Implications of a sediment-chemistry study on subalpine marsh conservationin the Lake Tahoe basin, USA

Sediment cores, representing a range of watershed characteristics and anthr opogenic impacts, were collected in 11 subalpine marshes in the Lake Tahoe basin, USA and vicinity in order to trace the effect of anthropogenic impac ts on sedimentation and sediment accumulation rates. Cores were Pb-210-date d using a CRS model, and these data were used to calculate sedimentation ra tes and sediment accumulation rates, which were compared with environmental variables. Age-depth profiles indicated that sedimentation rates changed a t different times in different marshes, and we separated sediment cores int o two parts to show sediment characteristic and sediment-accumulation-rate changes. Ranges of dry mass accumulation rates and sedimentation rates were 0.081-1.356 kg m(-2) yr(-1) and 0.94-4.64 mm yr(-1) in the upper core and 0.094-0.563 kg m(-2) yr(-1) and 0.57-1.75 mm yr(-1) in the lower core. In g eneral, the cores showed recent increases in sediment accumulation rates. S pecies composition was related to water quality, with the dominant species changing in a predictable series with decreasing water quality: Nuphar, Nup har and Carex/Scirpus, Eleocharis, Potamogeton, Ranunculus, and Chara. The effect of a rough road used for timber transport was observed as the change of sediment accumulation rates in Upper Grass Lake. Sediment accumulation rates since the improvement of the rough road decreased to ca. 80 % of thos e prior to the improvement. Road-salt use was related to relatively high wa ter cation concentrations and conductivity in Meyers Grade Marsh adjacent t o Hwy 50. Increased lead accumulation rates and concentrations in more rece nt sediments at Meyers Grade Marsh and Snow Creel, reflect the increasing u se of fossil fueled vehicles in this region. Urbanization and animal farmin g were related to increased N and P concentrations in marsh water. The numb er of exotic plant species within a marsh was highly correlated with water quality and C and N accumulation rates. This study showed that 8 of the 11 marshes studied are in relatively undisturbed condition, while the remainin g 3 have been heavily impacted. Urbanization and increased recreation facil ities, and activities are important anthropogenic impacts on subalpine mars hes. This study suggests that pristine subalpine marshes be protected from increasing human activities and that marshes impacted by rough roads can be restored to pristine conditions.