Biofilm responses to nutrient enrichment using salmon carcass analog in central Idaho streams

Background/Question/Methods

In the Pacific Northwest, salmon returns have declined due to harvest, hatcheries, habitat degradation, and hydroelectric dams. As a result of population declines, nutrients delivered to streams via spawning salmon are currently 5-7% of historical levels. Nutrient mitigation is used in salmon recovery efforts because it is assumed that added nutrients stimulate biofilm production, which is passed up the food web, eventually stimulating juvenile salmon growth and survival. Yet, studies demonstrating biofilm responses to nutrient mitigation are limited. Our objective was to assess changes in biofilm responses (standing crop, nutrient limitation, metabolism, and community composition) to experimental additions of salmon carcass analog in six tributaries of the upper Salmon River, Idaho. We used a multiple before-after-control-impact (M-BACI) design with upstream control and downstream treatment segments (3km) in six streams with three analog levels: 30 g (low), 150 g (high) analog/m²and no analog. We deployed nutrient diffusing substrates to determine nutrient limitation and collected biofilm standing crop samples in each segment before and after analog addition. Additionally, we collected biofilm samples to determine community composition via RT-Q-PCR with primers specific to common microbial groups. Finally, we estimated biofilm and ecosystem metabolism using light/dark chambers and open channel metabolism techniques, respectively.

Results/Conclusions

Biofilm standing crop, measured as chlorophyll a concentration and ash free dry mass, did not respond to either level of analog addition (all differences are significant at α = 0.05). Biofilms in our study streams were primarily nitrogen-limited and the limiting nutrient did not differ between control and treatment segments. However, chlorophyll a concentrations were higher on nutrient diffusing substrates deployed in treatment segments than control segments. Respiration and gross primary productivity (GPP) measured using chambers was three and four-fold higher in treatment vs. control segments under high analog levels, respectively, but did not differ under low or no analog levels. Net primary productivity did not differ under any analog level. Open channel GPP and ecosystem respiration were higher, and net ecosystem productivity was more negative in treatment segments than control segments of our high analog stream. Our study suggests that biofilm metabolic activity may change in response to nutrient mitigation treatments, but that these changes may not lead to changes in standing crop. Discrepancies between biofilm activity and standing crop may occur if biomass accrual is constrained by streambed hydraulic characteristics or if grazers quickly remove additional biofilm standing crop, providing a potential link from nutrients to juvenile salmon.