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The water quality of a lake is often described as a reflection of its surrounding watershed. The term lake collectively refers to reservoirs (man-made impoundments), natural lake systems and smaller ponds (man-made or naturally created). Water from the surrounding watershed enters a pond or lake as streamflow, surface runoff and groundwater. The water quality of these water sources is greatly influenced by the characteristics of the watershed such as, geology, soils, topography and land use. Of these characteristics, changes in land use (e.g., forested, agriculture, silviculture, residential, commercial, industrial) can significantly alter the water quality of ponds and lakes.
Nutrients (e.g., phosphorus, nitrogen, carbon, silicon, calcium, potassium, magnesium, sulfur, sodium, chloride, iron) are primarily transported to ponds and lakes via streamflow, surface runoff and groundwater, while sediments are mainly conveyed by streamflow and surface runoff. As streamflow and surface runoff enter a pond or lake, their overall velocity decreases, which allow transported sediments to settle to the pond or lake bottom. Many of these incoming nutrients may be bound to sediment particles and subsequently will also settle to the pond or lake bottom. Very small sediment particles such as, clays, may resist sedimentation and subsequently pass through the lake without settling.
Once within the pond or lake, water quality is further modified through a complex set of physical, chemical and biological processes. These processes are significantly affected by the pond’s or lake’s morphological characteristics (morphology). Some of the more important morphological characteristics of ponds and lakes are surface area, shape, depth, volume and bottom composition. In addition, the hydraulic residence time (i.e., the pond’s or lake’s flushing rate) also greatly affects these processes and is directly related to the pond’s or lake’s volume and the annual volume of water flowing into the pond or lake.
With respect to nutrients, phosphorus and nitrogen are generally considered the most important nutrients in freshwater ponds and lakes. Phosphorus and, to a lesser degree, nitrogen typically determine the overall amount of aquatic plants present. Aquatic plants adsorb and convert available nutrients into energy, which is then used for additional growth and reproduction. In ponds and lakes, aquatic plants are mainly comprised of phytoplankton (free-floating microscopic plants or algae) and macrophytes (higher vascular plants). The most readily available form of phosphorus is dissolved orthophosphate (analytical determined as dissolved reactive phosphorus), while ammonia (NH3-N) and nitrate (NO3-N) are the most readily available forms of nitrogen.
The transfer and flow of energy in ponds and lakes is ultimately controlled by complex interactions between various groups of aquatic organisms (both plants and animals). A simplistic diagram of these interactions among aquatic organisms is shown as Figure 1. In Figure 1, algae (phytoplankton) and aquatic macrophytes (plants) capture energy from the sun and convert this energy into chemical energy through the process known as photosynthesis. During photosynthesis, carbon dioxide, nutrients, water and captured sunlight energy are used to produce organic compounds (chemical energy), which are then used to support further growth and reproduction.
Energy continues to flow upward through the food web. Algae are primarily grazed upon by zooplankton. Zooplankton are tiny aquatic animals that are barely visible to the naked eye. Next, zooplankton serve as prey for planktivorous (plankton-eating) fish and larger invertebrates (macroinvertebrates). In turn, plankitvores are consumed by piscivorous (fish-eating) fish. Overall, these aquatic organisms (zooplankton, macroinvertebrates and fish) derive energy by breaking down organic matter through the process known as respiration. During respiration, organic matter, water and dissolved oxygen are converted into carbon dioxide and nutrients.
At the bottom of the food web (Figure 1), particulate organic waste products (excrement or feces) from aquatic organisms along with dead aquatic organisms settle to the pond or lake bottom and are subsequently feed upon by other organisms. Organisms that live or reside along the lake bottom are referred to as benthivores. After settling to the pond or lake bottom, dead organic materials and organic waste products are now called detritus. Some benthivorous fish (catfish and carp) and microorganisms (bacteria, fungi and protozoans) feed upon detritus. Aquatic organisms that feed upon detritus in ponds and lakes are referred to as decomposers. Decomposers obtain energy by breaking down detritus (dead organic matter) via the process of respiration. During decomposition, some of the nutrients are recycled back into pond or lake water and can now once again be used by algae and aquatic plants for growth and reproduction. Any unused detritus will accumulate and eventually become part of the lake sediments, thereby increasing the organic content of these sediments.
