AME 36:285-291 (2004)  -  doi:10.3354/ame036285

ABSTRACT: We investigated fluctuations in the nitrogen stable isotope ratio (δ¹⁵N) of Chlamydomonas acidophila (Chlorophyceae) by short-interval sampling during a bloom that occurred from 11 May to 29 July 2001 in Lake Katanuma, Japan. In this lake, C. acidophila uses NH₄⁺ as the primary inorganic nitrogenous nutrient, since only trace amounts of other inorganic nitrogenous compounds are present. The δ¹⁵N values obtained for C. acidophila ranged from -7.8 to -3.4‰ during the study period. The lowest values occurred during periods characterized by high concentrations of NH₄⁺ (>15 μmol l^-1), and our results showed that δ¹⁵N values for C. acidophila were negatively correlated with concentrations of NH₄⁺. The results suggest that this nutrient plays a major role in the variation of the δ¹⁵N value for this species. The nitrogen isotope fraction factor (ε) of C. acidophila was much lower than that of many NH₄⁺-based primary producers from other eutrophic environments.

KEY WORDS: Nitrogen · Stable isotope · Phytoplankton · Isotope fractionation · Inorganic nutrient · Acidic lake · Spring bloom · Lake Katanuma