Volume 52, Number 1, May 2005:
Stream Ecology

Text-only version


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ABOUT THIS ISSUE
- about KSN
- about the author

IN THIS ISSUE
- introduction
- stream communities
- physical characteristics of streams
- biological features of streams
- common groups of stream organisms

Stream Ecology
by Carl Prophet

BIOLOGICAL FEATURES OF STREAMS

If anyone were asked to name some examples of animals that could be found in a Kansas stream, it is highly probable that the examples given would include fish, frog, turtle, crayfish or clam in some manner or the other. Certainly, each of these groups of animals is well represented among the stream macrofauna, but they represent only a fraction of the many diverse organisms present which play an important role in the ecology of a stream. If noticed at all, many other types of animals commonly found in a stream are often dismissed as some kind of worm or bug and therefore of little value. Yet, when carefully studied, each creature reveals something about how it meets the challenges of its environment and where it fits in the web of life in a stream.

Ecologists have a term for everything. To draw attention to differences in their spatial distributions and modes of life the various organisms inhabiting an aquatic community may be grouped into one of several categories. Large aquatic animals such as fish that can orient and actively swim against the water currents are collectively termed nekton. Conversely, microscopic organisms with relatively limited or no powers of locomotion and which are suspended in the water column are known as plankton. Plankton may be further divided into either phytoplankton or zooplankton. In general, plankton is less diverse and less abundant in lotic systems than in lentic systems. This condition is especially noticeable in low order streams. Organisms that are associated with the substrate in an aquatic habitat are known as benthos. Invertebrates which are found on the bottom of a stream and which are visible with the naked eye are collectively known as benthic macroinvertebrates. The neuston consists of organisms which move about on or just below the surface of the water.

Photo caption: Many kinds of benthic invertebrates can be found on the surfaces of large rocks taken from a riffle in a limestone stream.

Depending on behavior and life cycle stage some aquatic organisms may fit into more than one of the above described categories at different times. It is also possible to recognize additional subdivisions, but greater detail is unnecessary for our purposes. The point of interest here is to emphasize that each member of the biota occupies a specific physical position within the community that in turn enables it to utilize the ecological resources in varying ways. The end result is a more diverse and efficient biological community. Because there are distinct differences in the physical conditions between lentic and lotic communities there are some distinct differences in the diversity and relative abundance of the organisms in each of these categories.

It was previously mentioned that, to a large extent, the physical features of a stream greatly influence the types of organisms which inhabit them. For example, there are some obvious differences between the species composition of fishes and benthos inhabiting a sandy stream and those found in a rocky limestone stream. Mussel populations in a limestone stream tend to be more diverse and more abundant than in either a sandy or silty stream. Macroinvertebrates dominate the benthos in limestone streams while microscopic burrowing forms are abundant in silty streams. If a sample of the substrate from a sandy stream is examined microscopically, an abundance of tiny animals may be found which live in the spaces between the sand grains. Aquatic ecologists term these types of organisms psammon. Most people are probably unaware that such creatures exist.

Benthic organisms are vital links in stream food webs, the complex pathways whereby energy is passed from lower to higher levels in an energy pyramid. Some species filter or strain tiny suspended organic particles from the water thereby converting some of this potential energy into their bodies. When a filter feeder is eaten by a predator some of the energy represented by the suspended organic material is transferred to the next level, and so on. A significant part of the total energy moving through a stream is derived from allochthonous (outside) sources. In other words, it enters as organic material washed into the stream from its watershed and as leaf fall from bordering riparian woods. Some aquatic ecologists have shown that the breakdown of a leaf is brought about by the combined efforts of many different microorganisms and benthic macroinvertebrates. Feeding as shredders and scrapers, many of the macroinvertebrates help reduce big pieces of organic matter into smaller and smaller particles which, in turn, may be utilized by collector (filter and strainer) feeders and which can be more rapidly reduced by decomposer microbes.