When to sample adult caddisflies: data from a five-year study of a first-order Michigan (USA) stream

Nearly 20,000 adult caddisfly specimens were collected during May through September 2010–2014 from approximately weekly ultraviolet light samples of Fairbanks Creek in northern Lower Michigan. Detrended correspondence analysis determined four distinct seasonal assemblages corresponding to spring (May to late June), summer (late June to late July), late summer (late July to early September), and autumn (early to late September). Of the 109 total species caught, 5 indicated the spring assemblage, 13 the summer assemblage, 1 the late summer assemblage, and 4 the autumn assemblage. Species richness, specimen abundance, and number of unique species were highest during the summer. Most summer samples contained ∼50% of all species caught during the year; whereas the spring and autumn assemblage contained a smaller but unique fauna, and the late summer assemblage was characterized by a decreasing richness of summer species. Pollution tolerance and shredder to filtering collector ratio metrics varied by factors of 5–30x; both were highly dependent on the abundance of particular species. Overall mean values of all metrics varied seasonally but not annually. The specific date of peak species richness varied based on annual weather, but occurred at a similar degree-day value for all 5 years. These results suggest the importance of sampling within a season when comparing streams for biological monitoring purposes. Moreover, they confirm the comparability of samples taken from different years, provided such samples are collected during the same season.