THE SELECTIVE ADVANTAGE OF BUOYANCY PROVIDED BY GAS VESICLES FOR PLANKTONIC CYANOBACTERIA IN THE BALTIC SEA

Observations were made on the vertical distribution of colonies of Aph anizomenon flos-aquae for 9 d at a drift-station east of Bornholm Isla nd in the Baltic Sea. The buoyant colonies were dispersed in the upper layers of the water column during periods of wind-induced mixing but floated up during calm periods. From measurements of the vertical ligh t extinction, surface irradiance and the photosynthesis versus irradia nce curve, calculations were made of the changes in the daily integral of photosynthesis with respect to time and depth throughout the water column. From these calculations it is demonstrated that net photosynt hesis by the population of Aphanizomenon flos-aquae increased nearly t hreefold by floating up after a deep mixing event. It is estimated tha t, averaged over alternating periods of calm and mixing, the buoyancy provided by gas vesicles in this organism will result in a nearly twof old increase in photosynthesis. A quantitative analysis has been made of the relationship of the daily integral of photosynthesis by the Aph anizomenon population with the mean depth of the population in the wat er column and the daily insolation. The analysis shows that the integr al decreases linearly with respect to mean depth.