Seasonal dynamics of carbon stable isotope ratios of particulate organic matter and benthic diatoms in strongly acidic Lake Katanuma

Lake Katanuma is a strongly acidic volcanic lake (average pH 2.2) located in NE Japan in which only 2 algal species are found: Pinnularia acidojaponica, a benthic diatom, and Chlamydomonas acidophila, a phytoplankton species. Although the δ 13 C of phytoplankton generally varies seasonally in lake ecosystems, in Lake Katanuma the mean δ 13 C of particulate organic matter (POM, mainly phytoplankton) is constrained to a narrow range from -26.4 to -23.7 ‰. A major reason for this is the continuous supply of dissolved CO 2 gas available to C. acidophila from fumaroles at the lake bottom. The δ 13 C of P. acidojaponica in Lake Katanuma varied seasonally and was positively correlated with P. acidojaponica abundance at 1 and 4 m depths. This suggests that the higher P. acido-japonica biomass at 1 and 4 m produced the 13 C-enrichment in the high-density algal mats because of the limited dissolved CO 2 gas. The mean δ 13 C of benthic diatoms was higher than that of phytoplankton in Lake Katanuma, although the diatoms seemed to assimilate the same carbon source (CO 2 gas) in the lake water.