Print PageLogjams are large accumulations of fallen and drifting wood that alter the flow of water. These natural structures were initially thought to cause lateral expansion of streams and obstruct fish migration, and consequently were regularly removed by local government agencies. However, recent studies have discovered growing evidence that logjams perform critical roles in stream health. They create habitat complexity such as plunge pools and riffles that can accommodate a diverse community of species. In addition, they offer overhead coverage, low-stress velocity shelters, and fixed surfaces for colonization. Another important function is their ability to entrap a variety of food and resources that fish and invertebrates rely on. However, the manner in which logjams affect the composition of the benthic invertebrate community is poorly understood. To address this knowledge gap, we examined the proportion and evenness of feeding groups and genera in the benthic invertebrate community as a function of distance from a spanning logjam. The study was conducted summer of 2010 at Cabin Creek in the Superior National Forest. Surber samples were collected from 50 sample sites randomly assigned within a 40-meter stretch centered on a spanning logjam.
Results/Conclusions
Data from the first sampling period (late May) has displayed a number of trends. Regression analysis shows that richness and the number of functional feeding groups appears to increase downstream of the logjam, but decrease above it (r2=0.1669, p=0.002306; r2=0.09973, p=0.01640). However, evenness showed the reverse trend, increasing upstream of the logjam and decreasing below it (r2=0.001773, p=0.006215). In addition, t-tests reveal that there was a larger proportion of herbivorous shredders and greater Shannon Diversity within 0-10 m from the logjam as opposed to 10-20 m from the logjam (p=0.09119; 0.09853). Furthermore, the community 0-20 m upstream of the logjam exhibited less richness (p=0.03737), less diversity (p=0.04252), greater herbivorous shredder count (p=0.06029), lower detritivore count (p=0.0606), greater proportion of macrophyte piercers (p=0.09196), and fewer individuals (p=0.09115) than the community 0-20 downstream of the logjam. The largest mean counts of detritivores and herbivorous shredders were both located 15-20 meters downstream of the logjam. Our findings show that diversity was highest immediately upstream from the logjam and tapered off in both upstream and downstream directions. Our findings also suggest that the habitat complexity downstream from logjams may accommodate a greater number of species.
