MEPS 220:119-130 (2001)  -  doi:10.3354/meps220119

ABSTRACT: Exposure to desiccation during heat wave conditions (≥35°C) is considered the most likely cause of a sudden dieback of 12700 ha of intertidal and shallow subtidal seagrasses along 95 km of coast in Spencer Gulf, South Australia. To investigat 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.

KEY WORDS: Seagrass dieback · Pulse amplitude modulated fluorometer · Chlorophyll fluorescence · Inhibition · Recovery