Photosynthetic response of Scandinavian kelp forests to stratospheric ozone depletion

The periodic ozone hole over northern Scandinavia raises concern whether increased ultraviolet radiation (UVR) will adversely affect primary production in Norwegian kelp forests. Quantitative predictions of UVR effects on kelp forest production require knowledge of spectral sunlight, its attenuation in marine waters, and the formulation of a high-resolution biological weighting function (BWF) that reflects the wavelength dependency of UV photoinhibition. A full spectrum irradiance model is presented which describes the temporal and spectral character of underwater light in a Laminaria hyperborea kelp forest at Finnøy, Norway (62.8˚ N, 6.5˚ E). The measurement-validated irradiance model was formulated based on a solar radiation transfer model, measured attenuation coefficients at the kelp forest, and astronomical and simulated tides. The least canopy exposure to UVR occurred when spring high waters coincided with solar noon, and the greatest UVR exposure occurred during neap low waters at noon. Tides altered daily photosynthetically active radiation (PAR) available for photosynthesis by < 9% whereas overcast skies reduced PAR by as much as 70%. The irradiance model was then combined with a BWF to produce a daylight- and depth-integrated production model. BWFs were determined experimentally for L. hyperborea collected from high light (0 m) and low light (10 m) environments. Photosynthetic oxygen evolution and carbon fixation were measured concurrently employing two emerging procedures: a fiber-optic optode system for measuring dissolved oxygen and stable isotope (13C) labeling of tissue C-uptake. The final production model was tested under different ozone depletion, tide, and cloud cover scenarios. Model simulations demonstrate that ozone depletion will have negligible impact on kelp forest production. Coastal waters shield the subtidal population from UVR, that is biologically effective UVR only penetrates < 3 m, even when low neap tides occurs at noon. Clouds vary daily production up to 20%, which suggests that available PAR primarily determines production in the kelp forest.