Induced turbulence in rotating bottles affects phytoplankton productivity measurements in turbid waters

Enclosing phytoplankton in bottles to measure photosynthesisimposes an artificial environment on the sample that can influencerate processes. The absence of turbulence inside incubationbottles can affect productivity measurements by altering boththe irradiance level and sedimentation rates of confined cellsand particles. In Fourleague Bay, LA, a turbid lagoon-estuary,we compared productivity measurements in motionless bottlesto those in bottles that were continuously rotated at 15 r.p.m.to prevent the sedimentation of bottle contents. Productivitydiffered significantly in rotating and non-rotating treatments.At low light levels, rotation often enhanced production, probablydue to increased average irradiance. At high light levels, rotationreduced productivity by 10–83%, probably due to increasedphotoinhibition because the average position of the planktonwas closer to the light source than those in non-rotated bottles.Furthermore, in non-rotated bottles a greater proportion ofsediment particles and cells settled to the bottom, potentiallyshielding cells from photoinhibition. Cell density of the 8µm fraction was nearly 100% higher at the bottom of non-rotatedbottles (2.5+10⁸ cells l^–1) than at the surface aftera 3.5 h incubation. In quiescent water samples, light increasedby >35% in 20 min as a result of rapid particle settlingwhen stirring ceased. Integrated production rates were overestimatedin non-rotated samples by up to 95%, demonstrating the importanceof maintaining a cell-particle suspension which approximatesthat in situ when measuring productivity in turbid systems.