Seagrass productivity, resilience to climate change and capacity for recovery in the Torres Strait: 2011-2012 report

• The Tropical Water & Aquatic Ecosystem Research (TropWATER) Seagrass 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, temperature, salinity and pH. • The first twelve months of the study show that seagrasses at Mabuiag Island undergo distinct seasonal and inter-annual changes in biomass as a result of complex interactions with natural drivers of change. Water temperature, daytime tidal exposure, salinity and rainfall were identified as the environmental variables contributing most significantly to the observed seasonal and inter-annual 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 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 have to rely more heavily on colonisation by sexual propagules and therefore may take years to recover, if at all. • Productivity levels of Mabuiag Island seagrasses appear to compare highly with other globally important ecosystems. The net primary productivity of the intertidal study meadow at Mabuiag Island at its peak in October (0.71 g C m⁻² day⁻¹) was higher than that of tropical coastal seagrasses at Abbot Point, Queensland (0.65 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 monitoring and research program at Mabuiag Island to allow better correlations and further identify 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 will be incorporated into modelling of the consequences of climate change on Torres Strait seagrass distribution to develop appropriate dugong and turtle management strategies that respond to potential shifts in seagrass distribution and communities.