Growth, Survival and Reproduction of Two Species of Marine Algae at Different Densities in Natural Stands

(1) Naturally-occurring monospecific stands of two subtidal fucoid species, an annual, Sargassum sinclairii and a perennial, Carpophyllum maschalocarpum, were monitored from recruitment, at high and low densities, in northern New Zealand. (2) Individuals of both species grew faster and attained a larger size in the high-density stands. (3) Survivorship rates fluctuated throughout the year for lowand high-density Sargassum sinclairii, but a higher proportion of low-density plants survived at the end of the year. Carpophyllum maschalocarpum plants showed higher survivorship at low density, but all plants in these stands which remained at the end of the study were stunted. (4) Size-frequency distributions through time tended to be positively skewed for both lowand high-density stands of Sargassum sinclairii. For both species, however, there was little indication that most plants at high density were being trapped under the canopy of a few large individuals. (5) Ninety-two per cent of the S. sinclairii plants became reproductive in high-density stands, compared with 46% at low density. Forty per cent of the Carpophyllum maschalocarpum plants became fertile after the second year of growth at high-density, while none of the low-density plants became reproductive. (6) The bearing of receptacles is clearly related to plant size, and most plants of both species at low density did not attain a large enough size to reproduce. For both species, the proportion of plant weight invested in receptacles increased with plant size. In addition, the greatest recruitment for Sargassum sinclairii occurred near dense adult stands. (7) High density aggregations may be important for the establishment and maintenance of populations of large brown algae. There may be important differences between terrestrial and marine systems with respect to some density effects.