Ecology is the study of organisms and their dwellings. You could study forest ecology, urban ecology, or even the ecology of the Johnson's household - with their permission, of course. In all cases you are considering the interaction of the living organisms - the biota - with their nonliving environment. And remember, the biota does not only refer to creatures that wriggle and crawl and do the two-step (as some animal chauvinists might have you believe). The living aspects of an ecosystem also include plants, even as small as algae, and microorganisms like protozoans and bacteria.
The focus of New Hampshire's Biomonitoring Program has primarily been the ecology of streams and rivers. Historically, streams were largely ignored from a resource management perspective. It was thought that streams were basically conduits, carrying water to rivers, which in turn served only to feed lakes. Today streams are recognized as magnificent ecosystems in their own right. The fact that they ARE conduits to rivers and lakes makes their study even more significant, given the current focus on watersheds and drainage patterns. Events occurring in a stream 20 miles upstream from Lake Clean-as-a-Whistle could turn that lake to Mud. Incidentally, there are 28 ponds in New Hampshire named Mud.
What is involved in studying the ecology of streams? There are flow considerations. Unlike lentic ecosystems (standing water), stream organisms must adapt to lotic, or moving water conditions. Imagine spending your days in a raging current, clinging to a tree to prevent you from traveling far downstream, leaving hearth and home behind. While many streams may not look like they are raging, if you are mayfly nymph (baby) weighing only a few milligrams, the smallest flow can sweep you away. All organisms fated to stream life have adaptations to flow. They may be simply smaller to slip between rocks to escape current. Fish and insects that dwell in currents frequently have a more streamlined shape, which reduces surface friction.
Insects especially have evolved ingenious means of surviving and even exploiting stream currents. Some actually have suction-cup type extensions that tether them to the substrate. To capture food they manufacture tiny nets to drape into the current. Several caddis fly species construct fixed retreats that are attached to the substrate to escape the current. Then there are other stream-dwelling organisms whose strategy is to NOT to fight the current. What better way to get the kids out of your hair than to toss them into the current and away they go downstream?
Another consideration of stream ecology is permanence. "We may be enjoying a lush aquatic environment today, but tomorrow we may all dry up." Many smaller streams are ephemeral - they exist intermittently as rainfall allows. Organisms either must make adjustments for this, by burrowing in mud or crawling, hopping, flying, swimming to wetter digs, or make long-term, evolutionary plans for the future. Some organisms realize their days are numbered by the spring snow melt and lay eggs resistant to desiccation, or produce encapsulated cases, or make some plan for future generations.
This brings up another aspect of stream ecology, and that is substrate. This describes the type of bottom any given stretch of stream has. Typically this changes continually along the stream's gradient. Also, the stream substrate is reflective of the geology of the area in which it flows. In headwater streams, with a high gradient, it's common to find rocky bottoms. As the terrain flattens and the current decreases, bedload is more easily deposited. Huh???? The bedload is the amount of sediment being carried by the moving water of a stream. In areas where the water slows, this sediment drops out and creates a silty bottom. In areas with extremely low flow or extremely large bedload, the stream bottom may be comprised of a heavy clay. Here you could expect to find vastly different types of organisms - those with more permanence. Aquatic vegetation can easily take root and in turn creates wonderful protective areas for fish and insects looking to escape any current.
Another physical aspect of stream ecology is the meander. What a fun little word to describe the path of the stream channel. Rarely is it a straight shot from headwaters to the mouth - in fact this never occurs without manmade influences. Here again, stream gradient - how far it drops and how fast, and regional geology have a big influence. But the path of a stream is important to the type of organisms that live there. An extreme meander creates many alternating pools and riffles, and a great variety of habitats.
One final aspect to consider in stream ecology studies is the riparian zone. It's not exactly the twilight zone but for many years it could just as well have been. There are terrestrial areas running along the stream bank and only recently has it received the deserved attention in stream studies. Consider all the impacts that stream-side vegetation has on in-stream activities. Leaf-fall contributes enormous amounts of litter - wonderful building materials for that two-bedroom bungalow Mrs. Caddisfly has been dreaming of. And vegetative material that is not utilized directly will be eaten or "decomposed" by microorganisms - the very base of the food chain. The riparian regions also dictate the amount of sunlight any given reach of stream receives. Shading is a big deal for all aquatic life, and can influence temperature and visual accuracy. Why do you suppose large mouth bass spend their afternoons basking in the cool recesses of a shaded pool?
There are many more ecological considerations for stream ecologists that have not been explored here. What types of animal populations inhabit streams? What types of plants? Where do they go when it's freezing? So much periphyton, so little time.