ABSTRACT: Knowledge of the factors regulating bacterioplankton growth is of considerable importance in attempts to understand the functioning of biogeochemical cycles and particularly their evolution after perturbation. We first tested the effect of nutrient additions on the growth of bacterioplankton assemblages collected from 10 atoll lagoons of various morphologies in the Tuamotu Archipelago and in oceanic surface waters surrounding them. Then, we examined these responses in relation to ambient nutrient concentrations and bacterioplankton community turnover rates in these lagoons. The lagoons studied exhibited a range of NH₄ (~0 to 0.7 µM), NO₂+NO₃ (~0 to 0.5 µM), and PO₄ (~0 to 0.3 µM) concentrations. Responses were assessed on bacterioplankton grown in seawater cultures during 2 seasons (end of rainy season, end of dry season) and amended with NH₄-N, PO₄-P or glucose-C in all possible combinations. The 10 lagoons investigated exhibited various responses to nutrient additions. Including co-stimulations, bacterioplankton growth was stimulated by C, N and P additions in 4, 10 and 4 cases, respectively. One atoll lagoon showed inconsistent responses during both cruises. Another one showed no response to any nutrient addition. On the other hand, the oceanic subsurface samples showed a systematic stimulation by glucose, in one case with a concomitant positive effect of N. Expressed as percentages of the unamended controls, responses to ammonium and phosphorus additions in the different lagoons investigated appeared to decrease with increasing ammonium and phosphorus concentrations, respectively. This correspondence suggests that in situ concentrations reflect the nutrient status of bacterioplankton in the sites visited. NH₄-N appeared as the most frequently stimulatory nutrient in the atolls investigated. This is in agreement with the close log-log correlation (p < 0.001) observed between bacterioplankton turnover rates and DIN/PO₄ ratio.

KEY WORDS: Atoll lagoon · Bacterioplankton · Nutrient limitation · Ammonium · Phosphorus

Published in AME Vol. 21, No. 2 (2000) on March 31
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 2000