Phymatolithon foecundum is a coralline growth found in the coastal benthic environment. Benthic surveys indicated that P. foecundum is abundant throughout the deep gradients (10-30m depth). A range of approaches were taken to determine the light vs photosynthesis characteristics of this growth form including both in situ and laboratory experiments. A diving-PAM fluorometer to log photosynthetic ... yield of a chosen piece of coralline algae attached to a rock was installed and made measurements at 20 minute intervals over a 15 hour period. Three deployments were made at depths of 16-20m. In the laboratory, coralline covered rocks were incubated in -1 °C water and used in two experiements. In the first, the surface of the coralline crust was exposed to irradiance intensities similar to those expected when ice-out occurs. Using a PAM fluorometer, the short term response of yeild vs irradiance characteristics were measured. In the second, oxygen micro-electrodes were installed to measure changes in oxygen gradients in the layer of water above the coralline crust under a range of irradiance conditions. This data will allow prediction of coralline algae to irradiance conditions during ice-free periods.