INDUCED TURBULENCE IN ROTATING BOTTLES AFFECTS PHYTOPLANKTON PRODUCTIVITY MEASUREMENTS IN TURBID WATERS

Enclosing phytoplankton in bottles to measure photosynthesis imposes an artificial environment on the sample that can influence rate processes. The absence of turbulence inside incubation bottles can affect productivity measurements by altering both the irradiance level and sedimentation rates of confined cells and particles. In Fourleague Bay, LA, a turbid lagoon-estuary, we compared productivity measurements in motionless bottles to those in bottles that were continuously rotated at 15 r.p.m. to prevent the sedimentation of bottle contents. Productivity differed significantly in rotating and non-rotating treatments. At low light levels, rotation often enhanced production, probably due to increased average irradiance. At high light levels, rotation reduced productivity by 10-83%, probably due to increased photoinhibition because the average position of the plankton was closer to the light source than those in non-rotated bottles. Furthermore, in non-rotated bottles a greater proportion of sediment particles and cells settled to the bottom, potentially shielding cells from photoinhibition. Cell density of the greater-than-or-equal-to 8-mu-m fraction was nearly 100% higher at the bottom of non-rotated bottles (2.5 x 10(8) cells l-1) than at the surface after a 3.5 h incubation. In quiescent water samples, light increased by >35% in 20 min as a result of rapid particle settling when stirring ceased. Integrated production rates were overestimated in non-rotated samples by up to 95%, demonstrating the importance of maintaining a cell-particle suspension which approximates that in situ when measuring productivity in turbid systems.