Short- and long-term reactions to ultraviolet radiation (UVR) of marine planktonic algae were investigated with respect to the influence of an enhancement of ultraviolet-B radiation (UVB) caused by stratospheric ozone depletion. In order to assess these reactions under ecologically relevant irradiances, realistic irradiances were applied by simulating solar-like spectral distributions under various ozone concentrations. As previously observed UVR had a pronounced influence on microalgal photosynthesis. Reduction in photosynthesis was observed with a simulation of enhanced UVB corresponding to 30% ozone depletion. Under a light-cycle simulating a variable irradiance (such as induced by vertical mixing) photosynthesis was significantly reduced at simulated 13% ozone depletion. Under both conditions, the total effect induced by enhanced UVB irradiance was mainly related to the applied maximum incident irradiance not to the applied cumulative dose.

There were reductions in photosynthesis and growth by enhanced UVB irradiance for various temperate and polar algal species after long-term exposures. UVB enhancement reduced photosynthesis and growth simultaneously, which indicates a close-coupling of photosynthesis and growth. On the basis of this close-coupling the possibility and capacity of a UVB-related photoacclimation in algae that could counteract the effects of changes in UVB irradiance by ozone depletion was evaluated. Photoacclimation to irradiance of ultraviolet A (UVA) and photosynthetically available radiation (PAR) was observed by pronounced changes in photosynthetic and photoprotective pigments, including UV-absorbing compounds, and by changes in photosynthetic capacity and growth. On the other hand, enhanced or reduced UVB irradiance did not change the pigment contents or pigment composition, but influenced photosynthesis and growth. However, effects on photosynthesis and growth were the same during short and long exposures, and long-term pre-exposure did not reduce the UVB-sensitivity. Hence, there were no indications of photoacclimation to UVB irradiance.

Under natural conditions vertical mixing will induce photoacclimation to low irradiance by subjecting phytoplankton to a low mean UVA/PAR irradiance. Phytoplankton of high turbulent waters will be acclimated to a low irradiance and, hence, be more sensitive to UVB irradiance at the surface. Additionally, vertical mixing will not reduce the effects of UVB radiation by limiting the time of exposure to high surface irradiances. Ambient UVB radiation influences plankton productivity instantaneously and this influence will only be mitigated by photoacclimation to the concomitant UVA and PAR irradiance. In contrast, effects of ozone depletion which only increases UVB irradiance, will not be counteracted by an acclimation to the enhanced UVB irradiance.