Habitat-forming intertidal algae across wave-exposures: An experimental evaluation of plant and herbivore interactions

Abstract

Ecological factors influencing the distribution, survival and growth across wave exposures of different life stages of some intertidal habitat-forming fucoid algae in southern New Zealand and Oregon were examined. Transect and quadrat sampling in southern New Zealand showed changes in patterns of habitat-forming intertidal algae across wave-exposures. Biomass and species richness across exposures varied greatly and was concentrated in the lower tidal zone. On wave-exposed platforms lower tidal zone biomass was often dominated by the bull kelp Durvillaea antarctica, mussels Perna canaliculus or suites of red algae (Gigartina species). Biomass in the lower tidal zones at moderately exposed platforms was dominated by perennial brown algae like Cystophora torulosa, C. scalaris and was dominated by the fucoid alga Hormosira banksii and the blue mussel Mytilus galloprovincialis at sheltered sites. Invertebrate herbivore abundance and biomass were similar across exposures but the greatest species richness was found in the upper tidal zones. Predator abundance was greatest at sheltered and exposed sites that were dominated by filter-feeder habitats. Settlement experiments compared the relative ability of Hormosira banksii, Cystophora torulosa and Durvillaea antarctica, to remain attached when exposed to a low energy wave after post-settlement attachment times of 1, 6 and 12 hours. Results showed the exposed shore species D. antarctica had better attachment capabilities than C. torulosa and H banksii which are more abundant at wave protected sites. In another set of experiments zygotes were subjected to a full 12-hr tidal-cycle in the field after post-settlement attachment times of 1, 6, and 12 hours. H banksii survival was again dependent on post-settlement attachment time and showed a negative relationship with wave exposure while D. antarctica survival was not affected by wave exposure or post-settlement attachment time. In identical experiments in Oregon, survival across exposures of Fucus gardneri and Pelvetiopsis limitata was not significantly different and was largely dependent on sites within exposures. However, a post-settlement time of at least six hours was required for greatest survival of P. limitata and F. gardneri. These results suggest that sheltered shore species may be constrained in their distribution and abundance across exposures from the outset due to required attachment times. Transplant experiments using early life stages of habitat-forming large brown algae were done across wave exposures at different times and at different spatial scales in different hemispheres. Specific hypotheses of models of community structure relating to the effects of grazers across exposures were tested using Durvillaea antarctica and Hormosira banksii in New Zealand, and Fucus gardneri and Pelvetiopsis limitata in Oregon. Invertebrate grazer effects were similar across exposures but interactions with ephemeral algae, which were most abundant at exposed sites, and sedimentation at sheltered and intermediate sites were important in both hemispheres. Experiments showed that all species were able to reach the visible recruit stage at all exposures if protected from invertebrate grazers. For all species growth was greatest during spring and summer at wave-exposed sites. Factors affecting the fine scale distribution of Durvillaea antarctica were tested in a canopy removal and substratum clearance experiment repeated at different times over a year. The results showed that the time of clearance affected recruitment of D. antarctica. Greatest recruitment was observed under adult canopies at one site and generally recruitment was greatest in coralline removal treatments. Transplant experiments using Durvillaea antarctica recruits were used to test growth and survival across exposures and across coastlines. I found that the distribution of D. antarctica across exposures and across coastlines is constrained by the selective grazing activities of the herbivorous fish Odax pullus. Traditional models of intertidal systems often do not reflect changes in the importance of biological and physical factors at different life stages. The innovation of this study is that even over hemispheric scales these intertidal habitat-forming algae are constrained by their life-history characteristics and demographic processes across wave exposures. There were clear differences in the relative abilities of algae to attach quickly to the substratum. At early life stages, the effects of invertebrate grazers were the same across intertidal wave exposure gradients but these affects interacted with growth rates, the environmental effects of sedimentation and the biological interaction with ephemeral algae. For Durvillaea antarctica, after reaching the recruit stage its distribution and abundance across exposures and coastlines was determined by the grazing effects of the butterfish Odax pullus. I propose a community structure model that incorporates changes in the importance of biological and physical processes at different life stages.