Ultimately, the amount of nutrients in lakes controls the overall degree of aquatic productivity (Figure 2). Ponds and lakes with low levels of nutrients and low levels of aquatic productivity are referred to as oligotrophic. Oligotrophic ponds and lakes are typically clear and deep with low quantities of phytoplankton and rooted aquatic plants. In these ponds and lakes, the deeper, colder waters are generally well-oxygenated and capable of supporting coldwater fish such as trout. Conversely, ponds and lakes with high nutrient levels and high levels of aquatic productivity are referred to as eutrophic. Eutrophic lakes are generally more turbid and shallower due to the deposition of sediments and the accumulation of detritus.
In some instances, the flow of energy through the aquatic food web may be disrupted. In hyper-eutrophic (highly eutrophic) ponds and lakes, aquatic productivity is extremely high and is dominated by very large numbers of a few, undesirable species. The phytoplankton community is typically comprised largely by blue-green algae during the summer months. Many species of blue-green algae are not readily grazed upon the zooplankton community. Under these conditions, the blue-green algae community is allowed to flourish due to the lack of predation, while the zooplankton community collapses. Decreases in zooplankton biomass in a pond or lake may in turn adversely affect the lake’s fishery. In addition, shallow lake areas may be completely infested with dense stands of aquatic macrophytes and dominated by common carp, catfish or other rough fish.
Algae -Aquatic, non-vascular plants that are suspended (free-floating) in the water or attach to larger plants, rocks, and other substrates. Also called phytoplankton, these individuals are usually visible only with a microscope. They form the foundation of the aquatic food web, but excessive amounts can make the water appear murky green and adversely impact pond and lake water quality.
Alkalinity - The acid-neutralizing capacity of water. It is primarily a function of the carbonate, bicarbonate, and hydroxide content in water. The lower the alkalinity, the less capacity the water has to absorb acids without becoming more acidic.
Ammonia (NH3) - A nitrogen-containing substance, which may indicate recently decomposed plant or animal material.
Benthos - The communities of aquatic life, which dwell in or on the bottom sediments of a water body.
Chlorophyll - Pigments (mostly green) in plants, including algae, that play an important part in the chemical reactions of photosynthesis. A measurement of chlorophyll-a (one type of chlorophyll) is commonly used as a measure of the algae content of water.
Conductivity - A measure of water's capacity to convey an electric current. It is related to the total amount of dissolved charged substances in the water. Therefore, it can be used as a general indicator of the quality of the water and can also suggest presence of unidentified material in the water. It is often used as a surrogate for salinity measurements.
Dissolved oxygen - Oxygen that is dissolved in the water. Certain amounts are necessary for life processes of aquatic animals. The oxygen is supplied by the photosynthesis of plants, including algae, and by aeration. Dissolved oxygen is consumed by animals and plants at night and bacterial decomposition of dead organic matter (plant matter and animal waste).
Epilimnion - The warmer, well-lit surface waters of a lake that are thermally separated from the colder (hence denser), water at the bottom of the lake when a lake is stratified.
Eutrophication - The acceleration of the loading of nutrients to a lake by natural or human-induced causes. The increased rate of delivery of nutrients results in increased production of algae and consequently, poor water transparency. Human-induced (cultural) eutrophication may be caused by input of treated sewage to a lake, deforestation of a watershed, or the urbanization of a watershed.
Fecal Coliform Bacteria - Bacteria from the intestines of warm-blooded animals. Most of the bacteria are not in themselves harmful, so they are measured or counted as an indicator of the possible presence of harmful bacteria.
Groundwater - Water stored beneath the surface of the earth. The water in the ground is supplied by the seepage of rainwater, snowmelt, and other surface water into the soil. Some groundwater may be found far beneath the earth surface, while other groundwater may be only a few inches from the surface. Groundwater discharges into lowland streams to maintain their baseflow.
Hydrology - The science dealing with the properties, distribution and circulation of water. The term usually refers to the flow of water on or below the land surface before reaching a stream or man-made structure.
Hypolimnion - The dark, cold, bottom waters of a lake that are thermally separated from the warmer (hence less dense) surface waters when a lake is stratified.
Invertebrates - Animals without internal skeletons. Some require magnification to be seen well, while others such as worms, insects, and crayfish are relatively large. Invertebrates living in stream and lake sediments are collected as samples to be identified and counted. In general, more varied invertebrate communities indicate healthier water bodies.
