Emerald ash borer (EAB) has killed millions of ash trees since its 2002 detection, propelling this invader to the forefront of natural resources concerns. Many deleterious impacts following EAB invasion are documented, but studies investigating the effects of ash mortality on aquatic ecosystems are scarce. Mortality of ash trees along headwater stream corridors can result in canopy gaps, potentially altering riparian plant community structure and subsequent organic matter decomposition in streams. Our aim was to determine how leaf litter taxa and canopy gaps alter decomposition rates and stream invertebrate and microbial communities through both survey and experimental approaches and compare our results to historical data. We postulated faster decomposition in light gaps and leaves with more palatable structure. Additionally, we hypothesized a functional community shift with increased light in canopy gaps. Leaf litter and their associated microbial communities were sampled upstream, downstream, and at the center of 2 EAB-related gaps per stream before, during, and after autumnal leaf senescence in 9 streams over 2 years. Leaf packs of four types (ash, oak, buckthorn and sterile control) were subsequently introduced upstream, downstream, and at the center of one gap and collected monthly to quantify decomposition and colonizing macroinvertebrate and microbial communities.
Results/Conclusions
Species richness (number of leaf taxa) of in-stream leaf litter packs was not significantly altered at gap or downstream locations compared to upstream sites (P>0.05). Yet, diversity of aquatic leaf litter microbes was significantly higher at downstream locations, compared to upstream samples, before autumnal leaf fall (P=0.05) and was inversely related to aquatic leaf litter species richness (P=0.01). Preliminary analysis from the decomposition experiment indicated higher macroinvertebrate densities underneath the canopy gap, compared to upstream. Additionally, leaf litter of buckthorn, a shade intolerant invasive plant, decomposed faster than native leaves, with oak having the slowest decomposition. Studies conducted in the same stream pre-EAB invasion similarly reported differences between oak and ash leaf litter (e.g. Petersen and Cummins 1974), yet microbial genomics techniques were not developed at the time. Our study’s novel findings utilizing a riparian invader and microbial genomics techniques offer innovative insights in stream ecology. This represents the first significant evidence that EAB invasion alters aquatic microbial communities, and this change is influenced by the allochthonous resource of leaf litter, informing and bridging larger fields of invasive species and microbial ecology.