Abstract
Diffusive boundary layers, photosynthesis, and respiration in Phaeocystis colonies 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, 1994. The oxygen microenvironment of a Phaeocystis colony with a mean diameter of 1.4 mm was mapped from 346 O2 measurements and showed gradients of oxygen concentration in the water phase up to 1 mm distance from the colony surface. The effective diffusive boundary layer was 0.4–0.9 mm thick. Oxygen concentrations inside colonies reached 180% of the bulk water, and pH increased up to 0.4 units when measured in light at saturating intensities (>90 µmol photons m −2 s −1 ). The respiration in the dark was low, resulting in a 6% lowering in oxygen concentration and 0.04 units lowering in pH inside colonies, compared to the bulk water phase. Such colonies were net heterotrophic communities at light intensities up to 10 µmol photons m −2 s −1 . A week later, colonies were net heterotrophic at light intensities up to 80 µmol photons m −2 s −1 . The effective diffusion coefficient for oxygen in the gelatinous colonies was not significantly different from that in sea water.