Primary production potential of non-geniculate coralline algae at Cape Evans, Ross Sea, Antarctica

The abundance and photosynthetic characteristics of non-geniculate coralline algae were determined at Cape Evans, McMurdo Sound (Ross Sea), Antarctica, during spring 2003. At the time, the site was covered by 2.5 m of sea-ice through which only 0.48% of incident irradiance was able to penetrate. Coralline algae covered 4 to 60% of the seafloor from water depths of 13 to 26 m. Coralline algal crusts showed light-saturated rates of photosynthesis from 9.4 to 20 mmol O2 m -2 thal- lus d -1 and dark respiration rates from 0.15 to 0.96 mmol O2 m -2 thallus d -1 . The average irradiance at which the onset of light saturated photosynthesis occurred (E k) was 3.2 µmol photons m -2 s -1 , mea- sured using oxygen micro-electrodes, and 2.9 µmol photons m -2 s -1 , measured using pulse amplitude modulated (PAM) fluorometry techniques. Irradiance at depths of 15 to 20 m did not exceed 2 µmol photons m -2 s -1 during the study, but frequently exceeded the light compensation point for photo- synthesis (Ec), estimated as 0.10 µmol photons m -2 s -1 . The effective quantum yield (ΔF/Fm') of Photo- system II, measured in situ over diel periods at depths of 16, 18 and 20 m, averaged 0.691 (SD = 0.003), 0.639 (SD = 0.005) and 0.632 (SD = 0.007), respectively. These values were indicative of mini- mal down-regulation of photosynthesis, and were consistent with the observation that irradiance never exceeded Ek. Laboratory investigations, showed that on transfer from ambient irradiance to 23.5 µmol photons m -2 s -1 , a change of 2 orders of magnitude, the algae showed minimal photo- inhibition and a small, but statistically significant increase in Ek and maximum electron transport rate. Coralline algae at Cape Evans are able to persist with net carbon accrual under ice, while being able to tolerate periods of higher irradiance during ice-free conditions. This contrasts to Phyllophora antarctica which is dependent on ice-free periods to achieve net production.