Seasonal and interannual variability of plankton carbon isotope ratios in a subarctic lake

Abstract

1. Carbon isotope ratios (δ 13 C D ) of a herbivorous zooplankter, Daphnia middendorfiana and several environmental variables were investigated during four annual production cycles (1988–92) in Smith Lake, Alaska to determine factors that affect the seasonal and interannual variability of δ 13 C D . 2. δ 13 C D varied from −44.7 to −31.5 ppt and was significantly correlated with Chl a ( r = 0.66, P = 0.0001), weakly correlated with CO 2(aq) ( r = 0.31, P = 0.07) and uncorrelated with δ 13 C DIC ( r = 0.10, P = 0.70). Carbon isotope fractionation was minimal when Chl a appeared to be optimal. The largest 13 C fractionation was associated with the lowest Chl a during early and mid winter periods. 3. δ 13 CD was also significantly correlated with water temperature ( r = 0.480, P = 0.0001) and photoperiod ( r = 0.62, P = 0.0001), probably suggesting a critical role of physical forcing, particularly solar energy input, in affecting algal photosynthesis and δ 13 C D in this subarctic lake. 4. There was a large interannual variability of δ 13 C D among ice‐cover periods which was partly explained by interwinter differences in the amount of snowfall that affected the flux of solar irradiance to the ice‐covered lake. 5. Other explanations for δ 13 CD variability such as species succession, changes in algal cell size and differential use of CO 2(aq) and HCO 3 – were also considered, but cannot account for the observations reported here.