Acidification, stress, and detrital processing: implications for ecosystem function in headwater streams

Environmental influences like acidification promote stress at the ecosystem level that manifests as reduction in metabolic and biogeochemical efficiency. Headwater streams along a chronic acidity gradient were assessed to explore how stress alters microbial abundance and activity and their influence on ecosystem structure and function. Streams draining deciduous forests were investigated during autumn when channels were filled by leaf litter. Whole-system measures of respiration were coupled to estimates of fungal biomass in leaf biofilms to generate an ecosystem-level measure of metabolic efficiency (qCO_2E, g CO₂–C g C⁻¹ d⁻¹). Stable isotope releases of nitrate nitrogen (¹⁵N–NO₃) were performed to address nitrate uptake (UNO3) across streams. Fungal stocks decreased across five streams as pH declined (6.98–5.34). Whole-system respiration decreased fivefold with increasing acidity, while qCO_2E did not respond consistently to acidification, but was correlated with stream temperature. Across streams, concentrations of nitrogen (N) were low and UNO3 related to nutrient availability and not to stream acidity. Results illustrate that acidification alters ecosystem processes through influences on microbial abundance and metabolic activity, while scarce N availability and low UNO3 characterized biogeochemical behavior during autumnal periods of maximal detrital stocks.