Effects of wave exposure and habitat fragmentation on growth and grazing of rocky shore seaweeds: a mesocosm experiment

Abstract

Seaweeds are foundation species on rocky shores that are exposed to a series of stressors. These include wave exposure, desiccation, grazing, and human activities, with habitat fragmentation as a common result. As part of climate and biodiversity changes, we are expecting increased winds and more frequent storm events, warmer climate, and trophic downgrading of ecosystems, causing overgrazing events. Here, we present the results from a two-factorial mesocosm experiment on the relationship between wave exposure, habitat fragmentation and growth (branching and tip elongation) of Fucus vesiculosus in the intertidal and upper subtidal. We found that intertidal seaweeds branched significantly more at low than high wave exposure. There was no effect of habitat fragmentation on branching, and no effects on tip growth from either wave exposure or habitat fragmentation. The experiment suffered greatly from uncontrolled grazing. Gastropod coverage was higher inside seaweed patches than in the seaweed free areas between and outside of the patches. In basins with highly fragmented seaweed patches, we found more gastropods at low than high wave exposure, whereas in basins with low fragmentation we found more gastropods at higher wave exposure. The percent coverage of gastropods in the basins did not seem to impact seaweed growth.