|HOME SCHEDULE AUTHOR INDEX SUBJECT INDEX|
A hemispheric comparison of grazing and growth of habitat-forming algae across exposure gradients.
Taylor, David*,1, Dunmore, Robyn1, Schiel, David1, 1 University of Canterbury, Christchurch, New Zealand, New Zealand
ABSTRACT- Models of intertidal community structure highlight that the relative importance of processes like grazing changes over environmental stress gradients. A commonly used proxy for environmental stress is 'wave exposure'. Tests of these models often use mussel and barnacle assemblages, but are these relevant for habitat-forming seaweeds? We tested the importance of grazing in the survival of related seaweeds across wave exposure gradients in southern New Zealand and Oregon. Fucoid algae can dominate the lower and mid intertidal zones in New Zealand and in the mid and upper zones in Oregon. We tested grazing effects on early benthic life-stages (germlings) that are critical to establishment of algal populations but are vulnerable to invertebrate grazers. We transplanted germlings (c. 0.15 mm) of Hormosira banksii and Durvillaea antarctica in New Zealand and Pelvetiopsis limitata and Fucus gardneri in Oregon in a series of experiments across wave exposure gradients. The grazing effect on germling survival was often small (< 5% percent within the first 7 days) but varied considerably and accounted for up to 38 % of mortality at sheltered and exposed sites. However, no clear gradient in grazing effects was found across wave exposures. Overall, germling survival was low, usually < 20 % after 14 days, even when molluscan grazing was eliminated. Across exposures, sedimentation and ephemeral algae cover affected germling mortality. Growth differed considerably between species with Durvillaea growing up to three times faster than the other species within 14 days. It appears that during this critical establishment phase the direct effects of grazing do not change across wave exposure gradients, but instead factors like plant characteristics, sedimentation, smothering and desiccation are more important. This work is part of the PISCO/Mellon project.
Key words: algae, grazing, marine, exposure