Riverbed Bioclogging and the Effects on Infiltration and Carbon Flux under Climate Variability

Abstract:

In California and other regions with Mediterranean climates, losing rivers are common and can introduce dynamic feedbacks affecting total transport of infiltration and nutrients, as well as unsaturated zone formation. Permeability decline from hyporheic zone bioclogging is one such feedback mechanism that is thought to highly depend on the initial grain size distribution during each seasonal cycle. Variability from the El Niño Southern Oscillation (ENSO) can influence the initial grain size distribution demonstrating a large scale climate-control on biogeochemical fluxes. To address these questions, we simulated biological growth and carbon dynamics using 1D and 2D MIN3P numerical models, allowing a range of initial grains size distributions to represent ENSO control of riverbed scour. We quantified microbial growth and total carbon and infiltration fluxes. Our results showed that infiltration declines due to bioclogging were greatest in higher permeability sediments (typical of El Niño with greater probability of bankfull discharge). However, in these higher permeability sediments, cumulative carbon fluxes and infiltration volumes were lower than more drought-like, La Niña conditions (less probability of riverbed scour). Additionally, we found the initial sediment grain size distribution exerted strong control on the location of the bioclogging hotspot when an unsaturated zone developed. Our results demonstrate a flow and nutrient transport feedback mechanism where bioclogging and carbon cycling become limited from ENSO controlled initial conditions. These results provide a new understanding of nutrient cycling and hotspot bioclogging in losing rivers worldwide.