Assessing the response of watersheds to catastrophic (logging) and possible secular (global temperature change) perturbations using sediment-chemical chronologies
David T. Long, Matthew J. Parsons, Catherine H. Yansa, Sharon S. Yohn, Colleen E. McClean, and Ryan J.
Vannier
Geochemical profiles of sediment cores from two oligotrophic lakes
(Elk and Mullett) in northern Lower Michigan were studied to examine the
response and recovery of watersheds to large-scale logging that occurred between
1850 and 1920. Specific questions addressed were: can the impact of extensive
clear-cutting of forests be recognized in the sediment-chemical chronologies,
can states of system stability be identified prior to the logging, and are there
indications that the systems are recovering and possibly returning to a stable
state? To answer these questions, elements were put into four groups as proxies
for watershed runoff or export (e.g., Al, Mg), pollution (e.g., Pb, Cu), redox
(e.g., Fe, As), and productivity (e.g., Ca, P). It was observed that vertical
patterns of all proxies were influenced by logging and the early increases in
concentration of pollution proxies were due to increased watershed export, not
pollution. System stability might be recognized by relatively symmetrical
vertical patterns among all of the proxies or secular changes of slowly
increasing or decreasing vertical concentration trajectories. Some pre-logging
trajectories were punctuated by episodes of slightly elevated concentrations
that appear to be related to comparatively warmer periods during the Little Ice
Age. Iron and Mn enrichments caused by increased watershed runoff might be
misinterpreted as paleo-redox horizons. Results are interpreted to indicate that
(1) reference conditions may be better defined as the temporal trends among
proxy groups and not individual concentrations, (2) simply assuming pre-1800
conditions as a reference may not be appropriate, (3) inter-proxy group
comparisons are needed to help for interpretations of intraproxy group patterns,
(4) the possible regime shift identified here might be expected for other
ecosystems because of the intensity of human disturbances and secular changes,
and (5) without consideration of a possible regime shift, recovery from logging
is estimated to be on the order of 75130 a, but shorter if regime shifts are
considered.