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, δ¹³C and δ¹⁵N of OM, δ¹³C 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 δ¹³C and δ¹⁵N 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 δ¹³C_VPDB (−35.8 to −34.5‰) and δ¹⁵N_Air (−3.5 to −2.5‰) 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 δ¹³C remains constant or even decreases during this period, implying that increasing productivity did not elevate the δ¹³C of dissolved inorganic carbon and thus cannot explain the observed large increase in OM δ¹³C (7.4‰) and δ¹⁵N (5.8‰) over this period. Instead, δ¹³C, δ¹⁵N and C:N ratios of OM and differences in δ¹³C between calcite and OM suggest that the large increase in OM δ¹³C and δ¹⁵N 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 δ¹³C 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 ¹⁵N-enriched industrial pollutants has resulted in a steady increase of 1.9‰ in δ¹⁵N. Proxy records for lake (δ¹³C and δ¹⁵N 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.