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 delta(13)C of phytoplankton generally varies seasonally in lake ecosystems, in Lake Katanuma the mean delta(13)C of particulate organic matter (POM, mainly phytoplankton) is constrained to a narrow range from -26.4 to -23.7 parts per thousand. A major reason for this is the continuous supply of dissolved CO2 gas available to C. acidophila from fumaroles at the lake bottom. The delta(13)C of P. acidojaponica in Lake Katanuma varied seasonally and was positively correlated with P. dcidojaponica abundance at 1 and 4 m depths. This suggests that the higher P. acidojaponica biomass at 1 and 4 m produced the C-13-enrichment in the high-density algal mats because of the limited dissolved CO2 gas. The mean delta(13)C of benthic diatoms was higher than that of phytoplankton in Lake Katanuma, although the diatoms seemed to assimilate the same carbon source (CO2 gas) in the lake water.