the most important parameter used to characterize lake water quality.
Algae and aquatic plants produce oxygen as a by-product of photosyn thesis but also take in oxygen for respiration. Respiration occurs all the time, but photosynthesis occurs only in the presence of light. Consequently, a lake that has a large population of algae or plants can experience a great fluctuation in dissolved oxygen concentration during a 24-hour period.
During a sunny day, photosynthesis occurs and can supersaturate the water with oxygen. At night, plants no longer produce oxygen; however, they continue to consume oxygen for respiration. In some lakes after dark, dissolved oxygen can be depleted by the plants at a rate faster than it can be diffused into the lake from the atmosphere. In extreme cases, the oxygen in the water can become depleted. This lack of oxygen will cause fish and other aquatic organisms to suffocate.
Extreme fluctuations of dissolved oxygen concentrations place great stress on the oxygen-breathing creatures in the lake. Only tolerant species can survive in this type of environment. Unfortunately, tolerant species are usually the least desirable for recreational purposes. Carp are an example of a tolerant fish. Trout, on the other hand, are highly intolerant of fluctuating oxygen levels.
In addition to the impact on living organisms, the lack of oxygen in a lake also has profound effects on water chemistry and eutrophication. To explain this situation, one must understand the temperature cycle, how it affects water density, and the phenomena of lake overturn and thermal stratification .
The Temperature Cycle
Most U.S. lakes with a depth of 20 feet or more stratify into two tempera ture-defined layers during the summer season. The water in the upper layer (epilimnion) is warm, well lit, and circulates easily in response to wind action. The deep layer (hypolimnion) is dark, cold, more dense, and stagnant.
These two layers are separated by a transition zone ( metalimnion) where temperatures change rapidly with depth. The metalimnion functions as a barrier between the epilimnion and the hypolimnion.
The magnitude of the temperature difference between the two layers defines how resistant they are to mixing. A large temperature difference means that the layers are stable and that it would take a great deal of wind energy to break down the stratification and mix the layers.
In the fall, lowered air temperatures eventually cool the waters in the upper layer to a point where they become the same temperature (and density) as the lower layer. At this time, the resistance to mixing is removed and the entire lake freely circulates in response to wind action. This action is known as fall overturn.
Layers again form during the winter. However, it is the upper zone that is slightly colder than the deeper layer. In the spring, increasing air temperatures warm the upper layer to a point that it becomes the same temperature as the bottom zone. Wind action then mixes the entire lake and spring overturn occurs.
Oxygen Depletion in the Lower Layer
Bacteria, fungi, and other organisms living on the lake bottom break down organic matter that originates from the watershed and the lake itself. Algae, aquatic plants, and animals all provide food for these decomposers when they excrete, shed, and die. Like higher forms of life, most decomposers need oxygen to live and perform their important function.
The mixing action of spring and fall overturn distributes oxygen through out the water column. During summer stratification, however, the lower layer is cut off from the atmosphere. There is also usually too little light to support photosynthesis by algae or aquatic plants. Therefore, with no supply source, what oxygen there is in the lower layer can be progressively depleted by an active population of decomposers.
When the dissolved oxygen concentration is severely reduced, the bottom organisms that depend on oxygen either become dormant, move, or die. Fish and other swimming organisms cannot live in the lower layer. As a result, trout and other game fish that require deep, cold water and high oxygen levels may be eliminated from the lake altogether.
Other Problems Caused by Lower Layer Oxygen Depletion
Oxygen depletion in the lower layer occurs "from the lake bottom up." This is because most decomposers live in or on the lake sediments. Through respiration, they will steadily consume oxygen. When oxygen is reduced to less than one part per million on the lake bottom, several chemical reactions occur within the sediments. Notably, the essential plant nutrient, phosphorus, is released from its association with sediment-bound iron and moves freely into the overlying waters.
If wind energy breaks down a lake's stratification, this phosphorus may be transported into the upper layer where it can be used by algae and aquatic plants. This internal pulse of phosphorus (often termed internal loading) can thus accelerate algal and aquatic plant problems associated with cultural eutrophication.
Iron and manganese are also released from the sediments during anoxic (no oxygen) periods. These elements can cause taste and odor problems for those who draw water from the lower layer for drinking or domestic purposes.
Fortunately, many of the negative effects of anoxic conditions are eliminated during overturn. As the waters of the lake are mixed and re-oxygenated, many of the constituents released from the sediments chemically change and precipitate back on to the lake bottom. Others are reduced in concentration by their dilution into the waters of the entire lake.
Overturns do also bring nutrients back up to the surface where they become available to the algae. Therefore, it is not unusual to see algal blooms associated with overturns.
