A YEAR-ROUND ECOPHYSIOLOGICAL STUDY OF HIMANTOTHALLUS-GRANDIFOLIUS (DESMARESTIALES, PHAEOPHYTA) AT SIGNY-ISLAND, ANTARCTICA

Field studies carried out on the Antarctic brown macroalga Himantothallus grandifolius (A. Gepp et E.S. Gepp) A.D. Zinova (Desmarestiales, Phaeophyta) during two summers and an intervening winter season are described. Monitoring of plant growth by the punched-hole technique shows that elongation of the lamina stops a month before the onset of fast sea-ice in the austral autumn, but begins again a month before sea-ice break-out. In situ photosynthesis measurements carried out at 2-weekly intervals show that net carbon accretion also ceases as early as March, two months prior to formation of the sea-ice, and recommences before sea-ice break-out. A short period of high carbon accretion occurs immediately after the ice has gone, but this soon decreases abruptly due to reduction of underwater irradiance by a dense phytoplankton bloom during mid-summer. The high rates of carbon accretion do not recommence even after the bloom decays, because water clarity is by then dramatically reduced by terrestrial run-off and turbid glacial melt-water. Nutrient levels are sufficiently high throughout the year to support maximal rates of macroalgal photosynthesis, but water temperature seldom reaches even 1-degrees-C, resulting in relatively low metabolic rates. However, these are still sufficient to produce photosynthetic rates up to 9-mu-g C cm-2 h-1 at a saturation irradiance of 10 W m-2, and carbon loss due to dark respiration of up to 2-mu-g C cm-2 h-1.