10:45 AM - 12:15 PM
[MIS09-P02] Biogeochemical sulfur cycle in sediment and contribution of sulfide-derived sulfur for benthic food-web in Lake Biwa
Keywords:Lake Biwa, sulfur, benthic animal
Biogeochemical sulfur cycling has been well studied in the marine sediments, whereas earlier studies have received less attention to lacustrine sediments due to the low sulfate concentration in lake water. Recently, however, it has been suggested that microbial sulfate reduction and subsequent primary production by thioautotrophs support organisms on and below the freshwater lake bottoms. In this study, we investigated the microbial sulfur cycle in freshwater lake sediments and contribution of the sulfide-derived sulfur to sulfur source of benthic animals in Lake Biwa using stable sulfur isotope.
Sediment and animal samples were obtained from the central part of the northern basin in Lake Biwa (90 m water depth) and from the off Hikone (50 m water depth) in September 2021, when the thermal stratification was developing. Sulfur isotope ratio (δ34S) of acid volatile sulfide (AVS) in the sediments, sulfate in lake water, and animal tissue were also measured to investigate contribution of sulfide-sulfur to sulfur source for the animals.
High concentrations of AVS were detected in the sediments, indicating occurrence of microbial sulfate reduction. The vertical profiles of the concentrations and δ34S values of AVS in the sediments suggested that AVS was oxidized and assimilated by sulfur-oxidizing bacteria. The isotope fractionation factor during the oxidation was calculated to be about −4‰. Using this value, the contribution of sulfide-derived sulfur to the sulfur source of the animals was calculated. The results showed a various contribution from 0% for amphipods to ~100% for oligochaetes with the largest biomass in the lake. These results indicate that even in freshwater environments, microbial sulfur cycling is significant during the stratification period when anoxic water masses are developed in the stagnant bottom water, affecting nutrient sources for the benthic animals.
Sediment and animal samples were obtained from the central part of the northern basin in Lake Biwa (90 m water depth) and from the off Hikone (50 m water depth) in September 2021, when the thermal stratification was developing. Sulfur isotope ratio (δ34S) of acid volatile sulfide (AVS) in the sediments, sulfate in lake water, and animal tissue were also measured to investigate contribution of sulfide-sulfur to sulfur source for the animals.
High concentrations of AVS were detected in the sediments, indicating occurrence of microbial sulfate reduction. The vertical profiles of the concentrations and δ34S values of AVS in the sediments suggested that AVS was oxidized and assimilated by sulfur-oxidizing bacteria. The isotope fractionation factor during the oxidation was calculated to be about −4‰. Using this value, the contribution of sulfide-derived sulfur to the sulfur source of the animals was calculated. The results showed a various contribution from 0% for amphipods to ~100% for oligochaetes with the largest biomass in the lake. These results indicate that even in freshwater environments, microbial sulfur cycling is significant during the stratification period when anoxic water masses are developed in the stagnant bottom water, affecting nutrient sources for the benthic animals.