Changes in the trophic state of Lake Erie: discordance between molecular [formula omitted] and bulk [formula omitted] sedimentary records

Abstract

Prior to 1980, anthropogenic phosporus (P) loadings resulted in increased algal productivity and the deterioration of water quality within Lake Erie. We measured C and N elemental compositions, geolipid concentrations, and δ 13 C values for bulk organic material and individual alkanes in samples taken from a 40-cm sediment core from the eastern basin of Lake Erie to infer recent trends in primary productivity and diagenesis. The predominance of autochthonous algal-derived C throughout the core is evidenced by low C/N values (9–10) and the persistent abundance of n-C 15 to n-C 17 alkanes throughout the core. The δ 13 C values of bulk organic C from Lake Erie vary between −25.9 and −24.9‰ with a maximum corresponding to the period of elevated historical P loading. Within Lake Erie, δ 13 C values of short-chained algal-derived alkanes (< n-C 18) are lower (approximately 1.0‰) than their terrestrial-derived homologs (> n-C 25). Down-core variations in the δ 13 C composition of algal-derived short-chained alkanes do not exhibit the same trend as that observed for bulk organic C. Down-core variation in δ 13 C values of individual alkanes likely reflect multiple diagenetic effects including kinetic fractionation effects, variations in the relative contributions of isotopically unique n-alkanes to the sediments, and/or temporal changes in 13 C enrichments of n-alkanes associated with a single source. High sedimentation rates characteristic of this core emphasize temporal variations in molecular δ 13 C values that might not otherwise be observed. Whereas δ 13 C data for bulk sedimentary organic C appear to reflect variation in trophic state, the isotopic record for algal-derived n-alkanes (e.g., n-C 17 and pristane) from Lake Erie must be considered from the perspective of molecular transformations.