Effect of phytoplankton growth on air bubble residence time in seawater

Laboratory experiments were carried out in a seawater mesocosm tank to investigate the influence of marine phytoplankton growth on air bubble residence time (BRT). Air bubbles of 10-1000 mu m in diameter were injected by flushing a water jet into the top of the tank and BRT was determined acoustically. The tank was filled with seawater containing a natural phytoplankton population and growth stimulated by irradiating with artificial fluorescent light. A second experiment was conducted using a monoculture of the diatom Cylindrotheca closterium. BRT and several phytoplankton growth-related parameters (chlorophyll concentration, dissolved inorganic nutrients, dissolved organic carbon (DOC), oxygen saturation and bacteria numbers) as well as the water viscosity were monitored over periods of up to 24 days. BRT showed a statistically significant covariation with oxygen saturation (r = 0.69, alpha = 0.01 for natural phytoplankton; r = 0.93, alpha = 0.01 for the Cylindrotheca closterium) and chlorophyll concentration (r = 0.69, alpha = 0.05 natural phytoplankton; r = 0.76, alpha = 0.01 Cylindrotheca closterium) during phytoplankton growth periods. Increases in BRT of a factor > 2 were found during the chlorophyll maximum, when the water was sufficiently supersaturated with oxygen (similar to>110%). No clear relationship was evident between BRT and measurements of DOC or water viscosity. Model experiments with highly oxygen-supersaturated water and artificial polysaccharide compounds indicated that oxygen supersaturation alone is not the main factor causing increased BRT during phytoplankton growth, but it is most likely a combination of the degree of gas saturation and the composition of the organic exudates derived from the microalgal population.