CHANGES IN THE TROPHIC STATE OF LAKE ERIE - DISCORDANCE BETWEEN MOLECULAR DELTA-C-13 AND BULK DELTA-C-13 SEDIMENTARY RECORDS

Prior to 1980, anthropogenic phosporus (P) loadings resulted in increa sed algal productivity and the deterioration of water quality within L ake Erie. We measured C and N elemental compositions, geolipid concent rations, and delta(13)C values for bulk organic material and individua l 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 through out 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 delta(1 3)C values of bulk organic C from Lake Erie vary between -25.9 and -24 .9 parts per thousand with a maximum corresponding to the period of el evated historical P loading. Within Lake Erie, delta(13)C values of sh ort-chained algal-derived alkanes (< n-C-18) are lower (approximately 1.0 parts per thousand) than their terrestrial-derived homologs (> n-C -25). Down-core variations in the delta(13)C composition of algal-deri ved short-chained alkanes do not exhibit the same trend as that observ ed for bulk organic C. Down-core variation in delta(13)C values of ind ividual alkanes likely reflect multiple diagenetic effects including k inetic fractionation effects, variations in the relative contributions of isotopically unique n-alkanes to the sediments, and/or temporal ch anges in C-13 enrichments of n-alkanes associated with a single source . High sedimentation rates characteristic of this core emphasize tempo ral variations in molecular delta(13)C values that might not otherwise be observed. Whereas delta(13)C data for bulk sedimentary organic C a ppear to reflect variation in trophic state, the isotopic record for a lgal-derived n-alkanes (e.g., n-C-17 and pristane) from Lake Erie must be considered from the perspective of molecular transformations. (C) 1998 Elsevier Science B.V. All rights reserved.