Effect of particle size and heterogeneity on sediment biofilm metabolism and nutrient uptake scaled using two approaches

Numerous studies have examined the effect of sediment particle size and distribution on community structure, but few have focused explicitly on how physical habitat characteristics influence biogeochemical functions of freshwater biofilms. In this study, we evaluated the effect of particle size and heterogeneity on rates of biofilm metabolism and nutrient uptake in colonized and native sediments normalized using two different scaling approaches. Coarse, pebble‐ to cobble‐sized sediments were sorted into four homogeneous particle size treatments plus one heterogeneous treatment. Each treatment was deployed, in replicate, within one riffle and one run habitat feature in three different high‐latitude stream reaches with contrasting hydrological and physicochemical characteristics. A treatment of native, homogeneous sediment was also evaluated at each deployment location. After incubating for approximately five weeks, metabolism and nutrient uptake of biofilms in all treatments (n = 69) were measured in recirculating microcosm chambers. For each treatment, functional rates were normalized by projected surface area and sediment surface area scaling approaches, which account for the surface area in plan view (looking top‐down) and the total surface area of all sediment particles, respectively. This comparison was designed to determine whether treatment effects were independent of increased surface area associated with smaller particle sizes or heterogeneous sediments. Community respiration and uptake of ammonium‐nitrogen and phosphate‐phosphorus by biofilms decreased significantly as the particle size of homogeneous treatments increased when normalized by projected surface area, but significantly increased with increasing particle size when normalized by sediment surface area. Sediment particle size had a limited influence on production rates evaluated across treatments. Heterogeneous and homogeneous treatments with similar median particle sizes did not differ significantly from one another for most biogeochemical functions measured. Our findings indicate that rates of biogeochemical function in heterogeneous habitats were directly related to the total sediment surface area available for biofilm colonization. The significant interactions between sediment surface area and rates of respiration and nutrient uptake suggest that information about the size and distribution of sediment particles could substantially improve our ability to predict and scale measurements of important biogeochemical functions in streams.