Caspar Creek macroinvertebrate assemblage responses to forest management and hydrologic disturbance

We analyzed data sets from the Caspar Creek Watershed study, with a 55-year comprehensive record of hydrologic regime in two sub-watersheds with different logging treatments, and three separate instream biologic studies conducted since 1990. Long-term data sets of instream biota are rare, and we used them to investigate sediment regime response to forest harvest and extremes in flow regime. Increases in sediment transport after logging were observed in the North Fork during second experiment at Caspar Creek in the 1990’s. Our analysis found turbidities were higher after logging across the range of flows including high magnitude/short duration as well as low magnitude/long duration events. However, few sediment impacts to macroinvertebrate assemblages were observed above and below tributaries with upstream harvest. Specifically, there were no differences in presence/absence of sediment sensitive taxa before and after logging. Moreover, North and South Forks sensitive taxa distributions were similar. With the South Fork, we compared 2016 and 2017 macroinvertebrate assemblages collected in July and May respectively. Statistical classification separated South Fork communities clearly by year and watershed location, reflecting a dynamic community. A six-grouping solution of 81 taxa divided based on relative abundance, watershed distribution, and functional feeding group. Natural disturbance has been high in the watershed recently with discharge regime varying five-fold since 2014, a drought year, and with higher than normal winter flooding in 2017. Sediment sensitive taxa were distributed in two groups of the six and showed no distinguishable patterns. However, overall patterns of the whole macroinvertebrate community structure between years and watershed locations were observed in the classification.