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 a two-story
fishery, with warm and cool water fish in the epilimnion
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.
grow so thickly in some eutrophic
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
oxygen-related problem in eutrophic lakes is winterkill.
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
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 termed biomanipulation,
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.