The fish community and the stream habitat were evaluated at selected reaches in each of the seven long-term monitoring basins (see map on page 7) to determine the distribution of fish populations and the relations of fish populations to stream habitat (Bilger and Brightbill, in press). Studies of fish-community composition were done annually from June 1993 to June 1995. As with other parts of the NAWQA study, each basin represented an environmental subunit. Environmental characteristics for the selected reaches consisting of instream and riparian habitat, hydrology, and water quality were determined.
A total of 33,143 fish were collected from the 28 samples from multiple reaches in the 7 basins during the 3-year intensive sampling period from 1993 to 1995. Thirty-nine species were collected from eight families. The Cyprinidae (minnows) were represented by the greatest number of species (17), followed by the Centrarchidae (sunfishes) with 7 species, and by the Percidae (perches and darters) with 4 species. The most abundant and frequently collected species were the blacknose dace, white sucker, and mottled and slimy sculpins. Together, these species made up 49 percent of the total fish collected. Statistical analysis determined that the composition of fish communities for each stream did not differ between years and multiple reaches.
Fish communities inhabiting the seven streams were related to the bedrock type. Limestone and dolomite bedrock are associated with limestone streams (see table on page 6). Sandstone, shale, and crystalline rocks are associated with freestone streams (see table on page 6). Limestone streams were located in valley areas and receive much of their flow from large springs (Shaffer, 1991). Limestone springs discharge cool water to the stream throughout the year. The limestone (calcium carbonate) dissolved in the spring water provides for a stable pH. These factors make the conditions favorable for sensitive fish such as trout species. Limestone streams are known for naturally low numbers of fish species and high abundances of aquatic plants and invertebrate life. The valuable limestone farmland is commonly cultivated to the edge of the streambank, leaving little or no riparian vegetation (canopy cover). This, in turn, affects water temperature. Agricultural areas with little or no riparian buffers can also have increased sedimentation.
Freestone streams, such as Stony Creek, tend to be supplied by runoff from small streams that flow off ridges. Riparian vegetation provides favorable habitat conditions by shading the stream and reducing sedimentation.
Limestone streams, like Bachman Run, are supplied by springs that have nearly constant ambient temperatures throughout the year. Agriculture is a dominant land use that can degrade habitat conditions by reducing riparian vegetation and increasing sedimentation. Habitat degradation can offset the natural benefits of having a supply of cool water from springs.
Freestone streams tend to be fed from runoff and by small feeder-type streams and gain water a little at a time. The flow and temperature in these streams is more variable. Freestone streams do not have as much dissolved calcium as the limestone streams and are vulnerable to changes in pH. These streams also tend to flow off ridges and through areas with hilly topography, making the riparian zones less likely to be cultivated. Although freestone streams do not have the large springs discharging to the stream, the absence of alterations to the riparian habitat is favorable for fish communities.
The habitat characteristics that proved most influential in defining fish communities in the seven long-term monitoring basins were mean channel width, mean water temperature, mean canopy angle, and suspended sediment. These four variables combined accounted for about 79 percent of the variation in the stream habitat-species relation.
Fish are sensitive to water temperature. Warm-water streams support different fish communities than cool-water streams. Streams with moderate water temperatures have species found in both the cooler and warmer streams. Canopy angle and channel width, which affect water temperature, influence the fish species that are able to inhabit a stream. Canopy angle determines the amount of sunlight that reaches the stream surface. A wide stream can have a well-established riparian zone, but the canopy cannot shade the entire stream; thus, the water temperature is typically higher than for a smaller stream with the same type of riparian zone.
Fish were sensitive to suspended sediment; therefore, erosional bank conditions also influenced fish communities. Steep, high banks with little vegetative cover have a greater chance of erosion during storms than lower banks with more vegetation. Banks consisting of finer sediment are more erodible than banks that consist of cobbles and boulders. The more tolerant fish species were present at sites that were warmer and where the banks were more eroded than at sites that were cooler and had more stable banks. These factors also influence the amount of oxygen in the stream water. Fish with high oxygen demands typically thrive in cooler waters with little to no erosion and with fairly high oxygen concentrations. Fish with lower oxygen demands can live in warmer waters where lower oxygen concentrations are common.
The health or general condition of the fish community was determined by examining the populations of pollution-tolerant and -intolerant species, the numbers of nonnative species, the percentage of omnivores, and the percentage of individuals with external anomalies. The assessment of the fish community, based on these factors, showed that fish populations were healthier in the three freestone streams than in the four limestone streams. This may be the result of a number of interrelated factors, such as riparian vegetation and canopy angle, which affect temperature and sedimentation. The intense agricultural activity in limestone areas can have an influence on the fish community. The influence of agriculture on fish communities is related to habitat degradation rather than nutrients in the water. The limestone agricultural settings appear to adversely affect the fish community in many ways. Although limestone streams have many characteristics that would support a healthy fish population, changes in the land use around the stream can adversely affect the native fish populations (Shaffer, 1991). The limestone agricultural streams chosen for this study were chosen to assess the effects of intense agricultural activity and do not represent the fish populations of all limestone streams.
In summary, the overall ecological condition of the Lower Susquehanna River Basin appears to represent good water quality for aquatic life and low contaminant levels. In addition to analyses of the fish community, preliminary analysis of benthic-invertebrate communities collected and analyzed at the seven long-term monitoring sites are mostly indicative of the natural conditions that would be expected. For example, headwater limestone streams have fewer species than larger freestone streams. No sites had benthic-invertebrate communities that indicated adverse effects from water quality. Examination of long-term retrospective data generated by local regulatory agencies confirms that ecological conditions in the basin are improving. Sensitive water-quality indicators such as mayflies are now considered a local nuisance species at nighttime sporting events along the Susquehanna River at Harrisburg, Pa.