Particulate Allochthonous Inputs: Relationships with Stream Size in an Undisturbed Watershed

Abstract

Four streams of a pristine boreal watershed in eastern Quebec were examined to assess the composition, seasonal pattern, and ash-free dry mass (AFDM) of particulate allochthonous input. Study sites on streams ranging from first to sixth order exhibited significant differences in the types and magnitude of litter input. Composition of litterfall varied according to vegetation and physical structure of the riparian zone. Although the forest in the watershed is dominated by conifers, deciduous leaves dominated annual budgets and seasonal input patterns. Annual litterfall per unit area of stream surface declined exponentially with increasing stream order (r2 = 0.98) from 307–534 g AFDM∙m−2∙yr−1 in a first-order stream to15–17 g AFDM∙m∙yr−1 in a sixth-order stream. Lateral inputs of 25–70 g AFDM∙m−1∙yr−1 were not related to stream size, but were strongly influenced by riparian structure and entrainment of organic matter during spring flooding. Total input per unit stream length increased logarithmically with increasing order (r2 = 0.87), approaching 1000 g AFDM∙m−1∙yr−1 in the fifth- and sixth-order streams. A watershed budget calculated from these results estimates a total input of 729 t∙yr−1 to streams of this 673-km2 watershed. When particulate allochthonous input is compared with primary production measurements in these streams, 81–95% of the organic carbon supplied to the first- and second-order streams is allochthonous, but 85–95% of the supply to the fifth- and sixth-order streams is autochthonous. While these trends agree with predictions of the synthetic continuum model, the importance of local effects in our results illustrates the need for a more mechanistic approch in allochthonous input research.