Bachelor thesis - Relationships between habitat fragmentation, wave exposure, grazing intensity and growth of the intertidal seaweed species Fucus vesiculosus L.

Littoral seaweed beds often dominate rocky shores worldwide, providing a range of important ecosystem services to human populations and housing rich biodiversity vital to the health of coastal systems. Seaweeds are well known ecosystem engineers, and are exposed to a series of pressures, including habitat fragmentation, wave exposure variation and grazing. Here, I present the results from a factorial mesocosm-based experiment which aimed to study the relationship between habitat fragmentation, wave exposure, grazing and growth of Fucus vesiculosus Linnaeus. Two levels of habitat fragmentation were defined, one consisting of a single patch of seaweed, and one of lots of tiny patches. Two levels of wave exposure – high and low – were also defined. Seaweed growth, measured both as changes in apical tip length and branching, was monitored weekly during 14 weeks in 12 mesocosm basins. Seaweeds were analysed separately at two different depth levels (intertidal and upper subtidal) in case there were differences due to depth. The grazing level of each seaweed was also recorded, and these data were supplemented with monthly recordings of snail density. I found no effects of habitat fragmentation or wave exposure on apical seaweed growth, neither in subtidal or intertidal seaweeds. However, intertidal seaweed individuals branched significantly more at low than high wave exposure (but this was not found for the subtidal seaweeds). Grazing (most likely from snails) was the main driver of the recorded seaweed growth pattern. However, after a threshold level of light grazing, increased grazing intensity did not significantly reduce tip growth. Interactions between wave exposure and fragmentation level significantly impacted snail density, with higher snail coverage occurring under low wave exposure in more fragmented habitat, and vice-versa. These results highlight the importance of taking into account interactions between multiple stressors in management, as human impacts on the world’s coasts intensify.