Oxygen microenvironment of coralline algal tufts and their associated epiphytic animals

Separate subhabitats are distinguishable in tufts of Corallina officinalis, including the surface of the algae itself and the interspace (areas of water) within the algal tufts. The oxygen microenvironment of each of these was investigated in laboratory investigations using oxygen microelectrodes to test the hypothesis that oxygen gradients form adjacent to the seaweed surface and that the oxygen concentration of the seawater between the branches of individual plants differs from that of the surrounding water body due the tuft forming nature of this seaweed. Regions of hyperoxia (up to 250% of saturation) were detected at the surface of Corallina branches in static conditions, with steep declining gradients of oxygen concentration through the diffusive boundary layer in the vertical plane to 100% of saturation a distance almost 2mm from the surface. Oxygen concentration at the surface did not vary with position along individual branches, or with position on any one branch segment. Concentrations were significantly higher on main branches than on peripheral branches of individual plants. Water flow was the dominant factor controlling the depth and oxygen supply of diffusive boundary layers and in moving water oxygen levels did not achieve such high saturation levels and the boundary layer was thinner. On a larger scale, oxygen concentrations in the interspace of C. officinalis tufts were highly variable and commonly in excess of air saturation. The oxygen environment was both temporally and spatially dynamic, and very rapid changes in oxygen concentration were observed in response to changing fl ow conditions. Despite the ranges of oxygen concentrations, and often hyperoxic conditions described, a thriving epiphytic community of animals smaller than 2mm, dominated by harpacticoid copepods and marine mites is associated with this extreme and dynamic environment.