Seasonal growth and recruitment of Himanthalia elongata (Fucales, Phaeophycota) in different habitats on the Irish west coast

Vegetative and reproductive growth of individually marked plants of the bro wn alga Himanthalia elongata was monitored over 25 years at two sites with different wave exposures on the Irish west coast. Macro-recruits were first visible to the unaided eye in February/March. About 65% of all buttons pro duced receptacles during autumn of the same year, whereas others remained s terile. Growth of receptacles started between late August and late November , when vegetative plants were 9-14 months old, but no further plants became reproductive after December. At both sites, length increase of receptacles was greatest between February and April, but only longitudinal growth occu rred and no more new dichotomies were produced. At the more wave-exposed si te, growth was fastest in plants growing in red algal turfs and slowest in plants on open rock. Growth rates were maximal when seawater temperatures w ere 10-12 degrees C in early spring but decreased from May onwards with the maturation of receptacles aged 7-9 months in June. Release of viable gamet es started in June and continued throughout the autumn and winter, until al l plants disappeared from the population in February. Plant density was hig hest in March after the appearance of macro-recruits in the population, and declined during summer when the number of buttons decreased. Percentage pl ant cover was highest in June immediately before the onset of gamete releas e and this was followed by the decay and disintegration of receptacles. Sin ce both survival and growth of young buttons was highest in red algal turfs and lowest on exposed rocks, the distribution of H. elongata on semi-expos ed shores in Ireland may be limited by the availability of suitable micro-h abitats for recruitment, and reduced by the early loss of fertile, adult pl ants from exposed sites.