The influence of historical forestry practices and climate on the sediment retention function of wetlands.

Wetlands provide beneficial functions and services (e.g. sediment retention, nutrient sequestration) to downstream aquatic environments. The resiliency of these functions under disturbance conditions is, however, not fully understood. Two wetland-lake systems (Boswell and Viewland) in the central interior of British Columbia whose contributing catchments have historically been impacted by forestry practices were selected to examine how wetland sediment retention responds to disturbance. Core chronologies and sedimentation rates were calculated from unsupported Pb measurements using the Constant Rate of Supply (CRS) model, and sediment source contributions were determined using a multivariate unmixing model, for both wetlands and their downstream lakes. Sedimentation rates did not significantly change post-logging in either lake however, the dominant source to Viewland Lake changed from channel bank material to subsurface material. The increase in the proportion subsurface material consistent with increase in dry density and magnetic susceptibility, and decreases in median grain size and C:N. The bordering wetland was not found to contain any material other than channel bank material. The ephemeral nature of the wetland channel, as well as the length of the channel and the significant decrease in median grain size are thought to have prevented sediment deposition, or increased the potential for resuspension and further transport. Sedimentation rates were greatest near the inflow of Boswell wetland, however, the strongest responses to forestry practices were observed near the wetland outflow. Similarly significantly lower median grain sizes could have limited deposition in the upstream areas of the wetland. Increases in precipitation as snow and stream discharge in addition to effects associated with forestry practices are thought to have been responsible for driving sedimentation rates in both catchments however, changes in source contributions were likely the result of active forestry practices. --P. ii.