The community types described here are restricted to flowing water habitats, such as rivers and streams. Communities are identified within watersheds, which are commonly defined as an area of land where all water drains to the same point (www.epa.gov). In watersheds, the water moves through a network of drainage pathways, both underground and on the surface. Generally, these pathways converge into streams and rivers, which become progressively larger as the water moves on downstream, eventually reaching an estuary and ultimately the ocean. All land is part of a watershed and every stream, tributary, or river has an associated watershed. Small watersheds join to become larger watersheds, just as small streams join to become larger streams.

Mussels, macroinvertebrate and fish datasets were analyzed to describe the organisms that most commonly occur together in communities and have similar habitats. We determined the communities from the results of multivariate analysis of species and sites. The communities were associated with water quality, habitat, and landscape information. We reviewed species associations and communities with a panel of aquatic biologists from the region. Details about the analyses and data for the ACC project are available in the Methods Report. The community types, habitat and environmental associations are described in the ACC User's Manual report.

All three of these types of organisms hold unique niches in Pennsylvania's streams and rivers. Macroinvertebrates include aquatic insects, worms and crustaceans (e.g., crayfish and scuds), which generally occupy the lower levels of food webs in aquatic systems. The presence of certain macroinvertebrates reflects differences among stream locations in food availability, water quality and habitat type. Perhaps most importantly, macroinvertebrate communities provide an overall picture of stream health; macroinvertebrate taxa generally respond to environmental stress in predictable ways, based on their levels of tolerance to different stressors.

Macroinvertebrates are an important prey source for many fish. Food resources and spawning habitats can be specific for different species of fish as different species will have different habitat requirements and habitat needs. Just like macroinvertebrates, fish are influenced by stream quality and the condition of the watershed. For example, sediment from erosion at a mismanaged construction site near a stream may cover substrates that are necessary for fish such as brook trout to lay their eggs. Layers of fine particles from sedimentation such as this can also smother the habitats that developing fish require, preventing them from reaching adult life stages.

Mussels are filter feeders, which means that they siphon water through internal gills to extract particles of food from the water column. They require relatively clean water to survive, and are particularly sensitive to industrial discharge, abandoned mine drainage and urban runoff pollution. Mussels generally require gravelly, sandy or muddy habitats where they can burrow into the stream bottom. They typically occur in larger streams and in rivers that contain sufficient nutrient levels to supply them with food.

Many factors influence the occurrence of aquatic communities, including natural variations in stream environments. Fast-flowing, cold streams flowing from ridgetops provide different habitat types than slow, warmer rivers meandering through valleys. Aquatic communities reflect these differences in stream type and environment. Geology also varies across Pennsylvania, and flowing water may have unique chemical compositions based on the types of rocks that it contacts.

Human alterations to aquatic environments can exert much stronger effects than any type of natural variation discussed above. Many changes within a watershed can be detected within its streams and rivers. If implemented improperly, timber harvest, agriculture, urban development and road management are among some watershed alterations that may cause changes in water quality and stream habitats from non-point source pollution. Additionally, a number of pollutants can enter aquatic systems from point sources, such as discharges from sewage treatment plants, abandoned mines and other industrial sources.