A new approach to historical reconstruction: Combining descriptive and experimental paleolimnology

Abstract

Here we introduce a combined experimental and descriptive approach (termed resurrection ecology) to reconstructing historical perturbations, pointing out how direct tests with sediments and hatched resting eggs complement the traditional descriptive calculation of microfossil fluxes. In the Keweenaw Waterway, a freshwater estuary off Lake Superior, turn‐of‐the‐century copper mining impacted the resident biota. Remain fluxes document that diatom, rhizopod, and Bosmina production all declined during stamp sand discharges but recovered rapidly after World War II, moving above background levels due to developing eutrophication. In addition to biogenic silica, we discovered that bromine flux holds promise as an indicator of diatom production and confirmed that this element is present in several genera. Fluxes of Daphnia resting eggs also increased dramatically since the 1940s, dominated by a hybrid apparently produced from crosses between offshore and interior Waterway species, after channeling promoted greater mixing of water masses. Toxicity studies with sediments and Daphnia clones directly tested recovery of environ ments after cessation of mining activities. The studies document that increased concentrations and fluxes of copper in the Waterway during mining discharges were toxic to invertebrates. Once stamp sand discharges ceased, the biota recovered rapidly due to a combination of decreased copper cycling and organic complexation. Although sedimentation has returned to near‐background conditions and surficial sediments in much of Portage Lake are no longer toxic, eutrophication and faunal exchange with Lake Superior make it unlikely that the original zooplankton community composition will return to the Waterway system.