Complex trajectories of aquatic and terrestrial ecosystem shifts caused by multiple human-induced environmental stresses

Large shifts in the isotopic compositions of organic matter (OM) in lake sediments, over the last few hundred years, are commonly interpreted as representing changes in photosynthetic productivity corresponding to eutrophication or in the input of terrestrial OM due to human disturbances. Based on multiple-proxy data (C:N ratio, delta C-13 and delta N-15 of OM, delta C-13 of calcite, lithology and fossil pollen) from a 700-year sediment core at White Lake, New Jersey (USA), we propose a new explanation that relates these large shifts in OM delta(13) C and delta(15) N to human-induced changes in aquatic OM producers. Combined records of geochronology, fossil pollen and lithology from White Lake reveal that the upland forest was cleared by European settlers for farmland beginning around 1745 A.D. and has gradually reforested since 1930 after the abandonment of the farmlands. For the pre-agricultural period, OM had relatively constant but extremely low delta C-13(VPDB) (-35.8 to -34.5 parts per thousand) and delta(15) N-Air (-3.5 to -2.5 parts per thousand) and high atomic C:N ratios (13.7 to 16.7), indicating a stable anoxic lake environment with prominent microbial producers. Following the human disturbance (since 1745), high OM mass accumulation rates and abundances of the green alga Pediastrum indicate an increase in aquatic photosynthetic productivity due to enhanced nutrient input from disturbed uplands. However, carbonate 613 C remains constant or even decreases during this period, implying that increasing productivity did not elevate the delta C-13 of dissolved inorganic carbon and thus cannot explain the observed large increase in OM delta C-13 (7.4 parts per thousand) and delta N-15 (5.8 parts per thousand) over this period. Instead, delta C-13, delta N-15 and C:N ratios of OM and differences in delta C-13 between calcite and OM suggest that the large increase in OM 813 C and 815 N can be attributed to a human-induced ecological shift in the predominant organic source from anaerobic bacteria to autotrophic phytoplankton. During the post-agricultural period, mass accumulation rates of OM, carbonate and silicate, and the delta C-13 of OM and calcite all decreased significantly, corresponding to stabilization of the uplands. However, over the last 70 years, an intensifying aquatic stress from the deposition of N-15-enriched industrial pollutants has resulted in a steady increase of 1.9 parts per thousand in delta N-15. Proxy records for lake (delta C-13 and delta(15) N of OM) and upland conditions (pollen and silicates) at White Lake show complex trajectories of the aquatic and terrestrial ecosystems in response to past human disturbances. Published by Elsevier Ltd.