Evaluating environmental response and recovery using a multiproxy paleolimnological approach.

摘要

Human activity has influenced ecosystems since antiquity, with the health and function of aquatic ecosystems being significantly impacted since the industrial revolution. A better understanding of the functioning of modern aquatic ecosystems can be gleaned by reconstructing an environmental history of a system. An effective way to do this is to merge the record of historical perturbations with the overprint of modern stressors, which can be evaluated using lake sediment cores as they archive both historic and recent influences.;In a paleolimnological study of Muskegon Lake, Michigan, USA, multiple environmental proxies were analyzed to evaluate a system's response and recovery to human perturbation through time. The overarching hypothesis was that ecosystems highly disturbed by human activity cannot be expected to return to a pre-disturbance state; and further, with the influence of climate change, and other continuing and emerging stressors, ecosystems will not obtain a new state of balance (steady state and/or equilibrium). To investigate this hypothesis, geochemical and biological proxies from a 150 cm sediment core were evaluated and compared to a chronology of human activity in the watershed. If true, the proxies will correspond to the chronology of human perturbations in the watershed.;The geochemical data from the core revealed suites of elements that corresponded to the source of the material, including terrestrial, productivity and anthropogenic related inputs. These elemental groups closely tracked the history of human activity and identified three phases of human influence. The profiles of anthropogenic elements were used to evaluate geochemical reference conditions and showed that the modern concentrations have not decreased to pre-historical values, thus indicating a scenario of a continually adapting reference condition state.;The biological data, inferred from fossil diatoms, also identified three distinct paleoecological phases corresponding to human influence. The ecological preferences of individual diatom taxa suggest that phase changes were driven by temperature, cultural nutrient inputs, and recently emerging stressors (e.g. climate and invasive species). Further, the benthic v. planktonic community structure of diatoms was used to reconstruct productivity regimes in the lake through time, which identified a significant shift from planktonic to benthic dominated productivity occurring in recovery phase of the core.;Finally, to better understand ecosystem function (e.g. time-lags and feedbacks), the proxies were integrated. Geochemical and biological phases were compared, with good agreement among them of overall system response. Phase one represented pre-disturbance (reference) conditions, phase two identified anthropogenic influence, and phase three suggested partial system recovery from phase two; though it also indicated furthered transition to up-and-coming stressors---of which little is known.