Discontinuity in the insect assemblages of a northern Lower Michigan stream

Abstract

We assessed the physicochemical and biological continuity of a 2nd-4th order reach of the Little Manistee River in northern Lower Michigan. Contrary to typical woodland streams, the downstream sites of the river were covered with a dense riparian canopy, whereas the upstream sites were devoid of this canopy due to historical (≥10 ybp) agriculture. Other than slight changes in water temperature and dissolved oxygen, there were no appreciable differences in measured water physicochemistry between the canopied and non-canopied sites. The stream, however, appeared biologically discontinuous as indicated by lower shredder abundance and higher filtering collector abundance at the upstream (non-canopied) sites for both benthic macroinvertebrate and adult caddisfly assemblages. Benthic scraper abundance was, likewise, higher in the upstream sites. Our results suggest that changes in riparian canopy alone can lead to changes in biological assemblages, even without obvious changes in water physicochemistry.