Dam and river ecosystem basics module: introduction | hydrology | wetlands | morphology and sediment | water quality | biology | timing of dam repair or removal | references | appendix

Biology

Free-flowing and impounded systems host a wealth of biological activity, from single-celled organisms to large predatory fish, mammals, and birds. Just as water quality, especially the temperature and dissolved oxygen content, differs greatly between free-flowing and impounded river systems, the types of fish and other aquatic life also differ in each. Certain species favor the still water of impoundments over the flowing water in rivers and vice versa.

In impounded and free-flowing systems, the base of the food chain is made up of phytoplankton. The types of phytoplankton found in both systems are relatively similar. Phytoplankton is composed of small, free-floating plant and plant-like organisms that convert inorganic compounds into organic compounds. Phytoplankton, by converting nutrients and inorganic compounds to complex organic compounds, provide the primary biological productivity of the water body. Aquatic invertebrates (insects and crustaceans) feed on the phytoplankton. Fish, in turn, feed on these aquatic invertebrates, and larger fish prey on smaller fish. Some fish feed on aquatic invertebrates floating in or on top of the water. Other fish feed on aquatic invertebrates that live on the bed of the river or impoundment. Free-flowing and impounded systems also support populations of other types of wildlife, including amphibians (frogs and salamanders), reptiles (turtles and snakes), waterfowl (ducks, geese, herons), predatory birds (eagles and ospreys), and mammals (otters, beaver, and humans).
 

Impounded systems

Impoundments have three different zones of biological communities (Cooke and others, 1986): near shore, open water, and bottom zones. The near-shore zone contains large beds of aquatic plants rooted to the bottom of the impoundment. Two factors that determine the extent of this zone are the area of shallow, fertile sediments that provide the nutrients for the plant beds and the greatest depth that light can penetrate the water. The plant beds also provide habitat and food for fish and waterfowl. The open-water zone is inhabited by phytoplankton and some fish species. In eutrophic impoundments, this zone can be dominated by blue-green algae. The bottom zone contains organisms that live in and on the bottom sediments, including microbes, insects, and mussels. In eutrophic impoundments (fig. 11), this zone may be reduced to only species that can survive in low-oxygen environments.
 

 

Figure 11. Algae bloom in Mounds Dam impoundment caused by eutrophication. (Photo courtesy of Stephanie Lindloff, River Alliance of Wisconsin.)

Because nutrients and organic matter have relatively long residence times in impoundments, as well as abundant sunlight and higher temperatures, algae and plants may flourish. This food source may lead to large numbers of fish and aquatic invertebrates, provided that the impoundment does not have so much algae that impoundment is eutrophic (see also Water Quality section of this module). A eutrophic impoundment has undesirably high concentrations of phytoplankton and supports fewer aquatic invertebrates and fish because of the associated turbidity and low dissolved oxygen. Eutrophic impoundments tend to only support rough fish, such as carp.

The still water of an impoundment allows plants to grow easily, without the need to be strongly rooted. Some aquatic plants found in impoundments may even be free floating. Impoundments support the types of invertebrates that do not require flowing water for feeding or respiration purposes. Invertebrates found in impoundments also tend to be able to tolerate warm water temperatures and relatively low dissolved oxygen. Studies have documented that the construction of a dam eliminates the riverine types of plants and invertebrates, especially insects and mussels, replacing them with those commonly found in lakes and ponds (Hynes, 1970). Mussels are among the most endangered aquatic organisms in North America, partly due to the fact that the greatest mussel diversity is found in flowing water habitats, not impoundments.

The types and abundance of fish found in impounded systems depends on the characteristics of water found in impoundments. As previously mentioned, some fish prefer fast currents; those species that prefer slow currents favor impoundments. Different types of fish prefer different temperatures of water, especially for spawning. Many fish have narrow ranges of water temperature at which spawning occurs. Because impoundments may have relatively warm temperatures, warm-water species tend to dominate impoundments. An impoundment with cold water, however, will support cold-water species. Some fish, such as trout, need cold water because cold water holds more oxygen; however, if the water is too cold in winter, the eggs do not develop quickly enough and a large percent of the eggs die.

Dams are major obstacles to the movement of fish, either upstream or downstream. This can result in the disappearance of migratory fish from the river in and upstream of the impoundment (Hynes, 1970). Although the free movement of fish can sustain a healthy fishery, a dam may also be a barrier to the movement of unwanted invasive species. For example, the Orienta Dam on the Iron River in northwest Wisconsin prevents the further movement of sea lampreys up the Iron River from Lake Superior. The dam–removal proposal for this location recommends a barrier designed to restrict movement of unwanted species. If such a dam is to be removed, appropriate measures must be taken to protect the river upstream for the unwanted invasive species (installation of a structure that allows fish passage, but prevents passage of sea lampreys).
 

