Description of Benthic Macroinvertebrate Assemblage It was first necessary to determine if all data from the study could be pooled for analysis, or if the spring data and summer data (April versus September, respectively) should be analyzed separately. This was done by using paired t-tests to analyze densities of 16 individual taxa that appeared to be abundant in at least one season, density of total organisms, and 12 assemblage metrics that represented a variety of ecological information. Nine of the 16 taxa densities were significantly different between April and September (Table 3). Of those nine densities, six were significantly higher in September and three were significantly higher in April. Density of total organisms was not significantly different between seasons. Of the 12 assemblage metrics, 9 were significantly different between April and September (Table 4). Since more than half of the taxa densities and three-fourths of the assemblage metrics were significantly different, it was concluded that the data from each season should be analyzed separately. It is most likely that the seasonal differences reflect the natural life cycles of the invertebrates, especially the insects.
A total of 158,882 organisms belonging to 91 taxa were collected during this study. A greater number of taxa occurred in April than September (Table 5). More taxa were found at the Bruce Farm (BR) reaches than the Mixed Use (MU) reaches (84 versus 63), though twice as many samples were taken at BR.
BR than MU in September. The most abundant organisms in the benthic macroinvertebrate assemblage were insects in the order Diptera (true flies), primarily Chironomidae (non-biting midges). Chironomidae exhibited much higher density at BR than MU in both seasons. Tipulidae (crane flies) were another group of abundant Diptera, especially Antocha at BR on both dates. There were relatively few taxa of Diptera, most of which were rare in occurrence.
The order Trichoptera (caddisflies) was the second most abundant component of the benthic macroinvertebrate assemblage. Most of their abundance was accounted for by two genera, Cheumatopsyche and Hydropsyche, in the family Hydropsychidae (common netspinners). Density of Hydropsychidae was slightly higher at BR than MU in April, but was much higher at BR than MU in September. Many Trichoptera taxa other than Hydropsychidae were collected in this study, many of which are considered to be sensitive to environmental stressors. Four genera of these other Trichoptera had appreciably higher density at BR than MU (Micrasema, Helicopsyche, Hydroptila, Goera), while the reverse was true for three genera (Glossosoma, Chimarra, Neophylax). Of these seven genera, Hydroptila exhibited conspicuously higher density at BR than MU.
The insect orders Coleoptera (water beetles) and Ephemeroptera (mayflies) occurred at about equal density and were the third most abundant groups of macroinvertebrates. Within the Coleoptera, Elmidae (riffle beetles), especially Optioservus and Stenelmis, accounted for most of the high density. Psephenus herricki in the family Psephenidae (water pennies) also occurred at an appreciable density. All of these water beetles were much more abundant at BR than MU, especially in September.
Among the Ephemeroptera, one genus, Ephemerella, in the family Ephemerellidae (spiny crawlers) accounted for most of the density in April. Ephemerella occurred almost exclusively in April, and density was much greater at BR than MU. The genus Seratella was the most abundant in September, and also occurred at higher density at BR than MU.
The only other groups that occurred at moderately high density were non-insect taxa: Oligochaeta (aquatic earth worms), Planariidae (flat worms), and Pleuroceridae (snails). All of these non-insect taxa were much higher at BR than MU. Oligochaeta and Pleuroceridae had their highest density in April, whereas Planariidae had their highest density in September.
Plecoptera (stoneflies) demonstrated much lower density and richness than what would be expected in a pristine stream comparable to Smith Creek. The only stonefly with moderately high density was Amphinemura, which had higher density at BR than MU and was only collected in April. With the exception of Amphinemura in April, all Plecoptera taxa were considered rare for analyses.
The Virginia Stream Condition Index (SCI) was calculated for each sample at BR and MU. The maximum score at BR was 55.89, the minimum was 27.95, and the average score was 44.86. All BR samples were classified as impaired (score < 61.3) according to the SCI. The maximum score at MU was 64.2, the minimum was 37.83, and the average was 49.86. Though MU scored slightly higher than BR, all but one sample at MU were classified as impaired (VADEQ, 2008b).
