Relationships between bacterioplankton and phytoplankton biomass, production and turnover rate in Tuamotu atoll lagoons

As a part of a multi-disciplinary study of the biological production in 12 Tuamotu atoll lagoons, we assessed phytoplankton and bacterioplankton biomass and production. The only consistently significant differences were those existing between lagoons, without any other spatial or seasonal signal. Compared with the surrounding oligotrophic ocean, lagoons displayed an increase in trophic status related to their hydraulic regime (water renewal rate and atoll rim morphology). Bacterioplankton biomass was correlated to phytoplankton biomass (power function, slope < 1), and bacterial carbon exceeded phytoplankton carbon in the most oligotrophic atoll lagoons. Bacterioplankton production and turnover rate were strongly related to phytoplankton production and turnover rate, respectively, by power functions (both slopes > 1). Besides these quantitative differences corresponding to the classical ‘inverted trophic pyramid’, potential exoproteolytic activity in the lagoons increased, relative to bacterial production, with increasing oligotrophy, hinting at an increased reliance on polymeric dissolved organic matter (DOM). In increasingly oligotrophic lagoons, phytoplankton (although less abundant) may increase its ratio of exuded DOM, but sessile benthic fauna (especially corals) was also found to be more abundant and might enrich the water column with high-nitrogen DOM.