Pomme de Terre Watershed Project
When many people think about stream biology, they often time think about what kind of fish they are able to catch in a stretch of river. Will they be able to catch some walleyes, or will carp be the only thing they have a chance at catching? But stream biology is much more than what kind of fish are swimming in the stream, and river systems are much more than simply the narrow ribbon of water that makes up the stream itself.
Streams are complex networks of terrestrial and aquatic communities. Streams and their floodplains provide diverse habitats including uplands, riparian streambank zones, floodplain forests, marshes, oxbow lakes, riffles and pools. The diverse habitats and their plant and animal species are key to maintaining healthy ecosystems.
Among the different ecosystems, rivers are unique because of the one-way, continual flow of water, sediment, nutrients and organisms. Several forms of life spend most, if not all, of their lives in rivers and streams. These organisms include types of bacteria, algae, plants, insects, mussels, fish, amphibians, reptiles and mammals. The organisms in an ecosystem are interconnected to form a food web.
In rivers, primary producers compose the base of the food web. Primary producers are aquatic plants and algae, which use the energy from the sun, along with carbon dioxide and water, and convert it to organic matter through photosynthesis. Other organisms such as bacteria and invertebrates feed directly on these primary producers, in turn; fish and other vertebrate predators feed on these, and each other. In the "Lion King," Mufasa called it the "great circle of life."
The diversity of aquatic organisms depends on a variety of stream habitats. A sinuous stream provides more habitats than a straight channel. A streambed composed of rocks and sediments of various sizes provides a greater assortment of habitats than a streambed of uniform sediment. Woody material, such as fallen trees in the water and along the bank, provides beneficial structure. They become an integral part of the channel, deflecting current, forming scour holes, and providing substrate for attaching organisms, basking sites for reptiles and amphibians, and overhead cover for others.
Streamside plants are important sources of shade and energy in narrow headwater reaches. Throughout a stream's length, the vegetation along the stream intercepts flows of incoming runoff, nutrients and contaminates. Deep-rooted native plants anchor soil in place and stabilize streambanks. Plants at the water's edge also serve as buffers from bank erosion, absorbing energy of waves and swift currents.
Streamside habitats along the lateral and longitudinal gradients allow movement of traveling wildlife. Long wooded stream courses provide many species, especially mammals, important avenues of movement to other habitats. In altered and developed landscapes, the floodplain of a river is often the only stretch of remaining habitat for birds, mammals, reptiles, and amphibians.
Stream organisms of all kinds have physical and behavioral adaptations to varying water velocities. Consequently, the animals found in riffles differ from those in pools, even within the same stretch of river.
Fish species have adapted to different stream habitats by modifying their forms, habits and reproductive strategies. Fish in high-gradient upland streams have small, agile bodies that enable them to accelerate quickly and move through the swift, turbulent flow of these streams. They usually require water with a higher oxygen content and cooler temperature than downstream species. As one moves closer to the mouth of a river, the low gradient provides a greater variety of environments and therefore are populated with a wider variety of organisms. Fish in these rivers may be larger and more tolerant of wider ranges of temperature, and lower oxygen concentrations.
For some fish, like the sturgeon on the Red River, the upland streams provide habitat for spawning and young, while the lower gradient river is home to adults. For these species, it is essential that the longitudinal connectivity is maintained to allow the fish to move up and down stream to complete their life cycle.
Freshwater mussels can be the "canary in the coal mine" species of a river. Freshwater mussels are sedentary, long-lived mollusks that nestle in sediments while filtering particles out of the water to feed. Mussels are vulnerable to stream habitat disturbances such as dams, channelization and pollution, and therefore are good indicators of stream health. They are one of the most endangered animals in North America. Seventy species are currently federally listed as endangered or threatened. Mussels help clean the water of particles and chemicals during their feeding process, and are a source of food for fish and other animals. Empty mussel shells on a riverbank surrounded by raccoon tracks are a common site along many rivers. Freshwater mussels also use fish as hosts for their larvae, relying on fish abundance, and migration for dispersal. This demonstrates the interconnections of aquatic systems.
About the author: Shaun McNally is the Pomme de Terre Watershed Project Coordinator. He is located in the Stevens SWCD office in Morris and can be reached at 320-589-4886 ext. 109.
The Pomme de Terre River Association maintains a website: www.pdtriver.org.