Ecosystem Engineering in the Streambed: Net‐Spinning Caddisflies Influence Hydraulic Properties

Hydropsychid (net-spinning caddisfly) larvae are aquatic macroinvertebrate ecosystem engineers, altering streambed properties by creating silk nets and retreats. Recent research has shown that caddisfly silk binds stream sediments and alters streambed cohesion, yet potential influences of caddisfly activity on streambed hydrologic properties have not received similar scrutiny. Utilizing a novel downward flow permeameter, we demonstrate how net-spinning caddisfly colonization of the streambed interstitia at moderate but common densities (2000 m(-2)) can reduce vertical hydraulic conductivity (K-V) by up to 55% in streambed sediments (coarse sand-medium gravel; median diameter = 12.91 mm). Additionally, organic matter content within sediment columns showed that occupation by caddisflies resulted in twice the ash-free dry mass (AFDM) of noncaddisfly (primarily microbial) organic matter at the end of our experiments relative to control systems that lack caddisflies. These results suggest that net-spinning caddisflies may restrict hyporheic exchange, alter streambed hydrologic residence time distributions and increase per-unit-volume biotic metabolic demand for stream solutes in the hyporheic zone, with the potential to alter whole-stream biogeochemical processes.