Introduction to lakes
Each has a life of its own
Lakes are complex ecosystems with many species of animals and plants interacting with each other and their environment. Every lake is a unique body of water, reflecting many of the characteristics of the surrounding watershed and the climate, as well as the shape and volume of the lake basin. External factors such as sunshine, wind, air temperature and water inflows combine with internal forces such as evaporation rates, currents, nutrient release from sediments, nutrient uptake by algae, and plant-animal interactions to produce an intricate web of relationships.
Water chemistry, physical characteristics, and biological communities in lowland northwest lakes vary seasonally, as well as by water depth, throughout the year. An especially dynamic period for lakes occurs during the "growing season" of mid-spring through early autumn when lake-dwelling organisms increase their activities, and the water column is segregated by temperature differences into non-mixing warm, upper layers and cool, deeper layers. By contrast, lake waters mix continually through the late fall through early spring, keeping the temperatures constant from top to bottom. The cool temperatures and low light levels keep biological activities at a minimum.
Lake classification and eutrophication
Lakes can be classified by measurements of potential and actual biological activity, also known as "trophic state." Lakes with high concentrations of nutrients and algae, generally accompanied by low transparencies, are termed eutrophic or highly productive, because the algae grow and reproduce at a high rate. Lakes with low concentrations of nutrients and algae, most often accompanied by high transparencies, are categorized as oligotrophic or low in productivity. Lakes intermediate between eutrophic and oligotrophic are termed mesotrophic, or in the middle.
Each lake's productivity is influenced by a variety of natural factors, including watershed size and geology, lake depth and surface area, climate, catastrophic events such as earthquakes and volcanic eruptions, and the quality and quantity of water entering and leaving the lake. Lakes may be naturally eutrophic, mesotrophic, or oligotrophic based on the original character and stability of the surrounding watershed.
Increases in a lake's biological activity ("eutrophication") may occur naturally in some lakes, but can be hastened by human activities in others. At any particular time, lakes in a region may exhibit a wide range in algae levels without being impacted by human activity. Natural eutrophication is almost never observed in the scale of a human lifetime. However, effects of human-induced eutrophication can be seen in as little as a decade.
Land use effects
Land use activities, including home building, commercial and industrial development, agriculture, forestry, resource extraction, landscaping, gardening, and animal keeping all have the potential to contribute nutrients to surface and ground waters as well as cause change in sediment movement. Increases in impervious surfaces associated with land development also result in distinct changes in surface water runoff patterns. This surface water, as it enters lakes and streams, can increase biological productivity by increasing concentrations of nutrients that stimulate plant growth. Additional sediment input associated with increased surface water runoff can also impact lakes in various other ways.
Lakes in various trophic states are sometimes separated by the frequency of algal blooms and the type of algae present. Large amounts of algae can affect swimming, fishing, boating, wildlife, aesthetics, and other uses. Eutrophic lakes, for example, may have frequent nuisance algal blooms dominated by bluegreen algae (cyanobacteria). These blooms can form surface scums, give off noxious odors, and may occasionally produce toxins that have direct health impacts on animals as well as people.
Vigorous growth of rooted aquatic plants can also impact boating, fishing, and swimming. A lake need not be eutrophic to support a large amount of aquatic plant life. Many aquatic plants are rooted in the sediments, from which they draw nutrients. One very important factor is the depth to which light can penetrate in the lake, as well as how much of the lake bottom is within that depth range. Clear lakes with large areas of shallow water can support more aquatic plant growth than steep-sided or colored-water lakes.