Sediment cores from 12 locations throughout the Laurentian Great Lakes basin were analyzed for geochemical indicators of primary production. Sediment analytes included organic and inorganic contents, carbonates, sediment accumulation rates, total organic carbon and nitrogen concentrations, carbon and nitrogen isotope composition, and trends in spectroscopically inferred chlorophyll a (and its main diagenetic products). When multiple indicators were considered, production records related to recent cultural eutrophication and catchment activities were clear in the paleorecords. Indicators derived from loss-on-ignition (organic and inorganic content) were strongly associated with periods of human settlement in the catchments that increased overall sediment loads to the lakes. The ratio of carbon to nitrogen tracked catchment inputs of carbon, especially during periods of watershed development in western Lake Superior. Sediment records such as chlorophyll a and δ13Corg appear to be reliable indicators of trends in past algal abundance, particularly in Lake Erie, which has a well-known history of higher production and cultural eutrophication. These analytes also correlated well with past measured water quality surrogates for lake primary productivity and stressor data such as human populations in adjacent watersheds. A comparison among indicators revealed that several show redundancy as good proxies of production or productivity, though context was important. For instance, heavier isotopes of carbon and nitrogen are often prescribed as sedimentary indicators of lake productivity, but these two analytes were negatively correlated in Lake Superior, possibly due to long-term increases in cyanobacteria or changes in the nitrogen source. An increase in sediment carbonates can indicate summer blooms of cyanobacteria (a phenomenon that was clearly apparent in Lake Ontario) or catchment erosion. To make convincing geochemical inferences of primary production in the Great Lakes, it is recommended that a weight of evidence be built through the use of multiple indicators.
Bibliographical noteFunding Information:
This research was supported by a grant to E. Reavie from the U.S. Environmental Protection Agency under Cooperative Agreement GL-00E23101-2. This document has not been subjected to the USEPA’s required peer and policy review and therefore does not necessarily reflect the view of the Agency, and no official endorsement should be inferred. Isotopic analyses were supported by Molly O’Beirne, Julia Halbur, and Sarah Grosshuesch. We thank Kathleen Kennedy, Amy Kireta, Robert Sterner, Craig Stow and the Research Vessel Lake Guardian and Blue Heron field crews for their help collecting core samples. Sediment dating was supported by Daniel Engstrom and personnel at the St. Croix Watershed Research Station.
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- Great lakes
- Primary production