Winter favours growth and survival of Ralfsia verrucosa (Phaeophyceae) in high intertidal rockpools in southeast Australia

Abstract

The recruitment, growth and survival of the brown crust Ralfsia verrucosa occurring around the southern coast of Australia were studied. Up to 43 upper intertidal and supratidal pools containing just Ralfsia and sometimes Hormosira on a reef with a wave-cut platform at about 36°S were monitored for just over 5 years. With an extreme tidal range of 2.0 m, tidal height on the platform was increased by an average of 0.5 m by swell. In winter mean pool temperature differed little from the sea but in the summer when low water was near mid-day it was 12°C higher. Individual pool temperatures at one time of year were correlated with those at another. High pools were hypersaline in calm weather and neap tides. In Ralfsia unilocular sporangia became apparent at the end of each March for each of 4 years, clearly in response to short days. Using photographs of selected areas of pools, growth and degeneration (clear centres of discs) of Ralfsia crusts were measured and recruitment followed after clearance through sand cover or drying out. Radial growth rate increased with initial size up to, but was unaffected above, a radius of 1 mm. Growth was faster in winter (35 μm day-1) than summer and reduced by partial sand cover, while degeneration and mortality of whole discs was greater in summer. Invisible scraps of filaments could remain after crust disappearance and formed the early, and more successful, apparent recruitment when conditions improved in autumn before sporelings appeared. Peak settlement of spores was calculated (using development rates in culture) to take place 2 months after peak fertility. Settlement was influenced by the environment of the pool: densities at different times were correlated. Mortality was higher in summer than winter when some discs reappeared. Responsible factors for the summer stress were not identified.