Sinking speeds of free-living phototrophic bacteria determined with covered and uncovered traps

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Abstract
Sinking of phototrophic bacteria in Lake Cisó was followed for 2 years as a model for sinking of free-living bacterioplankton. A special trap system was deployed which included covered traps. This minimized problems created by turbulence in the water column. A correction for decomposition was also applied. Measured sinking speeds were then compared to expected sinking speeds according to Stokes' law. To calculate the latter, buoyant densities and volumes of the cells were measured on several occasions and their values substituted into Stokes' equation. Bacteria did not sink during periods of turbulence. During stratification, some periods of sinking could be detected. During such periods, bacteria sank with speeds in accordance with Stokes' law. At other times, bacteria remained in suspension. Use of covered traps was shown to be necessary to correct overtrapping during periods of turbulence.