Quantity and quality of autumnal litterfall into a rehabilitated agricultural stream

Litterfall from riparian vegetation is a major source of nutrients for stream invertebrates. However, replacement of riparian forests with agricultural crops may result in reduced litter quantity and quality and therefore severely affect stream trophic structure. Litterfall inputs and their associated N flux were measured in 1996 and 1997 at Washington Creek, an agriculturally degraded stream in southern Ontario, Canada. Litterfall varied significantly (p < 0.05) among three treatment areas,,vith varying tree densities (3.14, 2.43, and 2.17 trees m(-2) respectively) and land use. Litterfall was highest (1611 kg ha(-1) yr(-1)) in sections with a wide buffer zone, although it was significantly less (p cr 0.05) compared with litterfall in a mature riparian zone (= control, 3238 kg ha(-1) yr(-1)). Leaves represented similar proportions of total litterfall at both the rehabilitated (91.7%) and control (98.2%) site. Litterfall varied significantly (p < 0.05) among trap orientation for both 1996 and 1997 litter collections and was highest in traps oriented south-west (the predominating wind direction) and perpendicular to the stream. Total N flux varied significantly (p < 0.05) among treatment areas of the rehabilitated site and the mature riparian zone. A higher N flux was found in treatment areas with a wide buffer zone directly adjacent to intensely cropped agricultural fields. This suggests that trees in the riparian zone may be intercepting agricultural runoff and enhancing water quality by converting NO(3)-N runoff to organic forms of N in canopy components. A leaf litter C:N ratio of 32:1 st the rehabilitated site suggests that N may be readily available for stream invertebrates and higher trophic levels.