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Exposure to desiccation during heat wave conditions (≥ 35°C) is considered the most likely cause of a sudden dieback of 12 700 ha of intertidal and shallow subtidal seagrasses along 95 km of coast in Spencer Gulf, South Australia. To investigate this hypothesis experiments in a constant environment (CE) room were designed to test the tolerances of 2 shallow subtidal species, Amphibolis antarctica and Posidonia australis, to desiccation for a range of temperatures and exposure times. The first experiment compared the effects of increasing exposure time (15, 30, 45, 60, 80 and 100 min) on photosynthetic efficiency at a relatively mild summer temperature (CE room set to 24°C). The second experiment compared the effects of increasing temperature (CE room temperature set to 18, 24, 28 or 32°C) and exposure time (0, 20 and 60 min). Photosynthetic efficiency, damage and extent of recovery were determined by chlorophyll fluorescence measured using a pulse amplitude modulated fluorometer. The ability of both species to recover from desiccation decreased at higher temperatures and longer exposure time, with P. australis more susceptible to desiccation than A. antarctica. The results indicate that higher temperatures alone did not significantly affect photosynthetic efficiency for either species. Four main responses for maximal quantum yield over a time series were apparent: (1) no difference from controls, (2) a significant degree of photosystem II inhibition followed by complete or (3) partial recovery, and (4) complete inhibition of photosystem II with no recovery. Short exposures to desiccation at lower temperatures generally resulted in moderate stress followed by complete recovery, while at higher temperatures there was only partial recovery, particularly for P. australis. However, both species exposed for 1 h at the highest temperature showed no signs of photosynthetic recovery up to18 h after re-immersion, suggesting that significant seagrass dieback is plausible under these conditions in situ, particularly if the seagrasses were exposed to repeated desiccation on consecutive days.

Summary1. Natural resources and ecosystem services provided by the world's major biomes are increasingly threatened by anthropogenic impacts. Rehabilitation is a common approach to recreating and maintaining habitats, but limitations to the success of traditional techniques necessitate new approaches. 2. Almost one-third of the world's productive seagrass meadows have been lost in the past 130 years. Using a combined total of three seagrass species at seven sites over 8 years, we experimentally assessed the performance of multiple rehabilitation methods that utilize fundamentally different ecological approaches. 3. First, traditional methods of transplantation were tested and produced varied survival (0-80%) that was site dependent. Secondly, seedling culture and outplanting produced poor survival (2-9%) but reasonable growth. Finally, a novel method that used sand-filled bags of hessian to overcome limitations of traditional techniques by facilitating recruitment and establishment of seedlings in situ produced recruit densities of 150-350 seedlings m 4.Results indicate that facilitating seagrass recruitment in situ using hessian bags can provide a new tool to alleviate current limitations to successful rehabilitation (e.g. mobile sediments, investment of time and resources), leading to more successful management and mitigation of contemporary losses. Hessian bags have distinct environmental and economic advantages over other methods tested in that they do not damage existing meadows, are biodegradable, quick to deploy, and cost less per hectare (US$16 737) than the estimated ecosystem value of seagrass meadows (US$27 039 year )1 ). 5. Synthesis and applications. This research demonstrates how exploring alternate ecological approaches to habitat rehabilitation can expand our collective toolbox for successfully re-creating complex and productive ecosystems, and alleviate the destructive side-effects and low success rates of more traditional techniques. Moreover, new methods can offer economic and environmental solutions to the restrictions placed upon managers of natural resources.