Why does a lake
become green and stinky?
Pungent green lakes
are usually blooming with algae. Algae are simple, small aquatic plants.
An algal bloom is a dense concentration of these plants. Like grass
and trees, algae use sunlight, carbon dioxide, and nutrients to generate
energy and produce more algae. In most lakes, algal growth is limited
by the availability of the nutrients, nitrogen (N) and phosphorus
the term used to describe the process of nutrient enrichment leading
to excessive plant growth and the subsequent sedimentation of dead
and rotting vegetation to the lake bottom. This natural process is
often accelerated by human activities in the watershed, which introduce
unnaturally high quantities of nutrients into lakes. Two common bloom-forming
algae are diatoms and blue-green algae. Diatom blooms usually occur
in the late spring or early summer, turning the water a bright green
or brown but not causing surface scums or odors. Blue-green algal
blooms create greater problems for lake users. The most obnoxious
forms are buoyant during the day and can form thick surface scums,
especially on a calm sunny afternoon. This scum may be blown into
shallow water making the shoreline appear as though it has been slicked
with blue-green paint.
When algae die the
bacteria that break them down use up oxygen in the water. If enough
algae die at one time, decomposition may use up the oxygen faster
than wind mixing or photosynthesis can replenish it. This can lead
to anoxic (no oxygen) conditions and the build-up of hydrogen sulfide
gas (rotten egg smell) or ammonia in deep water. Certain species of
algae can also be toxic to domestic animals.
What causes surface
scum on a lake?
There are various
causes of surface scums on a lake or pond. Look more closely to determine
what is on the waters surface. An oily film or yellow-green
dust on the surface of a lake make it look contaminated but, in most
cases, nothing is wrong. In fact, something natural is probably occurring.
An oily film in mid-summer may be caused by organic compounds from
nearby wetlands, rotting vegetation, or insect cases that were concentrated
along the shore by wind after a hatch. Insects can hatch at any time
from ice-out in the spring until mid-September. As the insect cases
decompose, they sometimes produce an oily film. Yellow-green dust
floating on the surface in late spring and early summer is probably
pollen from nearby trees. In contrast, a scum from an algal bloom
is green to blue-green, might have an oily sheen that resembles a
motor oil slick, and can form a thick, soupy mass on the surface of
Does foam on the
shore of a lake mean it's polluted?
The foam found in
lakes and streams is usually natural. Wind-driven currents frequently
create parallel streaks of foam in open water that accumulate along
windward shores and in coves. Foam is created as decomposing plants
and animals release organic compounds into the water. The compounds
reduce the surface tension of water, causing bubbles to form. Many
people blame shoreline foam on detergents, but detergents dont
create long-lasting foam since they quickly lose their sudsing ability.
Industrial pollution effluents may have been a more common source
of foam on surface water in the past, but these days point source
discharges are more closely regulated through the National Pollutant
Discharge Elimination System (NPDES) permits.
Why does the water
quality of our lake seem to get worse throughout the summer?
Lakes change a great
deal over the course of a year. Changes are caused by seasonal weather
patterns, watershed influences, and the life cycles of the lakes
biota. During the winter, ice and snow severely limit the amount of
light available for photosynthesis under the ice, so there is not
much algal growth. In the spring, snowmelt washes nutrients into the
lake. Many of the nutrients are used by rapidly growing aquatic plants
(macrophytes) near the shoreline, resulting in a "clear water"
As macrophyte growth
slows in mid- to late-summer, incoming nutrients and nutrients from
decomposing aquatic plants become available for algae. Available nutrients,
combined with warm water and plentiful sunlight, can result in a period
of heavy algal growth, potentially making the lake green and scummy.
Mid-summer water quality problems may be particularly acute if you
live on a shallow lake where high winds can mix warm surface water
all the way down to the lakes bottom waters. When this happens,
nutrients are released from the mud and sediments up into the surface
water where light is plentiful and algae can flourish. In autumn,
the combination of decreased daylight, cooler temperatures, and more
zooplankton grazing on algae, reduces algal growth and yields clearer
water once again.
What can I do to
help improve the water quality of my favorite lake?
You can improve your favorite lakes water quality by becoming
educated and involved. You and your neighbors can monitor the lake
to learn why and how the water quality has changed and identify ways
to minimize impacts. For instance, if erosion and excess nutrients
are degrading water quality, follow the proven techniques for stabilizing
shores suggested in Protecting Our Waters: Best Management Practices
for Protecting Your Shoreline. Protecting Your Shoreline explains
how to minimize nutrient inputs, reduce human impacts, restore shorelines,
and monitor lakes. Several additional resources are listed below.
Who can I contact
if I have questions or a problem related to water quality?
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 water quality?
- A Primer on
Limnology. 1992. B.A. Monson. University of Minnesota Water Resources
- Minnesota Lake
and Watershed Data Collection Manual. 1994. Minnesota Pollution
- LakeSmarts: The
First Lake Maintenance Handbook. 1993. S. McComas. Terrene Institute
- A Citizens
Guide to Lake Protection. Freshwater Foundation and the MPCA
- A Guide for Buying
& Managing Shoreland. 1988. Minnesota Department of Natural
Resources, Division of Waters
- Lake and Reservoir
Restoration Guidance Manual. 1990. North American Lake Management
- Protecting Our
Waters, Shoreland Best Management Practices. 1998. University
of Minnesota Extension Service
- Sustainable Lake
Management Handbook. 2000. Minnesota Lakes Association