Print PageStream logjams have the potential to increase hydraulic heterogeneity and thus substrate diversity, entrap leaves as food for shredders, create surfaces that intercept insect drift and facilitate emergence, provide substrate for periphyton as food for grazers, and concentrate prey and overhead cover for fish. However, most of these mechanisms remain poorly understood, and their implications for macroinvertebrates are unresolved. For this study, we sampled a 40-m section centered on a spanning logjam in a small stream in northern Minnesota to determine influences of the logjam on benthic macroinvertebrates and insect emergence. In late May (2010) we assessed 50 random locations within the 40-m section, sampling benthic macroinvertebrates with mini-Surbers and emerging insects with 0.5-m2 floating traps. In each location we also measured physical characteristics of the stream by quantifying substrate types, depth, velocity, and taking various measurements on both submerged and emerged wood. Lastly, coarse particulate organic matter (CPOM) and fine particulate organic matter (FPOM) were taken from each mini-Surber sample to find organic weight. Data were analyzed with nonmetric multidimensional scaling (NMS) and linear regressions, and mechanisms will be examined using structural equation modeling.
Results/Conclusions
The benthic community included over 100 taxa, and emergence traps contained 29 taxa. Regarding emerging insects, NMS showed that locations with more fine substrates and CPOM had different community composition. Longitudinal patterns for emerging insects also showed that, when moving from the downstream end to the upstream end of the 40-m section, total numbers and species richness slightly decreased whereas evenness slightly increased. Regarding benthic macroinvertebrates, NMS showed that CPOM, FPOM, wood substrates, and distance from the logjam all influenced the community composition. Overall, total numbers and species richness for both emerging insects and benthic macroinvertebrates were most strongly associated with CPOM and FPOM. Our data support the notion that logjams concentrate aquatic insects, and agree with previous work that the most important effect of logjams may be through increased retention and upstream deposition of organic material. These findings can be used in future conservation and restoration projects to show the importance of adding wood to streams.
