Biofilm Development, Senescence, and Benthic Substrate Influence
Hyporheic Transport in Streams
Abstract
Understanding fate and transport within fluvial systems requires
accurate modeling of breakthrough curve (BTC) tails, which often display
non-Fickian behaviors. However, it is unclear how anomalous processes
relate to the physical and biological characteristics of the stream
ecosystem. We use the Stochastic Mobile Immobile model (SMIM) to
determine the impact of biofilm colonization among different substrate
types on reach-scale transport velocity (V) and dispersion (D), rate of
delivery to the subsurface (Λ), and retention within the subsurface
(reflected by power law slope; β). During summer 2020 and 2021, we
conducted a total of n=42 Rhodamine-WT releases in four experimental
streams lined with contrasting substrata (sand, pea gravel, cobble, and
a three-way mix) at the Notre Dame Linked Experimental Ecosystem
Facility (ND-LEEF) in Indiana (USA). To explore the effect of biofilm
colonization, we conducted releases under artificially shaded, early and
late biofilm development, and senescent biofilm conditions. We found
that replicated releases under constant conditions consistently
reproduced stream BTCs and modeled transport parameters. Biofilm
abundance, biofilm status (living versus dead), and substrate type
produced significant variations in BTC shape and transport
parameterizations. We found a non-linear relationship between algal
biomass and V, where increases in biomass produced decreases in V at low
biomass and increases in V at high biomass. Substrate type also
predicted patterns in transport, with sand producing higher V, Λ, and β
than larger substrata. These results suggest that substrate type acts as
the primary driver and biofilm development the secondary control on
transport in fluvial systems.