Citizen monitoring programs designed to characterize the dissolved oxygen condition in lakes have:
Other Lake Conditions
The gradual filling-in of a lake is a natural consequence of eutrophication. Streams, stormwater runoff, and other forms of moving water carry sand, silt, clays, organic matter, and other chemicals into the lake from the surrounding watershed. These materials settle out once they reach quieter waters. The rate of settling is dependent on the size of the particles, water velocity, density, and temperature.
The sediment input to a lake can be greatly accelerated by human develop ment in the watershed. In general, the amount of material deposited in the lake is directly related to the use of watershed land. Activities that clear the land and expose soil to winds and rain (e.g., agriculture, logging, and site development) greatly increase the potential for erosion. These activities can significantly contribute to the sediment pollution of a lake unless erosion and runoff is carefully managed.
Sediment material from the watershed tends to fertilize aquatic plants and algae because phosphorus, nitrogen, and other essential nutrients are attached to incoming particles. If a large portion of the material is organic, dissolved oxygen can decrease as a result of respiration of decomposers breaking down the organic matter.
Sedimentation also can ruin the lake bottom for aquatic insects, crustaceans, mussels, and other bottom-dwelling creatures. Most important, fish spawning beds are almost always negatively affected.
The input of sediments to a lake makes the basin more shallow, with a corresponding loss of water volume. Thus, sedimentation affects navigation and recreational use and also creates more fertile growing space for plants because of increased nutrients and exposure to sunlight.
Monitoring programs that focus on sedimentation generally measure sediment buildup over time at a few select sites (e.g., near the mouth of a stream).
Not all sediment particles quickly settle to the lake bottom. The lighter, siltier particles often stay suspended in the water column or settle so lightly on the bottom that they can be easily stirred up and resuspended with even slight water motion. This causes the water to be turbid and brownish in appearance. Sediment blocks light from penetrating into the water column. It also interferes with the gills of fish and the breathing mechanism of other creatures.
Programs that focus on sediment turbidity will usually monitor water clarity and the amount of suspended solids in the water.
Acidity is a measure of the concentration of hydrogen ions on a pH scale of 0 to 14. The lower the pH, the higher the concentration of hydrogen ions. Substances with a pH of 7 are neutral. A reading less than 7 means the substance is acidic. If the pH is greater than 7, it is basic (alkaline). Because the pH scale is logarithmic, each whole number increase or decrease on the 0 to 14 scale represents a 10-fold change in acidity.
Acidic lakes occur in areas where the watershed soils have little natural buffering capacity. Acidic deposition (commonly called acid rain) and other artificial or natural processes can further contribute to lake acidity. Most aquatic plants and animals are sensitive to changes in pH. Thus, acidic lakes tend to be clear because they contain little or no algae. Fish are also thought to be negatively affected by lowered pH. In fact, many acidic lakes have no fish populations.
Acid rain occurs in areas where the combustion of fossil fuels increases the concentration of sulfuric and nitric acids in the atmosphere. These acids can be transported thousands of miles and eventually deposited back to earth in rain or snow.
Acidity may also enter lakes from drainage that passes through naturally acidic organic soils. These soils may become more acid through land use practices such as logging and mining.
Acidic drainage from abandoned mines affects thousands of miles of streams and numerous lakes throughout Appalachia. Acid mine drainage also occurs in the Midwest coalfields of Illinois, Indiana, and Ohio, and in coal and metal mining areas of the western United States.
Monitoring programs that focus on acidification generally sample for pH and alkalinity. These are two measurements that provide an indication of the acid/base status and the buffering capacity of the water, respectively.
Bacteriological Conditions at Beaches
The sanitary quality of bathing beaches is a special concern to swimmers. There are a wide variety of disease-causing bacteria, viruses, parasites, and other microorganisms that can enter the water and be trans mitted to humans. Some are indigenous to natural waters. Others are carried from wastewater sources including septic systems and runoff from animal and wildfowl areas. Infected swimmers themselves are also a source of pathogens.
The ideal way to determine potential health hazards at natural bathing beaches is to test directly for disease-causing organisms. Unfortunately, the detection of these organisms requires very complex procedures and equipment. In addition, there are hundreds of different kinds of pathogens; to test for each one would be impractical. Most public health officials, therefore, simply test for the presence of an indicator organism. The relative abundance of the indicator organism in a sample can serve as a warning of the likely presence of other, more dangerous pathogens in the water.
Monitoring programs that focus on bacterial quality at bathing beaches as the lake condition to be monitored generally sample for one or more indicator organisms throughout the swimming season.
The indicator organisms most often chosen for monitoring are fecal coliform bacteria or enterococci bacteria. The latter group of bacteria is more disease-specific and may be most appropriate for routine sample analy sis. Usually, health departments recommend weekly sampling for bathing areas.