AME 33:87-94 (2003)  -  doi:10.3354/ame033087

ABSTRACT: 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 δ¹³C of phytoplankton generally varies seasonally in lake ecosystems, in Lake Katanuma the mean δ¹³C 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 CO₂ gas available to C. acidophila from fumaroles at the lake bottom. The δ¹³C 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. acidojaponica biomass at 1 and 4 m produced the ¹³C-enrichment in the high-density algal mats because of the limited dissolved CO₂ gas. The mean δ¹³C 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.

KEY WORDS: Carbon stable isotope · POM · Phytoplankton · Benthic diatoms · Inorganic carbon · Seasonal variation · Acidic conditions