of algae and the animals that
feed on them are lower in oligotrophic
lakes because of low nutrient concentrations. Thus the water remains
clear. Decay of the relatively small amount of organic matter in oligotrophic
lakes does not completely deplete the hypolimnetic supply of
dissolved oxygen. Therefore, lack of oxygen does not restrict animals from living
in the hypolimnion
of oligotrophic lakes. Lake trout, for example, require
cold, well-oxygenated water and primarily live in the hypolimnion of
oligotrophic lakes. Minnesota's oligotrophic lakes are found in the
northeast region of the state, where infertile soils are covered with
mixed conifer forests.
deep oligotrophic lakes such as Lake Superior and Lake Tahoe have hypolimnia
that remain completely saturated with oxygen the entire year. However,
many moderately deep lakes (with maximum depths greater than about 30
meters) may develop anoxia in the lower hypolimnion during late summer
but may still be classified as oligotrophic because of their very low
nutrient concentrations, low algal abundance, and relatively high transparency
(high secchi depth). These lakes may have
fishery, with warm and cool water fish in the epilimnion
and metalimnion and cold water fish (such as trout) in the cold, oxygen
rich portion of the hypolimnion. The cold-water fishery is therefore
very sensitive to increased inputs of organic matter from sewage or
erosion (external inputs), and to increased algal and macrophyte production
(internal inputs) due to eutrophication since these factors will accelerate
the rate and extent of hypolimnetic oxygen depletion in the summer.
macrophytes grow so thickly in
lakes that light penetrates
only a short distance and nutrients below that depth are not assimilated.
As discussed earlier, phosphorus is typically the limiting nutrient
in freshwater lakes, meaning that the plants deplete all available phosphorus
before depleting other nutrients. In a hypereutrophic lake, algae may
become so abundant that they suffer from self-shading. In those cases,
photosynthesis is limited by light rather than by nutrients. When a
great abundance of phosphorus is available in a lake, nitrogen may become
limiting. In such lakes, certain species of blue-green algae that can
fix atmospheric nitrogen have a clear competitive advantage and frequently
become dominant. They dominate the algal community until another nutrient,
or usually light, becomes limiting. In many infertile lakes in northeastern
Minnesota, both phosphorus and nitrogen may be extremely low during
midsummer. Since most sources of either point source or nonpoint-source
pollution involve increased inputs of both N and P, these
lakes are extremely sensitive to such pollution, irrespective of which
is technically "most" deficient.
lakes show wide seasonal changes in their biological and chemical conditions.
Because of the great amount of organic matter produced in these lakes,
the decay rate is high in the hypolimnion, causing oxygen to be depleted.
Therefore, eutrophic lakes frequently show a complete loss of dissolved
oxygen below the thermocline during summers. Clearly, fish and most
other animals cannot live in the hypolimnion of such lakes. Warm-water
fish that can live in the epilimnion, however, can be quite productive.
Bass, panfish, northern pike, walleye, carp, and bullheads thrive in
many of Minnesota's eutrophic lakes. Complete or nearly complete oxygen
depletion below the thermocline may also be a common feature of many
moderately deep (10 to 30 m) mesotrophic lakes, if deep enough to stratify
throughout the summer. Therefore, virtually complete anoxia below the
thermocline does not necessarily mean that the lake is eutrophic.
Lake, one of our WOW lakes, is an example of a mesotrophic lake
that becomes anoxic below the thermocline in the summer, (see
Ice Lake section) as is Hale Lake,
a somewhat less productive lake immediately downstream of Ice
Lake. Both are ~16-18 meters deep.
oxygen-related problem in eutrophic lakes
A dense snow cover over the ice reduces light penetration and keeps
oxygen-producing photosynthesis from occurring. The high organic content
of the water, however, provides considerable food for the decomposers.
If the decomposers succeed in using all the available dissolved oxygen,
a fish kill can occur.
cases, a winterkill may lead to a more balanced fishery and possibly
even improved water quality. Fish that survive a winterkill will have
reduced competition for food for a period of time and so may grow faster
and to a larger size. Fewer small fish reduces predation on the larger
zooplankton, such as the water flea, Daphnia sp., leading to
increased zooplankton grazing on algae and a resultant increase in water
clarity. This general scheme, involving fishery manipulations to reduce
the abundance of zooplanktivorous fish, has been
and is being tried in many urban lakes where it is economically impractical
to reduce nutrient inputs enough to significantly reduce algae. In these
situations the offending fish may be removed by intense stocking of
gamefish, by intensive netting and trapping, or even by poisoning the
entire fishery and starting over with greatly reduced planktivores.