THE EFFECT OF SUBSTRATE STOICHIOMETRY ON MICROBIAL ACTIVITY AND CARBON DEGRADATION IN HUMIC LAKES

Studies of bacterial growth and community respiration in water from tw o humic lakes indicated a balanced supply of C relative to N and P und er periods with low loads of allochthonous C (humus). A predominant mi neral nutrient limitation existed when external C supply was high. Dur ing autumn, both nitrogen and phosphorus additions stimulated bacteria l growth, while community respiration was stimulated by N and P combin ed, but not N alone. Additions of excess labile carbon (glucose) never stimulated either bacterial growth or respiration, while glucose and mineral nutrients combined gave rise to a vigorous production, suggest ing that a strict C limitation is unlikely for heterotrophic bacteria in these lakes. Increased anthropogenic loadings of N and P may thus s timulate heterotrophic activity in humic lakes, increase degradation o f humic matter, and finally increase CO2 outputs to the atmosphere. A prevailing mineral nutrient limitation of the bacteria would strongly affect the algal-bacterial competition and constrain phytoplankton pro duction.