Why are lakes classified
into different categories?
Lakes are classified
in many different ways to help identify similarities and differences,
to assess ecosystem health, and to help establish policies and regulations.
Lakes are most often classified based on their productivity, i.e.,
the amount of algae and aquatic plants that the lake can support.
They also may be classified based on size or hydrology (drainage),
plant or animal communities, or the way humans use them (water supply,
recreation, amount of development, and navigation). Classification
strategies are used to formulate shoreland regulations, fish stocking
plans, utility forecasts, flood control, or other management plans.
Categorizing, or classifying, lakes also helps prepare realistic plans
for restoration work.
What is trophic
status and why is it used to classify lakes?
One common method
for describing the health of a lake is to identify its trophic status.
Trophic status is an indicator of how much growth or productivity
occurs in the lake. Productivity is directly related to the availability
of nutrients. Low productivity, nutrient-poor lakes usually have clear
water and are called oligotrophic. Lakes of this type are common in
northeastern Minnesota. Moderately productive lakes are defined as
mesotrophic. Nutrient-rich lakes with high levels of productivity
are called eutrophic and often can be murky and green, because of
of nitrogen and phosphorus increase, one can see important changes
in the balance between algae in the water and rooted plants, such
as pondweeds and milfoil. In particular, increases in phosphorus can
lead to dramatic increases in floating algae, which in turn intercept
light needed by rooted plants. Consequently, rooted plants can decline
as lakes become enriched with nutrients. Such a change in lake condition,
known as eutrophication, is often viewed as undesirable, because the
green and murky water that results from excess algal growth is aesthetically
unappealing to many people. In addition, the loss of rooted plants
can be detrimental to fish and wildlife populations, which rely on
plants for habitat and food.
proceeds, plants and algae die and settle to the lake sediments, using
up oxygen as they decompose. This means that eutrophic lakes often
have low or no dissolved oxygen in the lower depths (hypolimnion),
at least seasonally. Over time, large quantities of decaying plant
material accumulating on the lake bottom can actually make the lake
shallower. In addition, the decaying material provides a large in-lake
reservoir of nutrients that becomes available for plant growth whenever
lake water circulates, mixing bottom water into the upper, light-rich
waters where plant growth occurs.
about lakes has been that eutrophication is a natural process, reflecting
the gradual transformation of deep, oligotrophic lakes to shallow,
more eutrophic ponds over many thousands of years. As the lake basin
slowly fills with sediment, the lake eventually becomes shallow enough
for nutrients stored in the sediments to be re-suspended into the
surface waters, becoming available to stimulate additional plant growth.
As a result, the lakes trophic condition changes. Human land
use practices have been blamed for accelerating eutrophication by
vastly increasing the rate at which sediments and nutrients are added
to lake basins. Recent research by Minnesota scientists suggests that
the natural process is more complicated. The research shows that lakes
recently created by glaciers are rich in nutrients and can become
less productive over time as vegetation develops in the watershed.
In other words, a lakes trophic status is dependent both on
human activities in the watershed and natural processes within the
How are Minnesota
lakes classified for shoreland management and planning?
The Minnesota Department
of Natural Resources (MDNR) groups lakes into three main categories
for making shoreland management decisions: natural environment lakes,
recreational development lakes, or general development lakes. Natural
environment lakes are small, often shallow lakes with uplands that
are difficult to develop. Recreational development lakes are generally
medium-sized, have moderate recreational use, and are surrounded by
seasonal and year-round lake homes. General development lakes vary
in size, are often heavily developed around the shoreline, and are
typically used for recreation.
The MDNR uses the
following characteristics to decide whether a lake should be classed
as a natural, recreational, or general development lake:
- Size and shape
- Amount and type
of existing development
- Road and service
- Natural characteristics
of the water and shorelands
- State, regional,
and local plans and management programs
- Existing land use
- Presence of significant
In addition to MDNR classification of the lake, zoning requirements
can specify what development will be allowed along the shoreline.
Local zoning may specify the allowable land use in categories ranging
from protected to high density residential to general use.
Doesnt the Minnesota
Department of Natural Resources also categorize lakes based on what
kind of fish could live in them?
Yes, the MDNR also
uses certain lake characteristics such as size, depth, hydrology,
water quality, degree of development, and indigenous fish populations
to identify which lakes might support certain populations of fish,
to set angling limits, and to develop fish stocking plans.
How is hydrology
used to classify lakes?
Lakes may also be
categorized based on how water flows into or out of them, including
the relative influences of groundwater and surface water, the position
of the lake in its watershed, and the number of inlets and outlets.
How much and how frequently water flows through a lake and the source
of that water can have an effect on water chemistry and biological
communities in the lake. The amount of time required to completely
replace a lakes current volume of water with an equal volume
of new water is known as hydraulic residence time or retention
time. Calculating residence time helps managers understand the potential
impacts of human activities on lakes, such as the effects of a pollutant
spill or other pulse of inflow.
How are lakes classified
for water use?
In Minnesota Rules,
Chapter 7050, water quality standards are listed for different lake
uses. Many municipalities in Minnesota use surface water for their
drinking water supply. Lake water is also used for industry, agriculture,
wildlife, aesthetic enjoyment, navigation, and recreation. The Minnesota
Pollution Control Agency (MPCA) categorizes uses into Classes A, B,
C, and D. Class A refers to high quality water, requiring the least
treatment, whereas Class D refers to poor quality water, requiring
the most treatment before use.
Why are there so
many different ways of classifying lakes? Which is the right one?
agencies have different responsibilities, they sort lakes according
to different types of characteristics. Just as you might be grouped
based on your profession, where you live, or whether you like to fish,
water ski, or canoe, so the special features of lakes are used to
group them in ways that allow them to be better managed or protected.
Who can I contact if
I have questions or a problem related to classifying lakes?
Check your local telephone
listing, the Who to Contact
section of the Minnesota Shoreland Management Resource Guide Web site,
or the Web sites listed below for:
What are some additional
resources related to classifying lakes?
- A Primer on Limnology.
1992. B.A. Monson, Water Resources Center
Natural Heritage: An Ecological Perspective. 1995. J. Tester
- Lake Prioritization
For Protecting Swimmable Use. 1997. S. Heiskary, Minnesota Pollution