Streambed inhabitants mix and mobilize sediments by actions such as burrowing, feeding, and excretion, a process referred to as sediment reworking. This sediment-organism interaction could modify the hydro-physical properties of streambeds and significantly influence the hyporheic flow regime in streams as highlighted by our recent research. In this work, we further advance the understanding of the sediment reworking process by investigating the influence of the organism size on the modification of streambed properties and hyporheic exchange. Laboratory experiments were conducted in long recirculating flumes to simulate streamflow environment and Lumbriculus variegatus of two different sizes (large worms were double the thickness of small worms) were used as model organisms. The organisms of both sizes were allowed to rework the sediment beds in their respective flumes for 10 days after which dye tracer tests were performed to characterize hyporheic exchange flows. Visual observations reveal that the burrow openings at the bed surface were readily visible in the flume reworked by larger organisms compared to the flume reworked by organisms of smaller size. The former also exhibits higher hyporheic flux and shorter residence times compared to the latter which could be attributed to the rapid exchange of solutes across the streambeds due to the presence of a dense network of voluminous burrows. The exchange depths in both the flumes were similar and a potential reason for this observation could be the reworking of sediment beds up to similar depths in the flumes. We suggest that further exploration must be done at both small and large scales to comprehend the role of sediment-organism interaction in modulating hyporheic exchange flows as it will have direct implications on critical stream ecosystem services such as natural processing of nutrients and contaminants.

The mobilization and mixing of sediments by the activities of streambed inhabitants, referred to as sediment reworking, constantly modify the physical and hydraulic properties of streambeds. However, limited progress has been made to explore the influence of this sediment-organism interaction on hyporheic exchange. In this work, we advance the understanding of the role of macroinvertebrate sediment reworking in altering the hyporheic exchange flows in clogged streambeds. Laboratory experiments are conducted in re-circulating flumes following a control (clogging) and treatment (clogging + sediment reworking) based design. The experiments involve studying the interaction of model organisms (Lumbriculus variegatus) with fine sediment (clay) deposits, and its subsequent influence on hyporheic flow regime in homogenous model streambeds comprising fine sand, coarse sand, and gravel sediments. We observe that model organisms burrowed extensively into the clogging layer, mixed the clay particles with underlying grains, and eventually eroded or disintegrated the clogging layer at the bed surface in the treatment flumes. As a consequence, the treatment flumes exhibited greater solute penetration depth, shorter median and mean residence times, and higher hyporheic flux compared to their respective control flumes. The results also suggest that the modification of hyporheic exchange flows depends on the overall reworking of the beds including both fine and substrate sediments. The alteration of hydro-physical properties of streambeds and subsequently the hyporheic flow regime due to sediment reworking has direct implications for the biogeochemistry of hyporheic zones and may impact the overall quality of surface and sub-surface waters, particularly in low flow environments.