Several statistical techniques were used to examine spatial differences among the nine reaches. First, two-sample t-tests were used to compare the means for the 12 selected assemblage metrics in the BR section versus the MU section in April and September (Table 6). The majority of the metrics were significantly different between the two streams in both seasons.
In April, nine of the twelve metrics were significantly different between the BR and MU sites. Number of Sensitive Taxa, % Clingers-Hydropsychidae, and % Scrapers were the only metrics that were not significantly different between the two sites in April. Four of the significantly different metrics portrayed BR as having better water quality. Taxa Richness, Number of EPT, Number of Clinger Taxa, and Number of Crawler Taxa, were significantly greater at BR than MU. Five metrics indicated better water quality at MU, including Simpson’s Diversity Index, Hilsenhoff Biotic Index, % Predators, % Diptera and Non-Insects, and % Scrapers.
In September, ten of the twelve metrics were significantly different between the BR and MU sites. Only SDI and % Clingers – Hydropsychidae were not significantly different. As with April, about half of the significantly different metrics tended to portray BR as having better environmental conditions, including Taxa Richness, Number of EPT Taxa, Number of Sensitive Taxa, Number of Clinger Taxa, Number of Crawler Taxa, and % Collector-Filterers. Other metrics tended to portray better environmental conditions at MU, including HBI, % Diptera and Non-insects, % Scrapers, and % Predators.
The metrics that indicated better environmental conditions at BR were mostly measures of richness, whereas the measures that indicated better environmental conditions at MU were based on relative abundance. EPT taxa are sensitive to environmental stressors.
Clinger and crawler taxa require microhabitats that are relatively free of fine sediment. Collector-filterers exhibited lower relative abundance at BR, possible indicating less suspended organic matter that comes from nutrient enrichment. The HBI is a biotic index that produces a lower score when sensitive taxa are more abundant. Diptera and non-insects tend to include taxa that are tolerant of environmental stressors. Scrapers require a healthy, productive, but thin layer of nutritious algae on rock surfaces.
ANOVAs were performed and followed by Tukey’s post hoc test if there were significant differences among sites. Tukey’s post hoc test was used for further analysis of spatial groupings among the three MU and six BR reaches in April and September (Tables 7 and 8). Since there were 12 individual metrics and 9 reaches, it was difficult to distinguish consistent groupings. Upstream BR reaches (BR1-3) frequently grouped together, but not exclusively so. Downstream BR reaches (BR5-6) also often grouped together, although BR6 also frequently grouped with the MU reaches. This is likely because BR6 has a narrow riparian zone with a few trees, unlike the other BR reaches. The three MU reaches usually grouped together but almost never exclusively separate from the BR reaches.
Since univariate ANOVAs with 12 assemblage metrics provided only limited insight into spatial patterns among the nine reaches, a multivariate ordination technique, Principal Components Analysis (PCA), was performed on densities of all common taxa. Rather than classifying groups of reaches, ordination arranges the reaches in all possible gradients (axes) of how they relate to one another based on “species space,” then chooses and displays the best two gradients (axis 1 and axis 2). Multivariate ordination techniques, such as PCA, offer the advantage of integrating information on all of the taxa simultaneously rather than interpreting many individual univariate analyses.
List of Figures
List of Tables
Materials and Methods
Results Description of Benthic Macroinvertebrate Assemblage
Relationships of Environmental Factors and Benthic Macroinvertebrate Assemblages
Visual Estimates Versus Measurements of Habitat Variables for Explaining
Benthic Macroinvertebrate Assemblage
Measurements Versus Visual Estimates of Substrate Composition
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Environmental Factors Determining the Pre-Restoration Benthic Macroinvertebrate Assemblage In A Stream Used By Cattle