Despite an increasing awareness of the impacts of animal behavioral traits on ecosystem functioning, animal behavior remains poorly integrated in ecosystem ecology. Further, few studies have examined the extent to which differences in behavior among populations of the same species can modulate the rates of biogeochemical processes. Here, we combine field and mesocosm studies to investigate the significance of intraspecific variation in behavioral traits on stream ecosystem functioning using two crayfish species, Faxonius rusticus and F. virilis, that are invasive in many regions throughout North America. Crayfish play a key role in streams by translocating nutrients and organic matter from benthic habitats to the water column. Results from a previous field study indicate that movement-related behavioral traits (activity, boldness) of rusty crayfish are strongly related to rates of water column metabolism (P = 0.005, R2 = 0.83) and leaf litter breakdown (P = 0.001, R2 = 0.84) in midwestern streams. We conducted 4-week mesocosm experiments with F. rusticus and F. virilis from populations that varied in movement-related behavioral traits to determine whether crayfish behavior could be driving these ecosystem functions in natural streams. Parallel to what we observed in the field, mesocosms with active crayfish had significantly greater rates of metabolism and amounts of organic matter in the water column (P < 0.001, R2 = 0.41 – 0.66) than those with inactive crayfish of the same species. Additionally, we found that crayfish movement-related behavioral traits can influence leaf litter breakdown rates, nutrient uptake rates, and net-N2 cycling pathways (fixation vs denitrification; P < 0.01, R2 = 0.21 – 0.62). Our results suggest that different populations of the same species can have substantially different effects on ecosystem functions due to their behavioral traits. Thus, examining behavioral trait variation in animals may be central to understanding the mechanisms shaping freshwater ecosystem functioning.