Free-flowing systems

Free-flowing systems contain phytoplankton and support aquatic plants that must be rooted strongly enough to the bed of the river that they are not uprooted by the flow of water. Aquatic invertebrates, such as insects and crustaceans (scuds, crawfish), feed on the phytoplankton and plants. The types of invertebrates present in a river depend on several factors, including water temperature, dissolved oxygen, current speed, streambed material, vegetation, and dissolved substances (Hynes, 1970). Just as different types of fish prefer different temperatures, so do different types of invertebrates. Many invertebrates rely on the river’s current for supplying food or for respiration. Different aquatic invertebrates also prefer different types of streambed materials. Some invertebrates attach themselves to rocks; others burrow in sand or silt. Mussels are sensitive to high turbidity. Riverine insects generally require more dissolved oxygen than lake-dwelling insects.

Aquatic plants and invertebrates provide most of the food source for fish in free-flowing rivers. Although in most river systems a free-flowing section tends to be cooler than an impounded part, free-flowing sections may support warm-, cool- and cold-water species of fish, depending on the location in the watershed and other factors. By seasonal movement of fish, a single river may support fish preferring all ranges of temperatures at different times of the year or at various life stages. Almost all game and sport fish in Wisconsin are migratory, not sedentary. They seek out different habitats at various life stages and times of the year; these habitats can be separated by tens or hundreds of miles along the river. As the river increases in size along its length, the characteristics of habitats in the river change. These different habitats are needed for optimal reproduction, feeding, rearing, or other factors we may not fully understand. These fish species could not sustain such large expenditures of energy if corresponding benefits did not result.
 

Biological restoration

Biological connections between the parts of a river upstream and downstream of a dam can be improved while keeping a dam. Fish passage may be installed at the dam to aid the upstream or downstream movement of fish. This may enable some fish species to access natural habitats. However, fish passages vary in their success rates, depending on factors including design and placement, and do not fully allow fish to migrate freely.

Restoration of the biology of impoundments depends greatly on the restoration of water quality in the impoundment. The most common water–quality problem in impoundments is eutrophication due to large influxes of nutrients. Dealing with eutrophication may be split into two approaches: reducing the inputs of nutrients and periodically removing the excess nutrients and organic materials that have built up in the impoundment. Reducing the inputs of nutrients is the best preventative measure.

Cooke and others (1986) describe in detail various methods for restoring impoundments in their book Lake and Reservoir Restoration. The following are short lists of possible restoration activities:

Methods for removing nutrients already in the impoundment include
 

• dilution and flushing of the impoundment;
• precipitation of phosphorous using aluminum;
• inactivation of phosphorous using chemicals;
• sediment oxidation;
• sediment removal by dredging; and
• removing phosphorus and other nutrients by harvesting plant material from the system.

Methods for removing excess plant material include
 

• shading plants near shorelines;
• harvesting the plants (control or removal of rooted aquatic plants can result in heavy blooms of planktonic algae);
• artificially circulating air;
• drawing down the level of the impoundment temporarily; and
• adding aquatic plant barriers.

The removal of the dam should encourage the ecosystem to return to its natural character, as the river flow conditions return to approximately their pre-dam state. However, other major changes in land use since construction of the dam will also have effects on the river system, so the river system may not return to exactly its original state. Dam removal causes changes in water quality and character of the flow of the river that have major impacts on the plants and animals present in the river. It will probably take a few to several years for the plant and animal populations to stabilize after removal of a dam, just as it took time for the current types of plants and animals to stabilize after the dam was built. The temporary release of sediments from dam removal is likely to stress the fishery, especially if not properly managed. It may take time for the lake type of invertebrates to leave and to be replaced by riverine species. It can also take time for fish species that prefer the temperature and still water of the impoundment to move away, and for the natural river species to move into the restored area. An ongoing study is exploring the changes in the fishery of the Baraboo River, Sauk County, before, during, and after the removal of four dams along the river (E. Stanley, verbal communication, 2000). Eighteen months after the removal of the waterworks dam, researcher found that the previously abundant carp population had been replaced by a diversity of fish species. Before the dam was removed 11 species were present. After the dam was removed, there were 24 species, including a healthy population of small mouth bass. In another example, below Prairie Dells in Lincoln County, brook trout reproduction increased 30-fold after the temporary effects of dam removal passed (J.C. Martini, Wisconsin Department of Natural Resources, written communication, 2000).

Removal of a dam will also have effects on the ecology of the land in the area. Dam removal will re-establish riparian corridors, the strips of shoreland along the length of a river, to their original locations. These areas are the transition between the aquatic environment and the dry uplands. As a result of this transition, riparian corridors are characterized by high total numbers and diversity of plants, ranging from grasses and shrubs to large trees. These ranges in plant life in turn supports large numbers and diversity of animals, semi-aquatic and land dwelling.
 

 
Next section: Timing of dam repair or removal

URL: http://www.ies.wisc.edu/research/wrm00/educbio.htm