Biofilm response to nutrient mitigation using salmon carcass analog in Central Idaho Streams

Nutrient enrichment is an increasingly popular mitigation technique for declining anadromous salmon runs in the Pacific Northwest, yet studies showing mitigation effects on biofilm production are limited. We are conducting a multi-year experimental study of biofilm biomass, metabolism, and nutrient limitation responses to salmon carcass analog (SCA; made from dried and pasteurized marine fishmeal) in six tributaries of the upper Salmon River, Idaho. Our experimental design includes upstream control and downstream treatment reaches (3 km) in 4 streams, with two treatment levels: 30 g (low) and 150 g (high) analog/m2. We also monitored two untreated control streams. Biofilm ash-free dry mass (AFDM; autotrophs and heterotrophs) on rocks increased 2-3 fold following treatment with high SCA, but responded variably to low SCA. While biofilm chlorophyll a also increased in high SCA treatments, the responses were not proportional to changes in AFDM, suggesting that algae and heterotrophs responded differently to SCA additions. In conclusion, biofilm responses to SCA fertilization depend on treatment level, and may be manifested as shifts in relative abundance of autotrophs and heterotrophs rather than altered biofilm standing stock