Shigeto Takagi

Differences in nitrogen and carbon stable isotopes between planktonic and benthic microalgae

We compiled published data on the nitrogen and carbon stable isotope ratios of phytoplankton and benthic microalgae from lentic systems and explored the primary factors determining the isotope values among systems. Also, we investigated seasonal changes in nitrogen stable isotope ratios of phytoplankton and benthic microalgae in the strongly acidic lake, Lake Katanuma, which has only two dominant species, Pinnularia acidojaponica as a benthic diatom and Chlamydomonas acidophila, a planktonic green alga. From the published dataset, it may be concluded that δ13C of benthic diatoms were more enriched than those of phytoplankton at the same sites, although the nitrogen isotope of phytoplankton and benthic microalgae were similar. This differences in δ13C between benthic microalgae and phytoplankton could be explained by the boundary layer effect. On the other hand, nitrogen isotope values of both benthic microalgae and phytoplankton were primarily controlled by the same environmental factor, and boundary layer effects are not the primary factor determining the nitrogen isotope values of microalgae. Also, we showed temporal dynamics in nitrogen isotopes of benthic and planktonic microalgae species in Lake Katanuma, and the trends of nitrogen isotopes are similar between benthic and planktonic microalgae, as concluded from the published dataset.

Isotopic (δ13C) evidence for the autochthonous origin of sediment organic matter in the small and acidic Lake Katanuma, Japan

Sources of sediment organic matter from the strongly acidic Lake Katanuma (0.14 km2; average pH 2.2), in Japan, were determined from an analysis of carbon stable isotope ratios (δ13C). Organic carbon was derived mainly from the benthic diatom, Pinnularia braunii, and particulate organic matter (POM) primarily from the phytoplankton, Chlamydomonas acidophila, and not from emergent or terrestrial C3 plants such as Phragmites australis, Sasa kurilensis or Fagus crenata. Although the sediment organic matter of most small lakes, especially strongly acidic lakes, is commonly thought to be of allochthonous origin, the sources of sediment organic matter in the small and acidic Lake Katanuma were clearly autochthonous.