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Throughout the experiments, erythemal UV radiation and PAR measurements were obtained using a Solar Light Company 501A Biometer (PA, USA) and a LI-COR LI-190SA Quantum sensor (Nebraska, USA) mounted on the roof of the minicosm container. The Biometer and LI-COR were sampled every second and the average irradiance logged at 10-minute intervals. Due to the failure of the spectroradiometer, we were unable to quantify the spectral irradiance of the light to which microbial communities were exposed during the experiments at Davis Station. Consequently, upon return to Australia, the transmittance of PAR and erythemal UV into each minicosms container was modelled and the model used to quantify the 10 min averaged irradiances received by the microbial community in each tank during each experiment. Stratospheric ozone concentrations and specific cloud conditions at Davis Station could not be replicated. However, measurements were performed in austral spring to ensure similar solar zenith angles and, like Davis, were obtained in conditions ranging from clear sky to heavily overcast. 2) Depth: The attenuation coefficient of PAR and erythemal UV with depth in the minicosm was determined from measurements made at a range of positions and depths in the tanks (above). The average transmittance at each depth was graphed and the attenuation coefficient (kdepth) for PAR and erythemal UV determined from the power term of the least squares exponential regression. Due to absorption of light by the natural polythene walls of the minicosm, kdepth for UV and PAR were 0.0326 and 0.0162 cm-1, respectively. 3) Solar zenith angle: Changes in the transmittance of light to the minicosms as a result of varying solar zenith angle were obtained (as above) over a period of 6 weeks. Results showed that solar elevation had little influence on the transmittance of UV and PAR into the water column. Any slight influence was incorporated into other attenuating factors (above) that were made under a variety of sun angles. 4) Microbial biomass: We used a theoretical approach to quantifying attenuation of the PSAR and erythemal UV by the microbial biomass in each tank, due to differences in the microbial composition between Antarctic and Tasmanian coastal waters and the inability to grow microbial communities sourced from Tasmanian waters in winter. The absorption spectrum of a filtered sample of suspended matter in the Weddell Sea from 0.5 m depth of (Gieskes & Kraay 1990) was scaled using an optical density (OD) of chlorophyll a (chl a) of 88.7 cm-1 for 1g l-1 in water at ~673 nm wavelength (Jeffrey & Humphrey 1975, Gieskes & Kraay 1990). The radiative transfer model UVSPEC (Mayer & Kylling 2005), was used to determine the incident spectral irradiance I0 for Davis station at noon local solar time on the 21st of December using an albedo of 0.5, stratospheric ozone value of 300 DU and a sub-arctic atmosphere file. The transmitted spectral irradiance (IT) between 280 and 700 nm was then calculated using the equation:

Temporal changes in effects of ambient UV radiation on natural communities of Antarctic marine protists