Do stream logjams enhance aquatic export to riparian habitats?

Background/Question/Methods

Benthic production in streams can be exported to terrestrial habitats via emerging aquatic insects and therefore link aquatic and terrestrial ecosystems.  Stream logjams could represent important nodes in this linkage because they enhance physical and hydrological complexity.  This in turn could: 1) expand benthic microhabitats to increase aquatic insect abundance, which would subsequently 2) increase aquatic insect emergence at and near logjams, resulting in 3) enhanced aquatic-riparian exchange.  To test these hypotheses, we sampled a 40-m stream reach centered on a logjam and its adjacent riparian habitat in and along Cabin Creek, a third order stream in northern Minnesota’s Superior National Forest during May of 2010 and 2011.  Benthic and emergent insects were sampled, and physical parameters quantified (e.g., current velocity, submerged wood area, FPOM and CPOM) at 50 random locations sited upstream, downstream and at the logjam.  Linkage to the adjoining riparian habitat was quantified by enumerating spider webs along the stream reach and 6-m inland.  Hypotheses were tested by examining benthic abundance within functional feeding guilds, counting adult insects in floating emergence traps, and documenting the spatial distribution of spider webs.  Data were analyzed using structural equation modeling.  

Results/Conclusions

Our hypothesized chain of causation was supported by the data; specifically, 1) functional feeding guilds responded to logjam-modified habitat parameters, 2) emergent insect abundance correlated with benthic abundance, and 3) riparian spider web density increased near the logjam.  We found that most benthic feeding guilds showed strong associations with wood and current velocity.  In particular, all but shredders were associated with wood, and velocity was positively related to abundance of collectors, filterers, gatherers, scrapers, and predators.  Emergence was directly related to benthic macroinvertebrate abundance, but also CPOM, which may intercept drifting insects.  Other factors, particularly wood and FPOM, had indirect effects on emergence via their influence on benthic abundance.  Our spider web data suggest that aquatic exchange along our 40-m reach was greatest at the logjam.  Webs were concentrated near the logjam and extended further inland, implying deeper penetration by aquatic emergence.  Although logjams and stream wood are known to influence lotic community structure and function, their effect on aquatic-terrestrial connectivity is poorly understood.  Our results suggest that logjams, and more broadly, physical heterogeneity in streams, may be an important factor controlling the lotic-riparian linkage.