Phytoplanktonic exudation, bacterial reassimilation and production for three diel cycles in different trophic conditions

Abstract

Three diel studies of algae-bacteria relationships were carried out in three French lakes of different trophic status. Primary production was measured by the 14 C incorporation method. Exudation and bacterial uptake rates of organic carbon excreted by phytoplankton were studied by selective filtration. The total photosynthetic production rates were 157, 560 and 831 mgC m -2 d - and the excretion rates represented 16.8 %, 3.6 % and 3.5 % of the total production in the oligotrophic, mesotrophic and eutrophic lakes, respectively. In these three systems, 37 %, 52 % and 79 % of the exudates were incorporated by the bacteria with kinetics that differed according to the lake: 90 % and 75 % of the uptake occurred fast (within 4 h) in the oligotrophic and eutrophic lakes, respectively, against only 15 % in the mesotrophic lake. Gross bacterial production, determined from 3 H-thymidine incorporation rates, was 0.45, 2.29 and 22.8 mgC m -3 d -1 for the oligotrophic to the eutrophic system, i.e. 10.8, 58.2 and 45.6 mgC m -2 d -1 , respectively. The results are examined from biological and methodological points of view.