A 140-year record of redox variation in the bottom water of the northern basin of Lake Biwa, Japan, based on geochemical analyses of the deepest sediment core

Abstract In Lake Biwa, the largest warm monomictic lake in Japan, suboxic bottom water conditions occur frequently due to weak or incomplete vertical circulation mainly caused by global warming. We analyzed the total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS) contents of a well-dated sediment core obtained from the deepest part of the northern basin of Lake Biwa to examine temporal variation in bottom-water redox conditions from 1884 to 2021. The stable lower TOC, TN, and TS contents of the core before 1950 indicated oxic bottom-water conditions in the northern lake basin, suggesting strong winter vertical circulation during this period. From the 1950s–1990s, TOC, TN, and TS contents increased in response to a decrease in bottom-water dissolved oxygen resulting from progressive eutrophication, as well as weakened winter vertical circulation. Despite continuous increases in TOC and TN contents, we detected a marked negative shift in TS content after 2000, when bottom water conditions repeatedly became suboxic due to weaker vertical circulation. The timing of this rapid decrease in TS content coincided with the expansion of sulfur-oxidizing Thioploca spp. bacteria on the deep lake floor since the late 1990s. Frequent suboxic conditions probably increased the abundance of sulfur-oxidizing bacteria in surface sediments, which constrained sulfide generation and decreased sediment TS content. Although TS variations are a useful proxy for redox conditions, caution is needed when interpretating whether a decrease in sediment TS content represents a switch from reduced/suboxidized conditions to oxidized conditions in near-bottom waters, or whether it is merely associated with sediment processes (i.e., the influence of sulfur-oxidizing bacteria).