|Abstract or Summary
- 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
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.