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

1. Carbon isotope ratios (delta(13)C(D)) Of a herbivorous zooplankter, Daph nia middendorfiana and several environmental variables were investigated du ring four annual production cycles (1988-92) in Smith Lake, Alaska to deter mine factors that affect the seasonal and interannual variability of delta( 13)C(D). 2. delta(13)C(D) varied from -44.7 to -31.5 ppt and was significantly corre lated with Chi a (r = 0.66, P = 0.0001), weakly correlated with CO2(aq) (r = 0.31, P = 0.07) and uncorrelated with delta(13)C(DIC) (r = 0.10, P = 0.70 ). Carbon isotope fractionation was minimal when Chi a appeared to be optim al. The largest C-13 fractionation was associated with the lowest Chi a dur ing early and mid winter periods. 3. delta(13)CD was also significantly correlated with water temperature (r = 0.480, P = 0.0001) and photoperiod (r = 0.62, P = 0.0001), probably sugge sting a critical role of physical forcing, particularly solar energy input, in affecting algal photosynthesis and delta(13)C(D) in this subarctic lake . 4. There was a large interannual variability of delta(13)C(D) among ice-cov er periods which was partly explained by interwinter differences in the amo unt of snowfall that affected the flux of solar irradiance to the ice-cover ed lake. 5. Other explanations for delta(13)CD variability such as species successio n, changes in algal cell size and differential use of CO2(aq) and HCO3- wer e also considered, but cannot account for the observations reported here.