Implication of atmospheric nutrient inputs and warming effects for the ecosystem of Lake Yamanaka, Japan, revealed by sedimentary analysis

To reconstruct the historical trend of lake productivity and trophic status in mountain lakes over the past 60 years, we examined the diatom and cladoceran zooplankton remains, fossil pigments, and organic geochemical proxies in a sediment core from Lake Yamanaka, Japan. The fluxes of fossil pigments and total diatom valves, as well as diatom assemblages, did not show any marked changes from the 1960s to the mid-1970s, suggesting that the anthropogenic nutrient input was insufficient for changing the lake trophic status, despite the increased anthropogenic activities around the lake along with the concurrent rapid economic growth in Japan. After the 1990s, the increased fluxes of fossil pigments and percentage of eutrophic diatom species were suggestive of nutrient enrichment in the lake water. After the late 1990s, the stable nitrogen isotope ratios of bulk sediment (δN) gradually decreased with concurrent increases in the total nitrogen (TN) flux, thereby suggesting enhanced atmospheric nutrient deposition. In addition, the reconstructed Chl-a flux considered for post-burial degradation was significantly positively correlated with the summer surface water temperature since 2006. These findings suggest that, in recent decades, not only the anthropogenic nutrient loadings, but also climate warming might have significantly affected the eutrophication in Lake Yamanaka.