The Concept of Eutrophication
Maintaining a proper balance among life forms in lakes and streams is a critical aspect of water quality management. Water bodies have a considerable capacity to absorb organic material. Extensive populations of biota can be generated in such processes.
Lakes are sometimes characterized in terms of their productivity, that is the amount of organic material synthesized per unit of surface area in a given time.
Water bodies with high productivity are sometimes termed eutrophic. This word was coined from Greek terms equivalent to nutrient rich. The similarly coined word for water with low productivity, oligotrophic, means nutrient poor.
Eutrophic lakes are usually less than 18 meters deep and the bottom contour is U-shaped. Fertility of the lake is high. Water color varies from green to yellow or brownish-green and there are large areas of shallow waters with marshy type vegetation.
Because of the rich bottom humus, the oxygen content of the hypolinmeon is greatly reduced during summer stratification. The CO2 content in the hypolimneon is accordingly high. The volume of the epilimneon is usually greater than that of the hypolimneon. Plankton are abundant. Midge fly larvae and Chironomides can be very numerous and the Culicidae larvae Chaoborus is usually present. The bottom fauna is rich and there can be a large fish population in the epilimneon.
Characteristic fish species are the large mouth bass, perch, sunfish and pike. These lakes occur in relatively mature river systems. Many lakes in Minnesota and Wisconsin are of this type.
Oligotrophic lakes on the other hand are usually deep, over 18 meters, with very little shallow water. The bottom contours are V-shaped. There is little vegetation around the shore lines.
They are low in fertility. Dissolved oxygen is abundant throughout the system including the hypolimneon. Low CO2 in the hypolimneon is encountered and the color of the water varies from blue to green. The volume of the epilimneon is usually less than the volume of the hypolimneon. The fish population is not as large as found in eutrophic lakes.
Characteristic species are lake trout, whitefish and cisco. Midgefly larvae, Tanytarsus predominates. Plankton are not abundant. Many of the wilderness lakes along the Continental Divide are of this type.
A third type of lake, termed distrophic, which are the bog like lakes They have very rich marginal vegetation and high organic content. Oxygen is likely to be scarce at all depths. The water is usually conspicuously colored yellow to brown and may be acidic because of organic acids and incompletely oxidized decomposition products.
Plankton, bottom organisms and fish are usually scarce, but blue-green algae are sometimes abundant. Tendipedides may predominate among the bottom forms, but at times only charborus is present.
Characteristic fish are stickle backs and mud minnows. Many lakes of northern latitudes are distrophic in type.
All gradations exist between these three types of lakes and individual lakes are often difficult to classify. Based the loss of oxygen in the hypolimneon during the summer
Oligotrophy is indicated if oxygen is not over 0.025 mg/square cm per day;
Eutrophy, if oxygen is over 0.055 mg/square cm per day;
Mesotrophy, if oxygen is between the two.
A lake may change from one type to the other as succession proceeds. Most lakes start as oligotrophic, but as they accumulate vegetation and gain organic matter, they change into eutrophic lakes, or if the organic matter does not completely decompose, into distrophic lakes.
Eutrophic lakes may later develop into ponds and marshes, distrophic lakes into bogs. These changes usually take centuries.
Lakes in environments where growing conditions are favorable and nonaquatic vegetation is abundant, generally are highly productive and may have short careers in the geologic sense. Such water bodies tend to evolve into marshland or peat bogs owing to accumulation of organic debris.
Lakes in environments less favorable to vegetative growth may fill in with inorganic sediment or may be drained by stream erosion at their outlets.
Obviously, the career of a lake involves other factors in addition to organic productivity. But, pollution with organic wastes can bring about extensive changes in properties of the water. In a relatively short time, the pollution may convert a clear oligotrophic lake to a relatively turbid eutrophic one.
The rates that lake changes might occur and the feasibility of reversing them represents areas that are receiving environmental scrutiny. The effects of enrichment on lake eutrophication are well documented and as almost all of effects are bad from a human point of view. These lake conditions attract a great deal of attention.
We know far less about the effects of nutrient enrichment on running stream water. Very eutrophication experiments have been performed on running waters in contrast to the large amount of work that has been done on the fertilization of lakes and ponds. It is certain that enrichment causes changes in the flora and encourages algal growth and rooted plants, but the extent to which this occurs depends upon many watershed and local circumstances.
If the enrichment is accompanied by reduction in flow during warm weather, it must, especially if riparian trees have been cleared, lead to increased primary production. There are few before and after studies which are free of the complication of organic pollution, but changes in the attached algae have been noted in some areas and they have probably occurred almost everywhere.
Presumably, enrichment encourages planktonic algae in large rivers, but there appears to be little information on this point. Perhaps this is not surprising because many factors limit the development of plankton in rivers.
Enrichment is usually accompanied by soil erosion and increased turbidity, where there is intensive land use activity. These two effects tend to cancel one another out. It is therefore usually only where the rate of flow of a river has been reduced that dense blooms of planktonic algae develop. Without human intervention in the flow of the water, it seems that these occur only in eddies, side arms and flood plains of lakes.
In most of the situations, it is probable that in the summertime the initial concentrations of nitrate and phosphate will be declining downstream or being maintained at fairly low levels by additions from land surfaces.
Similarly, the general level of the major ions will be rising from addition from the land and the streambed. Even further downstream in the large river, the acquisition of major ions continues and the diurnal fluctuations become less pronounced, although they continue to be measurable.
Seasonal fluctuations of discharge with the variations in total dissolved matter, turbidity, oxygen content, etc. tend to replace more irregular local weather fluctuations observed in the smaller streams.
The effects of nitrate and phosphate on running water are rather indirect. In a uniform reach with no additions along its length, these ions tend to decline in amount because of the uptake by plants. The numbers of algae growing on surfaces of rocks and rivers tend to increase downstream, presumably, because there are more nutrients. Any increase of growth caused by sewage is usually attributed to the increased supplies of nutrients.
The increases observed below lakes may be caused by nitrogen fixation in the lakes or by the slow release of phosphate from littoral deposits during warm weather. Some nitrogen fixation may occur in running water if some nitrogen fixing organisms are present.
However, it has been observed that the amount of algal growth increases at points where rivers flow through areas that are rich in nitrate and phosphate.
All these observations lead to the conclusion that in many running waters, one or both of these ions is in adequate supply for maximum growth of attached algae.