Size structure of the planktonic community in microcosms with different levels of turbulence

The evolution of a natural plankton community in two microcosms with well-differentiated turbulence levels (10(-3) W m(-3) and 10(-1) W m(-3)) was compared during artificially-generated blooms. Biomass-size spectra were used to assess the effects of the turbulent kinetic energy (TKE) on the plankton community. Picoplankton was the size range least affected. Indirect effects on picoplankton size structure were related to the TKE effects on the predators. Small nanoplankton represented a transitional size range in which small and variable TKE effects were observed. In the diatom size range, the positive TKE effect on these non-swimmers increased progressively with cell size. The results showed that TKE has a combined effect on both biomass and size structure because of the linkage between these community characteristics. On the whole, the changes observed in the community size structure would lead to the uptake of the maximum available free energy.