Seagrass productivity, resilience to climate change and capacity for recovery in the Torres Strait: 2011-2013 report

• The Tropical Water & Aquatic Ecosystem Research (TropWATER) Seagrass Ecology Group in collaboration with the Torres Strait Regional Authority Land and Sea Management Unit (TSRA LSMU) established a program to develop critical information for the management of dugong and turtle in the Torres Strait by understanding how their key food resource, seagrass, is affected by seasonal change, climate and their ability to recover from impacts. The project also provides key information on how seagrasses in the Torres Strait may be affected by climate change and how this may impact on turtle and dugong management. • Several experimental sites were set up on intertidal and subtidal seagrass meadows examining seagrass recovery, productivity and potential environmental and climate drivers of change including light, exposure, temperature and salinity. • Seagrasses at Mabuiag Island undergo distinct seasonal and inter-annual changes in biomass due to environmental conditions. Daytime tidal exposure and rainfall were identified as the environmental variables contributing most significantly to temporal variation in intertidal seagrass biomass at Mabuiag Island. • The recovery experiments found that there were strong differences between meadow locations (subtidal and intertidal) and species in their capacity for recovery and the mechanisms employed to recolonise from disturbances. For intertidal mixed species meadows in this study, asexual colonisation was the most important mechanism for early recolonisation of cleared plots (gaps), whilst in the subtidal, recovery via a combination of sexual and asexual means was evident. • Most seagrass species at Mabuiag Island would likely be able to recover from small scale disturbances over a period of months where adult plants remain by capitalising on their highly clonal nature. However, recovery from larger scale disturbances would depend more heavily on colonisation by sexual propagules and therefore take years to recover, if at all. • Productivity levels of Mabuiag Island seagrasses compare with other globally important ecosystems. The net primary productivity of the intertidal meadow at Mabuiag Island at its peak in October (0.88 g C m⁻² day⁻¹) was comparable to that of sub-tropical coastal seagrasses at Gladstone, Queensland (0.89 g C m⁻² day⁻¹), and far above terrestrial grassland systems (0.50 g C m⁻² day⁻¹). This provides evidence that intertidal seagrasses at Mabuiag Island make a major contribution to local productivity and in supporting dugong and turtle populations. • Predicted changes to climate variables in Torres Strait and the Pacific region could have far reaching consequences for local seagrass community distribution and structure, which in turn may have profound implications for local dugong, turtle and commercial fisheries species. Management of seagrass resources in the Torres Strait should be focused on reducing any anthropogenic impacts to seagrass so as to ensure resilience levels of local seagrass populations remain high. We recommend continuation of the research program at Mabuiag Island to establish a stronger predictive set of relationships between key climate impacts to Torres Strait seagrasses. These relationships require data to be collected over several seasons and years to be fully developed. Results will provide much needed information on how natural climate variability and future scenarios of climate change may impact seagrass meadows, and therefore dugong and turtle feeding opportunities. Information that is collected has the potential to be incorporated into models to predict the consequences of climate change scenarios on Torres Strait seagrass, and to develop appropriate dugong and turtle management strategies that respond to changes in seagrass distribution and communities