Limiting nutrient - The nutrient that is in lowest supply relative to the demand. The limiting nutrient will be exhausted first by algae which require many nutrients and light to grow. Inputs of the limiting nutrient will result in increased algal production, but as soon as the limiting nutrient is exhausted, growth stops. Phytoplankton growth in lake waters of temperate lowland areas is generally phosphorus limited.
Limnology - Scientific study of inland waters.
Littoral zone - portion of a water body extending from the shoreline lakeward to the greatest depth occupied by rooted plants.
Macrophytes - rooted and floating aquatic plants, larger (macro-) than the phytoplankton.
Mesotrophic - A condition of lakes that is characterized by moderate concentrations of nutrients, algae, and water transparency. A mesotrophic lake is not as rich in nutrients as a eutrophic lake, but richer in nutrients than an oligotrophic lake.
Nitrate & nitrite (NO3, NO2) - Two types of nitrogen compounds. These nutrients are forms of nitrogen that algae may use for growth.
Nitrogen - One of the elements essential as a nutrient for growth of organisms.
Nonpoint source pollution (NPS) - Pollution that originates from diffuse areas and unidentifiable sources, such as agriculture, the atmosphere, or ground water.
Nutrients - Elements or compounds essential for growth of living organisms (plants and animals).
Oligotrophic - A condition of lakes characterized by low concentrations of nutrients and algae and resulting good water transparency. An oligotrophic lake has less nutrients than a mesotrophic or eutrophic lake.
Pelagic Zone - Deep, open water area of a lake away from the edge of the littoral zone towards the center of the lake.
pH - Measure of the acidity of water on a scale of 0 to 14, with 7 representing neutral water. A pH less than 7 is considered acidic and above 7 is basic.
Phosphorus - One of the elements essential as a nutrient for the growth of organisms. In western Washington lakes, it is usually the algae nutrient in shortest supply relative to the needs of the algae. Phosphorus occurs naturally in soils, as well as in organic material. Various measures of phosphorus in water samples are made, including total-phosphorus (TP) and the dissolved portion of the phosphorus (orthophosphorus).
Photic zone - The lighted region of a lake where photosynthesis occurs.
Phytoplankton - Floating, mostly microscopic algae (plants) that live in water.
Point Source Polution - An input of pollutants into a water body from discrete sources, such as municipal or industrial outfalls.
Salmonids - Salmon, trout, char and whitefish species of fish.
Secchi depth - Measure of transparency of water obtained by lowering a 10 cm black and white disk into water until it is no longer visible.
Sewage - That portion of wastewater that is composed of human and industrial wastes from homes, businesses, and industries.
Stratification of lakes - A layering effect produced by the warming of the surface waters in many ponds and lakes during summer. The upper waters are progressively warmed by the sun and the deeper waters remain cold. Because of the difference in density (warmer water is lighter), the two layers remain separate from one another. Overall, the upper waters "float" on deeper waters and wind induced mixing occurs only in the upper waters. Oxygen in the bottom waters may become depleted. In autumn as the upper waters cool, the whole lake mixes again and remains mixed throughout the winter, or until it freezes over.
Stormwater - Water that is generated by rainfall and is often routed into drain systems.
Thermocline - Depth in a stratified lake where the greatest change in temperature occurs. Separates the epilimnion from the hypolimnion
Total suspended solids (TSS) - Particles, both mineral (clay and sand) and organic (algae and small pieces of decomposed plant and animal material), that are suspended in water.
Transparency - A measure of the clarity of water in a lake, which is measured by lowering a standard black and white Secchi disk into the water and recording the depth at which it is no longer visible. Transparency of lakes is determined by the color of the water and the amount of material suspended in it. Generally in colorless waters of the Puget lowland, the transparency of the water in summer is determined by the amount of algae present in the water. Suspended silt particles may also have an effect, particularly in wet weather.
Trophic status - Rating of the condition of a lake on the scale of oligotrophic – mesotrophic -eutrophic.
Turbidity - Cloudiness of water caused by the suspension of minute particles, usually algae, silt, or clay.
Wastewater -Total flow within the sewage system. In combined systems, it includes sewage and stormwater.
Water Column - Water in a lake between the surface and sediments. Used in vertical measurements used to characterize lake water.
Watershed - The areas that drain to surface water bodies, including lakes, rivers, estuaries, wetlands, streams, and the surrounding landscape.
Zooplankton - Small, free swimming or floating animals in water, many are microscopic.