| Summary: | Human beings can modify the abiotic conditions and biotic interactions that shape natural ecosystems. The primary aim of my thesis was to elucidate local- and global-scale anthropogenic changes of importance in shaping seagrass habitats, by examining how a number of both established and forecasted conditions can modify biotic interactions between seagrass and their consumers (i.e. grazing herbivores), as well as interactions between seagrass and their competitors (i.e. algal epiphytes). I approached this from both a top-down (i.e. consumer driven) and bottom-up (i.e. resource limitation) perspective. I begin by examining how urchin-epiphyte-seagrass interactions can modify habitat loss in local seagrass beds. This was motivated by the observation that the seagrass species Amphibolis antarctica appeared to suffer disproportionately under the influence of herbivore expansions. To this end, I found the recovery rate of the seagrass Amphibolis antarctica was much slower following grazing damage than contiguous meadows of Posidonia sinuosa, which appeared to result from the distinct morphological differences between these two seagrasses (i.e. elevated vs. basal meristems, respectively). Given these emerging urchin effects and the legacy of seagrass loss from nutrient enrichment, next I investigated how these processes (i.e. top-down vs. bottom-up) might interact to shape the maintenance or loss of seagrass habitats. I found that the independent negative effects of urchins and nutrients do not combine in an intuitive manner, but rather eutrophic conditions reduce the per capita grazing of urchins on seagrass, as they appear to acquire greater nutrition from increasing food quality. After documenting these emerging local changes to seagrass, I examined how forecasted global changes, in particular increases in dissolved CO₂ could modify future seagrass meadows. From a resource limitation perspective I examined the carbon physiology of three seagrass species, consistently finding they were reliant on energetically costly ...
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