Understanding Ecosystem Responses to Environmental Perturbations and Secular Changes Using Sediment Cores

Stressors such as perturbations from land use change and pollution and slower secular processes such as succession and climate change alter ecosystems. Since these stressors influence productivity and the flux of chemicals, temporal records of chemical and biological variables in lake sediments are used to assess the current state of systems and pre-perturbation states. These observations can be used to infer future changes such as recovery. Because of the secular processes and intensity of perturbations, ecosystems may not or can not recover to pre-perturbation states. Thus, pre-perturbation records in lakes may not represent the norm for “recovered” systems. However, pre-perturbation records do reveal the relationship among chemical and biological indicators prior to anthropogenic influences that might be used to recognize evolution of the systems to a new norm. We are examining the temporal (typically for the past 300 years, a time period with intense changes at regional and watershed scales) chemical records in inland lakes representing various land use and climate situations. Cores are sectioned immediately after collection at high resolution and sedimentation rates determined by ²¹⁰Pb/¹³⁷Cs methods. Data are corrected for focusing and interpreted in terms of bulk concentrations, loading rates, and anthropogenic/watershed influences. Selected results are that 1) elements are being identified as possible surrogates for different stressors/processes (e.g., Al and U erosion; P, Ca, productivity), 2) pre-perturbation state is typically characterized by temporal norms of either constant sediment concentrations and loading rates or similar changes among a suite of elements, 3) sediment P and N concentrations can provide interpretable records, 4) changes in state to possible new norms can be identified, and some chemical patterns might be correlated to global temperature change. These results, albeit preliminary suggest that new norms might better be defined by systematic relationships among suites of chemicals rather than by individual chemical behavior.