| We present two novel stable isotope methods for measuring lake metabolism and compare the results to traditional techniques. The delta 18O method measures planktonic gross primary production (GPP) from dissolved oxygen concentrations, isotopes and respiration (R) and the delta 13C method measures "whole-lake" GPP and R from dissolved oxygen and carbon concentrations and isotopes. All three methods showed GPP was greater than R over the ice-free season and estimates of GPP were not significantly different. There was also no significant difference in R as measured by bottle incubations and the delta13C method. However, the delta 13C method does not account for inputs of external carbon which will result in underestimation of R and overestimation of GPP. In systems with significant allochthonous carbon inputs, the delta13C method cannot be accurate unless these inputs are accounted for.; The delta18O method was used to measure metabolic parameters of twenty-one northern temperate lakes and showed GPP dominated over R during the ice-free season. GPP and R were most strongly correlated with lake temperature, which in turn is a function of the amount of solar radiation received by the lake. Our results imply that it is this solar radiation that drives planktonic gross primary productivity, which in turn drives the majority of planktonic respiration. Variation in dissolved organic carbon only explained 8% of the variation in planktonic R, while variation in planktonic GPP explained approximately 80% of the variation in planktonic R.; Despite general autotrophy in the lakes, they were generally oversaturated in CO2 during the ice-free season, on average 252+/-25%. However, we found little evidence to conclude that this was the result of an excess of in situ respiration over production. The magnitude of the annual excess of R over GPP was not sufficient to account for the flux to the atmosphere. Moreover, carbon evasion was not a function of respiratory flux, nor did the isotopic signature of dissolved CO2 in the lakes present evidence of respiration. Groundwater inputs of carbon dioxide represent a plausible source for carbon dioxide oversaturation in some but not all of the lakes sampled. |
