Variable Production by Different Pelagic Energy Mobilizers in Boreal Lakes

We studied production by three key pelagic energy mobilizer communities, phytoplankton (PP), heterotrophic bacteria (HB), and methanotrophic bacteria (MOB), in five boreal lakes of varying size and concentration of dissolved organic carbon (DOC). Production by PP was responsible for most (>55%) of the total pelagic energy mobilization in all five lakes. Production by HB and PP estimated for the whole water column during the ice-free period were positively correlated, but with the exception of the clearest and most eutrophic lake PP apparently could not support the total carbon demand of bacteria. However, the DOC concentration did not explain the variability of heterotrophic bacterial production (HBP) within or between the lakes. Thus, our results provide circumstantial evidence for the “priming effect” whereby labile organic matter from autochthonous production enhances decomposition of allochthonous DOC. However, HBP was only 10–23% of the total pelagic energy mobilization in the lakes, suggesting that only a minor fraction of allochthonous DOC became available for higher trophic levels. High MOB activity was detected in the water columns of the stratified lakes when the molar ratio of CH₄:O₂ varied between 0.5 and 12. In the small stratified lakes (area < 0.01 km²), MOB production contributed 13–52% of the total pelagic energy mobilization, being greatest during the autumn mixing period. Our results indicate that in small stratified lakes (area < 0.01 km²) bacteria, especially MOB, are potentially quantitatively important supplementary food resources for zooplankton. However, in larger lakes primary producers are the most important (>70%) potential food source for zooplankton.