Permeable Biofilms Can Support Persistent Hyporheic Anoxic Microzones

Experimental observations reveal the existence of anoxic microzones within oxygen-rich aquatic sediments. The microzones are hotspots of enhanced removal of nitrate and production of greenhouse gas nitrous oxide. Here we established a new microscale flow, reactive transport and biofilm growth model and explore how anoxic microzones form and evolve within hyporheic zones under dynamic interactions among pore-scale flow, reactive transport and biofilm growth. The microzones can form and evolve by two patterns as streambeds are progressively clogged due to biofilm growth: (a) physical and biogeochemical conditions-dependent perennial microzones for permeable biofilms, and (b) physical and biogeochemical conditions-independent ephemeral microzones for low permeability biofilms. During biofilm growth, persistent transport-dominated systems sustain for the former, whereas the transformation of transport-dominated to reaction-dominated systems occurs for the latter. These findings are significantly different from previous single pattern of ephemeral microzones.