Diffusive boundary layers, photosynthesis, and respiration of the colony-forming plankton algae, Phaeocystis sp.

Diffusive boundary layers, photosynthesis, and respiration in Phaeocystis c olonies were studied by the use of microelectrodes for oxygen and pH during a bloom in the Barents Sea, 1993, and in the Marsdiep, Dutch North Sea, 19 94. The oxygen microenvironment of a Phaeocystis colony with a mean diamete r of 1.4 mm was mapped from 346 O-2 measurements and showed gradients of ox ygen concentration in the water phase up to 1 mm distance from the colony s urface. The effective diffusive boundary layer was 0.4-0.9 mm thick. Oxygen concentrations inside colonies reached 180% of the bulk water, and pH incr eased up to 0.4 units when measured in light at saturating intensities (>90 mu mol photons m(-2) s(-1)). The respiration in the dark was low, resultin g in a 6% lowering in oxygen concentration and 0.04 units lowering in pH in side colonies, compared to the bulk water phase. Such colonies were net het erotrophic communities at light intensities up to 10 mu mol photons m(-2) s (-1). A week later, colonies were net heterotrophic at light intensities up to 80 mu mol photons m(-2) s(-1). The effective diffusion coefficient for oxygen in the gelatinous colonies was not significantly different from that in sea water.