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

1. Carbon isotope ratios (δ¹³C_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 δ¹³C_D. 2. δ¹³C_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 δ¹³C_DIC (r = 0.10, P = 0.70). Carbon isotope fractionation was minimal when Chl a appeared to be optimal. The largest ¹³C fractionation was associated with the lowest Chl a during early and mid winter periods. 3. δ¹³CD 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 δ¹³C_D in this subarctic lake. 4. There was a large interannual variability of δ¹³C_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 δ¹³CD variability such as species succession, changes in algal cell size and differential use of CO_2(aq) and HCO₃^– were also considered, but cannot account for the observations reported here.