Combined effects of nutrients and small-scale turbulence in a microcosm experiment. I. Dynamics and size distribution of osmotrophic plankton

Abstract

The response of phytoplankton and bacteria dynamics to turbulence and nutrient avail- ability interactions was studied in natural coastal waters enclosed in 15 l microcosms. The effect of turbulence was examined under 3 different nutrient-induced conditions: nitrogen surplus (N, with initial addition of an excess of nitrogen, N:P ratio = 160), nitrogen:phosphorus ratio balanced (NP, with initial addition of nitrogen and phosphorus as Redfield ratio, N:P ratio = 16) and control (C, no nutrient addition). Turbulence (e = 0.055 cm 2 s -3 ) was generated by vertically oscillating grids. The experiment lasted for 8 d and samples were generally taken daily for nutrient and plankton mea- surements. Turbulence increased the relative importance of phytoplankton to bacteria when nutri- ents were added, while in the control the effect of turbulence was negligible. Turbulence also influ- enced the species' composition and the size distribution of the phytoplankton community. The relative contribution of diatoms to total phytoplankton biomass and the average cell size were higher under turbulence, particularly in N and NP treatments. The results of these experiments indicate the importance of considering the hydrodynamic conditions in studies addressing competition for nutri- ents among different osmotrophic organisms in plankton communities.