Catastrophic landscape disturbance and its impact on lake ecosystem stability (Lake Crescent, USA)

Landscape geomorphic instability is common in tectonically active regions, but the ecological consequences of mass wasting are rarely considered. The Late-Holocene sediment from Lake Crescent (USA) exhibits a remarkable record of repeated catastrophic mass wasting events, each of which led to deposition of meter-scale mass transport deposits on the lake floor. We used high temporal resolution diatom analysis of selected intervals of a 14 C-dated sediment core to assess the effect of two disturbance events on the lake at different period in its development state. The two disturbance events produced mass transport deposit I (MTD-I) and mass transport deposit II (MTD-II) at 3100 ± 100 and 4000 ± 100 cal year BP, respectively. The MTD-II event dammed the lake and increased water levels, but also blocked the migration of spawning salmon, reducing marine-derived nutrient inputs. These changes resulted in a more oligotrophic lake environment. Following the MTD-II event, diatom assemblages were dominated by benthic species and exhibited low rates of compositional change. In contrast, the MTD-I event favored nutrient-demanding planktonic diatoms (e. g. Fragilaria crotonensis ) and was characterized by a more rapid turnover of community composition. In both cases, the diatom assemblages demonstrated weak resistance to these mega-disturbance events, and it took approximately 30 years for the communities to reach a new ecological equilibrium. Investigating the impacts of these catastrophic events in Lake Crescent provides an opportunity to assess their effects on ecosystem dynamics and recovery rates in the absence of human disturbance.