Spatial and temporal variability of green turtle and dugong herbivory in seagrass meadows of the southern Great Barrier Reef (GBR)

Megaherbivore grazing (e.g. by turtles, and sirenians) plays a major and well-documented role in structuring seagrass meadows around the world; however, we know little about local-scale (intra- and inter-meadow) variability in megaherbivore grazing. This is surprising given that megaherbivores are highly selective eaters who may feed by targeting certain meadows, or areas within a meadow. We ran an experiment in the Great Barrier Reef to test the question: How does megaherbivory vary on a regional scale? We used megaherbivore exclusion cages in 5 meadows along a 50 km region of coastline around Gladstone in the southern Great Barrier Reef World Heritage Area to understand variability in megaherbivory between meadows. We found differences in the impacts of megaherbivore grazing on seagrass biomass and shoot heights between meadows. There were also interannual differences in grazing impacts at one meadow that had been studied previously. These differences may be due to megaherbivore population and grazing dynamics, as well as the response of seagrass to grazing pressure. Our results show that seagrass meadows grazed by megaherbivores are dynamic systems that vary on regional spatial scales as well as over time. This is important for management measures that seek to consider the seagrass-herbivore system as a whole and understand the implications of monitoring efforts based on seagrass aboveground condition.