Evidence for ecosystem-level stress: Stream acidification impairs ecosystem respiration while altering nitrogen dynamics

Background/Question/Methods

Trend-based changes in environmental conditions at the global scale represent unique scenarios of ecosystem perturbation that stand in contrast to the short-term, high mortality events that characterize disturbance.  These gradual, unidirectional changes are better conceptualized as stressors to receiving ecosystems and recently ecologists interested in how global change influences ecosystem function (i.e., energy flow; nutrient cycling)  have been urged to consider the physiological stress responses of key biota (e.g., microbes). Our research question addresses how stream acidification influences ecosystem services related to carbon and nitrogen processing. We investigated the respiratory and nutrient uptake responses of both leaf biofilms and entire stream ecosystems to chronic acidification stress through a combination of laboratory microcosm experiments and whole-system measurements across a gradient of stream pH in Shenandoah National Park, Virginia, USA.  

Results/Conclusions

Respiration standardized to microbial biomass (i.e., qCO₂) increased with increasing acidity in both microcosm (r² = 0.83, P = 0.032) and whole-stream (r² = 0.66, P = 0.094) investigations.  A concomitant decline in fungal biomass (r² = 0.96, P = 0.003) suggests that increased qCO₂ represents a stress-induced decline in carbon-use efficiencies.  Unexpectedly, biomass-specific uptake of dissolved inorganic nitrogen (DIN) also rose with increased acidity (microcosms: r² = 0.90, P = 0.014; whole-stream: r² = 0.67, P = 0.088) interpreted to mean that DIN uptake was related to functions other than growth. The activities of two carbon-acquiring exoenzymes (β-glucosidase and β-xylosidase) and a N-acquiring exoenzyme (proline aminopeptidase) were higher on leaf biofilms from acidic streams indicating increased exoenzyme production as the potential cause of increased biomass-specific N uptake in more acidic streams.  While fungal biomass  and ecosystem respiration (r² = 0.99, P < 0.001) declined with decreasing pH, N uptake (U, g N m^-2 d^-1) at the ecosystem level was not directly related to stream pH possibly because of compensatory enzyme production, which implies altered fates of N taken up in acid streams.  We propose a conceptual framework for the influence of acidification stress on stream ecosystem function that emphasizes the stress response of primary biological compartments integral to the biogeochemical dynamics of aquatic systems.