Interacting Effects of Biofilm and Collector Density on Suspended Particle Capture: A Flume Experiment

The complex flow-vegetation-sediment interactions in low-gradient rivers, marshes, and estuaries are of particular importance due to these ecosystems' susceptibility to sea level rise and other climatic factors. While direct capture of suspended particles by submersed vegetation stems is thought to play a minor role in many of these systems' sediment budgets given prior work suggesting low capture efficiency (𝜂), the potential for biofilm present on collector surfaces to magnify this effect is not yet entirely understood. We conducted a physical simulation in a recirculating flume to reproduce and expand upon prior work investigating effects of collector area-density (#/m² ) and biofilm on particle capture across a range of Reynolds number values (60-180). We found that the presence of biofilm resulted in effective capture efficiency (𝜂') 2 to 4 times the non-biofilm control, in contrast to previous flume experiments where the effect of biofilm was also positive but small in comparison (<1.5 times control). However, estimated absolute effective capture efficiency both with and without biofilm remained relatively small (𝜂' < .01), similar to previous experiments' findings. We also tested lower collector densities than those tested in previous experiments (minimum density: 278 collectors/m² vs. 1013 collectors/m² ), finding a strong negative relationship between collector density and capture efficiency. Our work highlights the fact that many independent variables influence capture efficiency. Experimental work delving into biophysical properties of plant stems and biofilm, along with replication via in situ field experiments, will help us reach a conclusion about the importance of particle capture by submersed vegetation in different types of low-gradient systems, and help us to better understand potential feedback with other hydrological and ecological processes.