A fish community is a group of fish species that live in the same area and interact with one another (Meador and others, 1993). Community composition refers to the kinds of species present and their absolute and relative abundances. Fish community composition is significantly influenced by factors affecting water-quality conditions, including land use, land cover, point- and nonpoint-source pollution, channelization, and wetland destruction. In order to assess and describe these relations within the Coastal Plain of North Carolina, fish, habitat, and water-chemistry data were collected and analyzed for sites on 16 small (14- to 59-mi2 basins) and 5 large (602- to 2,700-mi2) Coastal Plain streams. Only results for the 16 small streams are discussed here.
The small Coastal Plain streams that were sampled had relatively diverse fish communities. Forty-five fish species from 15 different families were captured at the 16 sites. The number of species (richness) captured at each site ranged from 6 to 25 and was comparable to the number of species found in streams in other areas of the southeastern Coastal Plain (Paller, 1994).
Several species were present in nearly every stream and, thus, compose the nucleus of what might be called a typical Coastal Plain small-stream fish community. These species include pirate perch, redfin pickerel, American eel, creek chubsucker, yellow bullhead, and bluespotted sunfish. Other species that were slightly less widespread and abundant included golden shiner, eastern mosquitofish, tessellated darter, and redbreast sunfish. Nearly half of the captured species were members of either the sunfish (Centrarchidae) or minnow (Cyprinidae) families (11 species each), which together included about one-third of all the fish that were captured.
Although nearly every small stream site had many of the typical Coastal Plain species, there were several important characteristics of fish community composition that differed among sites. These characteristics included species richness, the relative abundances of typical Coastal Plain species, and the presence and abundances of species that are associated with particular kinds of stream habitats. In addition, the differences were strongly related to basinwide environmental characteristics, such as soil drainage, the amount of wetland buffer, and land use.
Numbers of species were significantly correlated (Spearman's rho; p <0.05) with (1) the percentage of the basin having poorly drained soils (negative correlation), (2) the percentage of a 100-meter buffer zone along the entire stream classified as forested wetland (negative), and (3) the percentage of the basin used for row-crop agriculture (positive) (figs. 19, 20). Numbers of species also were correlated with several water-chemistry variables that are associated with these basinwide conditions, including (1) dissolved organic carbon (negative), (2) minimum dissolved oxygen (positive), (3) pH (negative), and (4) dissolved nitrate (positive). North Carolina Index of Biotic Integrity (NCIBI) scores, which are commonly used to indicate the ecological status of the fish community (North Carolina Department of Environment, Health, and Natural Resources, 1997), were related similarly to many of the same environmental variables (figs. 19, 20).
Figure 19. Numbers of species increased in basins having more row-crop agriculture.
Figure 20. Areas having more forested wetlands had lower numbers of species.
Although numbers of species and NCIBI scores were correlated significantly with basinwide environmental variables, they were not correlated significantly with reach-level variables, such as the amount of instream cover and substrate particle size. This result indicates that fish community composition may be broadly determined by basinwide environmental conditions and only secondarily by local conditions. One implication of this is that mitigation and restoration strategies that focus on the local scale, while neglecting basinwide conditions, may not be fully successful.
The relations between numbers of species, NCIBI scores, and environmental characteristics indicated that, in general, more species of fish and higher NCIBI scores were found in agricultural areas with minimal wetland buffers. In fact, richer fish communities and higher NCIBI scores tended to occur in areas where human impacts have been substantial--opposite of the expectation that numbers of species, and especially NCIBI scores, should decline when streams are affected by human activities.
This unexpected pattern may reflect natural differences in fish community composition that are a consequence of soil type rather than human activities. Streams located in basins dominated by poorly drained soils generally tended to have lower numbers of species and NCIBI scores than streams in areas dominated by medium- or well-drained soils (fig. 21). Yet several of the streams in areas with poorly drained soils were among the least affected in the study area--those with natural channels, the greatest percentage of wetland buffer, and the smallest percentage of row-crop agriculture.
Figure 21. Streams in poorly drained basins had lower numbers of species and NCIBI scores than those basins having well- and medium-drained soils.
Many of the streams in areas where poorly drained, organic-rich soils prevail cease flowing during the summer, partly because the soil does not yield enough ground water to maintain streamflow. The most pristine streams have extensive floodplain forests that contribute to this condition by lowering the water table through transpiration. Floodplain swamps and organic-rich soils contribute large amounts of particulate and dissolved organic material to these streams, and when the water stops flowing, the decaying organic material reduces the concentration of dissolved oxygen. High concentrations of dissolved organic acids also lower pH. The natural combination of quiescent water, low dissolved oxygen concentration, and, in some cases, extremely low pH may limit fish communities in these areas to a relatively small number of species that can survive these difficult summer conditions. These include banded sunfish, swampfish, and mud sunfish. The harsh summer conditions may, in fact, act as a natural barrier to other species, thus resulting in a refugium for a distinct natural fish community characteristic of Coastal Plain streams in areas with poorly drained soils.
In contrast, streams in basins with well-drained, sandy soils receive more ground-water input during summer and are, therefore, less dependent on rainfall to maintain streamflow. They also tend to maintain higher concentrations of dissolved oxygen and have a pH that is neutral or only moderately acidic. As a result, these streams appear to be able to support a more diverse fish community that includes more minnow species and species that are associated with flowing water, such as satinfin shiner, dusky shiner, comely shiner, and shield darter. Fish communities of unchannelized streams in these areas also included many of the species found in the areas with poorly drained soils. Fish communities in channelized streams in well-drained soils tended to have an even greater proportion of flowing-water minnow species and often lacked the species found in streams in areas with poorly drained soils.
The current version of the NCIBI is not calibrated to account for natural differences in species richness and other aspects of fish community composition that are associated with soil drainage characteristics. One way to accomplish this might be to calibrate the NCIBI independently for the inner and outer Coastal Plain. The inner Coastal Plain typically has sandy soils that drain relatively rapidly, whereas the outer Coastal Plain has many areas with organic-rich soils that drain much more slowly (McMahon and Lloyd, 1995). Calibrating NCIBI metrics separately within each of these physiographic provinces would help water-quality assessments account for a major component of the natural variability in fish community structure. Unfortunately, the data collected for this study are not sufficient for developing and testing candidate metrics for a recalibrated biotic index. Data for additional streams in the outer Coastal Plain are needed particularly.