224 - Forestry Affects Food Web Structure and Mercury Bioaccumulation Along Longitudinal Gradients in Rivers

Headwater streams in catchments with forestry have increased sediment, nutrient, and mercury inputs as well as altered food web structure, yet our understanding of whether such impacts accumulate or dissipate downstream is limited. Herein we examined whether % autochthony, food web length and methylmercury (MeHg) levels in water and biota changed across 6 up- to downstream sites within three river basins in New Brunswick, Canada, that had different total catchment disturbances of 7.3%, 12.7%, and 23.0% from all types of harvesting and replanting. Analyses and modeling of biofilm, leaf litter, macroinvertebrates and fish revealed that forestry affected the autochthony (using delta13C) and MeHg levels of some taxa and food web length (using delta15N), with either longitudinal cumulative or dissipative effects in the basins with greater disturbance when compared to the basin with the lowest disturbance. In addition, MeHg levels in water and food sources were greatest in the basin with moderate disturbance (highest % clearcutting but no replanting) and its trophic transfer through the food web (slope of log MeHg vs. delta15N) was lowest when compared to those of the other two basins. Collectively, these results are likely due to changes in DOC quality and quantity and increases in nutrient and sediment inputs to the basins with greater disturbances from forest management. These findings suggest that both the magnitude and type of disturbance are important considerations in assessing the impacts of forest management on mercury fate in aquatic ecosystems.