Date of Award

12-2017

Degree Name

MS in Forestry Sciences

Department

Natural Resources Management

Advisor

Brian Dietterick

Abstract

Macroinvertebrate community structure was characterized along the channel gradient of a headwater stream in a coast redwood forest on Cal Poly’s Swanton Pacific Ranch in Davenport, California. The significance of physical habitat characteristics in describing macroinvertebrate assemblage structure was assessed in an effort to create a framework to better understand the expected biological response to riparian canopy manipulation. Seven study reaches were established in 2015. These study reaches were evenly spaced throughout the Little Creek watershed, an approximately 4.8 km2 drainage characterized by steep inner-gorge areas and dense riparian vegetation. Macroinvertebrate samples were collected seasonally (i.e., spring, summer, and fall) during 2015 and 2016 using the Reachwide Benthos procedure described by the Surface Water Ambient Monitoring Program’s bioassessment protocol and all captured organisms were identified at family level taxonomic resolution. Non-metric multidimensional scaling (NMDS) and Analysis of Variance (ANOVA) procedures were performed to describe longitudinal patterns in community composition and determine the significance of collected environmental variables as predictors of community structure. The majority of taxa collected belonged to the orders Ephemeroptera, Plecoptera, Trichoptera, and Diptera. Macroinvertebrate density and biomass were significantly associated with study reach, with relatively depauperate macroinvertebrate assemblages occurring in the upper study reaches and significantly larger, more diverse assemblages in the lower study reaches. Significantly higher density and biomass was observed during the summer sample period across all sites. A clear site level separation was observed at the South Fork study reaches where significantly higher abundances of Diptera taxa colonized the primarily bedrock channel at those sites. The most diverse and pollution-intolerant assemblages were observed in riffle habitat types. Stream shading and solar radiation were not significantly associated with any macroinvertebrate community metric examined, making it difficult to predict instream response to a riparian canopy manipulation. However, trophic interactions that influence secondary production in the study reaches could be inferred based on temporal patterns in feeding guild composition; the relative abundance of shredder taxa coincided with seasonal detrital inputs indicating that food webs largely depended on allochthonous energy sources. Therefore, there is significant opportunity for further investigation of energy production and utilization in the study reaches to guide riparian canopy management practices toward enhancing key trophic interactions. This study provides an extensive and novel biological baseline for macroinvertebrate communities in Little Creek.

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