Sediment-organism interaction as a control on hyporheic exchange flows:
effect of body size of the organisms
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.