The effect of climate on seagass in the Torres Strait: 2011-2014

The Seagrass Ecology Group from James Cook University’s Centre for Tropical Water & Aquatic Ecosystem Research (JCU TropWATER),in collaboration with the Torres Strait Regional Authority Land and Sea Management Unit (TSRA LSMU), established a research program from 2011 to 2014 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 environmental change. An intertidal seagrass monitoring site was established at Mabuiag Island where information on seagrass biomass and species composition, and environmental data including irradiance (light), global solar exposure, daytime tidal air exposure, mean and maximum daily water temperature, rainfall, wind speed and salinity, were collected. \ud \ud Seagrasses at Mabuiag Island undergo distinct seasonal changes in biomass and species composition during summer, while biomass and species composition remain relatively stable between autumn and spring. Changes in biomass were driven by changes in total hours of daytime tidal exposure, maximum daily water temperature, mean daily irradiance and total rainfall in the 30 days prior to when biomass estimates were made. Changes in species composition were driven by total hours of daytime tidal air exposure, global solar exposure, and maximum daily water temperature. The negative effect of periods of peak maximum daily water temperature on seagrass growth indicates that Mabuiag Island’s intertidal seagrasses already live at the upper limit of their thermal tolerance. Projected future increases in sea temperature are likely to have negative consequences for these shallow coastal meadows, 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 minimising any anthropogenic impacts to seagrass to ensure resilience to cumulative stressors of local seagrass populations remain high.\ud \ud Results from this study highlight the importance of environmental drivers of seagrass change, and the benefits of monitoring over longer time periods to gain a more accurate understanding of seagrass dynamics. We recommend: \ud \ud 1. Commence a monitoring program of subtidal seagrass meadows for a similar time frame (3-4 years) to assess the relationship between subtidal seagrass biomass and species composition with environmental variables.\ud \ud 2. Commence manipulative experimental research to establish how Torres Strait seagrasses respond to some of the predicted effects of climate change, particularly to determine the thermal tolerance of different seagrass species found in Torres Strait.\ud \ud 3. Incorporate information collected from the present study and (1) and (2) to model the consequences of climate change on Torres Strait intertidal and subtidal seagrass to develop appropriate dugong and turtle management strategies that account for predicted changes in seagrass meadows.\u