Consumption of labile dissolved organic matter by limnetic bacterioplankton: the relative significance of amino acids and carbohydrates

We studied the seasonal dynamics and consumption by planktonic bacteria of the labile pools of dissolved organic carbon (DOClab), dissolved amino acids (TDAA(lab)), and dissolved carbohydrates (TDCHOlab) in Lake Constance, Germany, between March and October 1994. The labile pools were defined as those fractions consumed by bacteria in 1 mu m filtered samples over 4 d. DOClab varied from 0.064 to 0.606 mg C l(-1) and constituted 5 to 36 % of bulk DOC, which ranged between 1.45 and 2.3 mg C l(-1). Proportion of DOClab >13 % occurred during the phytoplankton spring bloom, whereas later in the year <13 % of bulk DOC was labile. The bacterial growth efficiency, i.e, the ratio of biomass produced over DOC consumed, ranged from 8 to 57 % with a mean of 23 % and no obvious seasonal trend. TDAA(lab) constituted 10 to 73 % of bulk TDAA and 5 to 95 % of DOClab, respectively, with highest proportions during the spring bloom, the clear-water phase (not of DOClab), and a phytoplankton bloom in July. TDCHOlab constituted 24 to 73 % of bulk TDCHO and 5 to 86 % of DOClab, respectively, with highest proportions during the clear-water phase and in September. TDAA(lab) and TDCHOlab were positively correlated to bulk TDAA and bulk TDCHO, respectively. Even though both substrate classes were the major components of the labile DOC and together constituted 47 to 100 % of DOClab in 75 % of the experiments, they were not utilized to equal amounts but relative proportions varied considerably from May to September. Both substrates often were also utilized differently for biosynthetic and energy requirements as demonstrated by respiration measurements of C-14-amino acids and C-14-glucose. During the spring bloom amino acids were respired preferentially, whereas during the summer bloom glucose was respired preferentially. Turnover times of amino acids and glucose also demonstrated that both substrate classes were utilized differently because turnover times of glucose were consistently longer than those of amino acids except in surface waters from mid-July until September.