AME 29:51-61 (2002)  -  doi:10.3354/ame029051

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

Laura Arin1,*, Cèlia Marrasé1, Marie Maar2, Francesc Peters1, Maria-Montserrat Sala1, Miquel Alcaraz1

1Institut de Ciències del Mar (CMIMA-CSIC), Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain
2National Environmental Research Institute, Frederiksborgvej 399, 4000 Roskilde, Denmark

ABSTRACT: The response of phytoplankton and bacteria dynamics to turbulence and nutrient availability 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 (ε = 0.055 cm2 s-3) was generated by vertically oscillating grids. The experiment lasted for 8 d and samples were generally taken daily for nutrient and plankton measurements. Turbulence increased the relative importance of phytoplankton to bacteria when nutrients were added, while in the control the effect of turbulence was negligible. Turbulence also influenced the species¹ 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 nutrients among different osmotrophic organisms in plankton communities.

KEY WORDS: Small-scale turbulence · Phytoplankton:bacteria relationship · Phytoplankton size distribution · Phytoplankton composition · Microcosms

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