Streambed geomorphology is a key determinant of the benthic distribution of forest-to-stream resource subsidies

The food webs of streams and adjacent riparian forests are inextricably linked. Riparian forests can contribute substantial amounts of organic matter to streams, increasing the productivity of food webs in these systems.  We examined how streambed geomorphology (sediment grain size) controls the strength of riparian-stream linkages by determining the rate at which terrestrially derived organic matter (e.g. deciduous leaves, conifer needles) is retained by and stored within the streambed.  Experimental releases of deciduous (alder) leaves and conifer needles in 18 stream reaches (sediment grain sizes ranging from fine gravel to boulders) showed that while the retention of both organic matter types was strongly and positively related to sediment grain protrusion, leaves were retained more effectively than conifer needles.  The relationship between the amount of organic matter stored within the streambed and sediment grain size was unexpectedly non-linear, with maximum storage values recorded at intermediate levels of grain protrusion.  Leaf litter breakdown rate did not vary substantially with grain size.  These results suggest that geomorphological controls on retention and storage may determine detrital resource availability (cobbles > boulders > gravel), but not the rate at which these resource subsidies are incorporated into benthic food webs.