Allochthonous organic carbon and phytoplankton/bacterioplankton productionrelationships in lakes

Humic lakes with high inputs of allochthonous dissolved organic carbon have a pelagic food chain that, to a large extent, is based on bacterioplankton energy mobilization from allochthonous organic carbon compounds. This is i n contrast to clear lakes in which total pelagic production is based mainly on phytoplankton photosynthesis. The energy economy in humic lakes may be less efficient than in clear lakes, because it is Likely that one more link is included in the food chain. Lake data from Scandinavia and North Americ a demonstrate that shifts between food chains based on heterotrophic produc tion and food chains based on primary production can take place at moderate increases or decreases in the concentration of dissolved organic carbon fr om allochthonous sources. Large variations in the loading of allochthonous organic carbon (e.g., due to climatic variations) may have considerable eff ects on the biostructure and productivity of lakes.