CANCELLED - Phenology of aquatic insect emergence in a northern Minnesota stream

Background/Question/Methods

Aquatic insects represent a critical component of stream ecosystems, through their biomass itself and the varied trophic roles they can perform.  In addition, aquatic insects that emerge as adults can become subsidies of matter and energy to riparian ecosystems.  The timing of such emergence can thus have important implications for both the aquatic and terrestrial ecosystems, as other organisms (e.g., fish, birds) may migrate or shift foraging behavior in anticipation of emergence peaks.  The purpose of this study was to better understand the phenology of aquatic insects in a northern Minnesota stream.  Specifically, we sought to determine which taxa had patterns of emergence that were seasonal (i.e., those whose emergence peaks during a relatively short period) and which were non-seasonal (i.e., those whose emergence is relatively even throughout the open-water months).  Fifty floating, 0.5-m² tents were stationed at random locations in the stream within a forty meter stretch to trap emerging insects.  Emerged insects from each trap were then identified and compared against other collection periods (i.e., late May, mid-July, or late August) to determine whether taxa have seasonal or non-seasonal phenology. 

Results/Conclusions

Total emergence was much greater in May, with 1937 individuals compared to 708 and 678 in July and August.  The three most common taxa in May were Doliana americana mayflies (31%), Chironomidae midges (20%), and Tanyderidae crane flies (12%) whereas emergence in July and August was dominated (>50%) by Chironomids.  Examples of seasonal emergence included Perlodidae stonefies (emerging exclusively in May) and Leptoceridae caddisflies (emerging exclusively in July).  By contrast, Chironomidae showed a non-seasonal pattern, with fairly consistent emergence across May, July, and August.  These determinations of aquatic insect phenology have important implications for maintaining and preserving stream communities, and should be considered when resource managers are planning management and conservation efforts.  For example, culvert replacements are common in the study area in northern Minnesota, and represent anthropogenic disturbances which could be planned to avoid vulnerable life stages (e.g., emerging duns) for sensitive species.