Changes in the trophic state of Lake Erie: discordance between molecular δ13C and bulk δ13C sedimentary records

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 δ13C 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-C15 to n-C17 alkanes throughout the core. The δ13C 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, δ13C values of short-chained algal-derived alkanes (<n-C18) are lower (approximately 1.0‰) than their terrestrial-derived homologs (>n-C25). Down-core variations in the δ13C composition of algal-derived short-chained alkanes do not exhibit the same trend as that observed for bulk organic C. Down-core variation in δ13C 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 13C enrichments of n-alkanes associated with a single source. High sedimentation rates characteristic of this core emphasize temporal variations in molecular δ13C values that might not otherwise be observed. Whereas δ13C data for bulk sedimentary organic C appear to reflect variation in trophic state, the isotopic record for algal-derived n-alkanes (e.g., n-C17 and pristane) from Lake Erie must be considered from the perspective of molecular transformations.