Exercise

The purpose of today's lab is to sample two sections of a stream, and then to determine if differences between these two sections are caused by natural changes or by human influences. Paint Branch Creek is a small stream that flows nearby campus. Several miles upstream from campus, Paint Branch flows through an area that is relatively undisturbed by humans. In fact, to this day the upper portion of Paint Branch Creek is a very nice state-designated trout stream. Further downstream, Paint Branch flows through the metro D.C. area and then along the northern edge of campus. By this point, the stream has experienced numerous changes. You will notice that many areas have been channelized to ensure more predictable flow. This is particularly obvious around bridges where boulders have been placed to prevent erosion. Furthermore, you may notice drains that transport water from parking lots to the stream, and possibly areas along the stream bank where vegetation has been removed.

Today we will sample biota living in the stream in order to identify the various trophic levels and to develop a food web. Further, we will sample five abiotic variables (temperature, dissolved oxygen, acidity, nitrates and phosphates) that are very important to stream organisms. Here are some things you should know about each variable.

Temperature

Water is more thermally stable than air, and because of this, most aquatic organisms have evolved to tolerate only a narrow range of temperatures. In turn, they are extremely vulnerable to even small changes in stream temperature. You can imagine that streams warm up as they become wider and allow greater penetration of sunlight. Indeed, these natural changes explain why different species of fish inhabit streams of different sizes. But in addition, humans alter stream temperatures by channelization (which removes deep pool habitat), cutting of streamside trees, construction of dams, and many others.

Dissolved Oxygen

Almost all organisms require oxygen to survive. This includes both aquatic plants and animals whichbreathe dissolved oxygen from the water. As discussed above, there are two factors that contribute dissolved oxygen to the stream: (1) photosynthesis produces O2 as a metabolic byproduct, and (2) turbulent mixing of water and air in riffles. Thus, the amount of dissolved oxygen usually differs between riffles and pools, and also changes as streams become larger (see Figure 2). Humans alter the amount of dissolved oxygen in streams by channelization (elimination of riffles), the dumping of sewage which increases bacterial respiration, and eutrophication.