The Aquatic Communities of Pennsylvania
In the community description chapters (4-7 in the ACC
User’s Manual) fish, mussel and macroinvertebrate communities are described
by the species that indicate each community type and the stream habitats the communities
are commonly found in. Information about community rarity, threats and conservation
recommendations is also included.
Fish communities are described for two separate watersheds: Atlantic drainage basin
(Delaware, Susquehanna and Potomac River watersheds) and the Ohio-Great Lakes basin
(Ohio River, Genesee River and Lake Erie watersheds). Mussel communities are described
from three areas: 1) Delaware River basin, 2) Susquehanna and Potomac River basins
and 3) the Ohio River and Lake Erie basins.
Aquatic Communities and Watersheds: FAQ
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- What is an aquatic community?
Aquatic communities are assemblages or groups of organisms that occur together and
have similar habitat preferences. The habitat can be inferred for the waterways
in which a community occurs when there are known community-habitat associations.
- Where do these aquatic communities occur?
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.
- How were aquatic communities determined?
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 (link).
The community types, habitat and environmental associations are described in the ACC User’s
Manual report (link).
- What do mussels, macroinvertebrates and fish tell me about streams and watersheds?
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 predicable 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.
- What environmental factors influence aquatic communities?
Many factors influence the occurrence of aquatic communities, including natural variations in stream environments.
Fast-flowing, cold streams flowing from ridge tops 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 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.