A geochemical record of recent anthropogenic nutrient loading and enhanced productivity in Lake Nansihu, China

Total organic carbon (TOC), total nitrogen (TN), stable carbon and nitrogen isotopes (delta(13)C, delta(15)N), total phosphorus (TP) and organic phosphorus (OP) were measured in surface sediments and two short cores (DU-3 and WS-4) from Lake Nansihu, China to infer historical changes in anthropogenic nutrient inputs and corresponding shifts in lake primary productivity. Results indicate that organic matter preserved in the sediments is mainly autochthonous and that analyzed sediment variables were affected little by post-burial diagenesis. Increasing TOC, TN, OP and TP concentrations since the 1940s reflect increased P loading and elevated lake productivity. The delta(13)C values varied from -21.5 to -26.6aEuro degrees in the two sediment cores. Values were relatively more negative before the 1940s, but thereafter increased until the mid-1980s, reflecting elevated lake productivity. Since the mid-1980s, delta(13)C values remained relatively constant in core WS-4 and decreased in core DU-3, perhaps reflecting a change in the phytoplankton community. The delta(15)N values ranged from -0.5 to 1.3aEuro degrees in core DU-3 and from 1.2 to 2.5aEuro degrees in core WS-4 before the mid-1980s, and increased to between 2.1 and 8.0aEuro degrees and 5.2 and 7.8aEuro degrees, respectively, thereafter. Topmost sediments in the two cores display delta(15)N values similar to those recorded in the surface sediments (5.5-7.5aEuro degrees). Higher delta(15)N values in recent deposits correspond to greater nitrogen concentration in water, and likely indicate anthropogenic nitrogen input, mainly from human and animal wastes.