The effect of ocean acidification on plant-animal interactions in a Posidonia oceanica meadow

Abstract

Ocean acidification may be one of the greatest challenges facing 'Society over the next SO- 100 years. In the Mediterranean Sea, meadows of the seagrass Posidonia oceanica play a pivotal role in coastal systems, providing shelter and food for a highly diversified plant and animal community, and nursery for several commercial species of fish and bivalves. This thesis aims to provide a broad overview of the effect of acidification on P. oceanica, and its associated community, through observation of seagrass meadows associated with subtidal CO2 vents off the Castello Aragonese, Ischia, Italy. Control (mean pH 8.1) and acidified (mean pH 7.8) stations were situated in seagrass meadows on both the north and south side of the Castello Aragonese. Research was concentrated into three main objectives; to investigate the effect of acidification on 1) plant structure and function, 2) invertebrate community dynamics, and 3) macroherbivore grazing pressure. Results suggested that P. oceanica and its associated community should be robust to levels of acidification projected for the end of this century. Seagrass density, invertebrate abundance, and P. oceanica grazing pressure all increased in response to acidification. Although a slight decrease in invertebrate diversity was observed in 'acidified stations, species richness was maintained. Increased grazing by the fish Salpa salpa was most highly correlated to a decrease in epiphytic algal cover and C:N content of the [eaves, and an increase seagrass density. Whilst fish may select leaves with low algal cover and C:N content, previous research suggests that increased shoot density is a response to high grazing pressure. The abundance of many groups of invertebrate taxa was positively correlated to shoot density, whilst decapods decreased during months of reduced canopy height, suggesting changes in P. oceanica condition variables, in response to acidification, may lead to a cascade of indirect effects which have positive or negative influences on the abundance of the associated invertebrate community. One of the most striking findings of this thesis was the ability of calcifying species to persevere, and even flourish in acidified conditions. This may be due to the complex nature of the seagrass, and high levels of photosynthesis, leading to local maintenance of pH microclimates. Secondly, the high degree of pH variability that naturally occurs in these shallow water habitats may result in organisms that are be better equipped to withstand pH variation. This research suggests that highly productive, non-calcifying, biogenic habitats, such as seagrass beds, may provide a refuge from OA, and highlights the importance of their conservation.