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.
Lake Words:
Algae: Aquatic plants (usually microscopic in size) that can grow as single cells or long strands of cells.
Eutrophication: The natural physical, chemical, and biological changes that take place as nutrients, organic matter, and sediment are added to a lake. When accelerated by human-caused influences, this process is called cultural eutro phication.
Limnology: The scientific study of fresh water, especially the history, geology, biology, physics, and chemistry of lakes.
Lacustrine: Relating to, or growing in, lakes.
Productivity: The rate at which organic matter is formed, averaged over a defined period of time.
Thermal stratification: Layering of lake water caused by differences in water density. During summer months, deep lakes divide into three layers: the epilimnion (uppermost, warmest layer), hypolimnion (lower, cooler layer) and metali mnion (middle layer).
Go to Shoreline Practices for a Healthy Lake
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