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Effect of Reach-Level Changes in Ditch Morphology and Riparian Vegetation on Macroinvertebrate Communities in an Agricultural Landscape

Results

Findings

Bench Development

Bench Width to Channel Width Ratio: Bench width to channel width ratio for all study reaches. The mean for the Bench group (1.04 +/- SD 0.51) is significantly higher than both the Tree (0.26 +/- SD 0.14) and Control (0.20 +/- SD 0.08) groups.

Substrate Quality

Texture Class Composition of Bed Substrate: Mean relative percent composition of texture classes for each group. The % gravel is significantly higher in the Bench group than in both the Control and Tree groups. The % silt/clay is significantly lower in the Bench group than in the Control and Tree groups.

Habitat Quality

OEPA: Qualitative Habitat Evaluation Index (QHEI) Overall Score: Box plots of QHEI total score values for each group. Red dots indicate mean values, the center line is the median, the tops and bottoms of the boxes are the upper and lower quartiles respectively, and the whiskers represent the extent of the data. Both the Bench and Tree groups are significantly greater than the Control group.

Taxonomic Richness

Rarefaction Curves: Rarefaction curves for each group based on pooled samples. Curves have not been analyzed for statistical differences. Trend shows small differences in the taxonomic richness per number of individuals collected. The tree group is highest, followed by the Bench and then the Control groups.

Macroinvertebrate Metrics: Means and standard deviations of macroinvertebrate community metrics for each group. No differences are statistically significant. The % EPT (Ephemeroptera, Plecoptera and Trichoptera) is the relative composition of mayfly, stonefly, and caddis fly taxa. These taxa are typically pollution sensitive, and therefore are indicators of the quality of streams.

Community Assemblage

Feeding Guild Distribution: Mean feeding guild distribution across the three groups based on pooled samples. There were no statistically significant differences across the groups for any of the feeding guilds. Members of the Chronomidae (midges) were evenly distributed in the collector-gatherer and predator guilds.

Macro Invertebrate Community Index Scores: Box plots of modified Invertebrate Community Index (ICI) values for each group. Red dots indicate mean values, the center line is the median, the tops and bottoms of the boxes are the upper and lower quartiles respectively, and the whiskers represent the extent of the data. Both the Tree and Bench groups are significantly greater than the Control group.

Stepwise Linear Regression

Response and predictor variables and correlation coefficients for all significant forward and backward stepwise linear regressions. Macroinvertebrate metrics were related to morphology, substrate, and habitat. Habitat and substrate texture were positively influenced primarily by two-stage morphology. The (BE), or bench elevation, is the elevation in which the morphology metric was derived (i.e., the bench is inundated) . The other elevation used in deriving morphology metrics was the top of the channel, which is slightly lower than the bench elevation (i.e., the bench is not inundated).

Conclusions

• Bench development provided the best substrate quality and habitat for these ditches.
  
  
• Benthic communities had a marginal response to reach scale changes in riparian vegetation and ditch morphology.
  
  
• Density of individuals was weakly related to the amount of cobble in the substrate.
  
  
• The presence of either trees or benches resulted in an increase in relative composition of sensitive taxa.
  
  
• Feeding guild structure was not influenced by riparian vegetation or channel morphology.
  
  
• Taxa richness may not strongly reflect changes in benthic communities relative to riparian vegetation and channel morphology in heavily farmed areas.

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