Turf assemblage of a Macrocystis kelp forest : experiments on competition and herbivory Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5999n768b

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  • Early and later successional stages of the assemblage of turf algae and sessile animals of a Macrocystis kelp forest were studied off San Nicolas Island, California from 1980 through 1981, and 1983 through 1984. Kelps were manipulated to determine if differences in illumination could account for dominance by turf algae or sessile animals. Caging and algal-removal experiments were conducted to determine if the effects of herbivory and competition could account for the low or variable recruitment and abundance of foliose turf algae. Plots were covered for different time intervals up to 1 year to determine if survival of overgrowth could explain the prevalence of competitively subordinate crustose coralline algae. Increased cover and recruitment of turf algae and decreased cover and low recruitment of sessile animals were correlated with the removal of canopy. In the presence of canopy, existing cover of crustose algae and sessile animals changed little over time; sessile animals recruited at significantly higher levels than algae on cleared plots. Patiria (a common invertebrate grazer) removed certain ephemeral algae and appeared to slow recruitment by other algae, but had little effect on mature turf algae. These effects appeared dependent on a high density of Patiria. Algal recruitment on caged and open, near-bare plots was negligible, but sessile animals recruited heavily to these plots. These results were correlated with low illumination caused by a dense and persistent surface canopy of Macrocystis. Established foliose red algae increased significantly in caged plots compared to open plots implying effects due to herbivory. No changes in cover were associated with turf algal-algal competition on these plots; a major decline in cover of all arborescent turf algae was correlated with low illumination. On covered plots, crustose coralline algae alternately survived, died, and recruited at different sampling points, resulting in only a slight change in abundance after 12 months of coverage. Other sessile organisms also survived while covered, but declined substantially compared to crustose coralline algae. Patterns of distribution and abundance of turf algae and sessile animals were correlated with the Macrocystis canopy, as it affects illumination at the substrate. No pervasive effects were attributed to grazing by large invertebrates. Herbivory, probably by fishes, was a plausible explanation for the low abundance of foliose red algae. The ability of crustose coralline algae to survive and recruit under conditions of overgrowth was shown to be a contributing factor for the prevalence of these algae.
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