The communities in a stream ecosystem include bacteria, algae/diatoms, macroinvertebrates, and fish. These communities form a food pyramid with bacteria and other microscopic organisms at the bottom and fish at the top. The physical features within a stream play an important role by influencing the availability of habitat for food supply, reproduction, and predation avoidance.
The Microscopic Community
Bacteria are decomposers that break down the organic materials derived from plants and animals into nutrients that fuel the algal and diatom community. If a stream does not retain organic material and make it available to bacteria, the aquatic community will be very poor. To trap and retain organic debris, a stream must have appropriate physical features that establish the necessary hydraulic conditions.
Algae and diatom communities tend to use the hard substrates found in streams. Cobble and gravel in riffles, bedrock in a run or pool, woody debris, and even stable sand are all suitable substrates for colonization. The more surface area that is available within a reach, the greater the potential for species diversity. Algal and diatom community richness is vital to other components of the food web, such as the benthic macroinvertebrates.
However, some algae can be harmful. Didymosphenia geminata, in the photo above, is a freshwater diatom thought to be native to far northern regions in cold, oligotrophic waters of North America, Europe, and Asia. Within the last 10 or so years, Didymo’s range has expanded to more nutrient rich waters in Maryland. Didymo occurs in lakes and flowing waters, but nuisance blooms are known only in streams and rivers. Nuisance growths of Didymo have the potential to impact fish populations, but clear evidence of negative influences is lacking. We have a .pdf fact sheet about Didymo here.
The Macroscopic Community
A large portion of the aquatic life in streams is composed of benthic macroinvertabrates, including clams, crayfish, worms, and aquatic insects. The term "benthic" refers to the bottom of a water body. "Macroinvertebrate" refers to organisms that lack an internal skeleton and are larger than five microns (about the size of the head of a pin). Like bacteria, these organisms are important for processing and transforming organic matter into sources of food for other aquatic life. Macroinvertebrates have a variety of lifestyles and feeding modes. Shredders decompose large organic particles from leaves and twigs that fall into the stream. Grazers scrape algae and diatoms from rocks and other substrates. Collectors filter fine particles transported from upstream. Predators feed on animal matter. The more diverse the features of a stream, the more diverse the macroinvertebrate community.
Healthy stream systems generally have species from several feeding groups and lifestyle modes that inhabit different stream features available within a drainage network. Active filtering collectors, such as clams and several species of mayflies, are found in pools and runs. Passive collectors, such as the net spinning caddisflies, are found in riffles. Leaf litter in pools harbor shredders such as amphipods. Shredding stoneflies inhabit leaf packs trapped in riffles and runs. The slow-moving water of pools is home to predatory dragonfly larvae, while predatory stoneflies inhabit riffles and runs. Scrapers, such as snails and many mayflies exist where there is a hard substrate colonized by algae and diatoms.
Increases in fine sediments that fill pools and clog the interstices in coarse bottom sediments reduce habitat diversity. As the diversity of physical habitat features decreases, the diversity of the macro- invertebrate community decreases. Bottoms composed of fine sediment generally have lower species diversities and larger populations of worms and midge larvae.
The Finfish Community
The physical habitat characteristics of a stream also influences the fish community. Like macroinvertebrates, fish species have adapted to specific habitats by using different feeding methods and other morphological characteristics. A number of species, such as sculpin, have evolved attributes, such as stiff pectoral fins and flat profiles, that allow them to survive well in the fast flows associated with riffle features. Other species, such as white suckers, have evolved specialized mouths to take advantage of the food sources in the soft sediments of pools and runs. Large predators, such as trout and bass, optimize hiding places in order to ambush their prey while avoiding the main stream flow, thereby conserving energy. Boulders and logs in the channel or undercut stream banks with extensive root mats can create these kinds of habitat conditions.