| Summary: | The aims of this work (MTSRF Project 1.1.3b) were to identify the role of light and water temperature as drivers of change in seagrass meadows of the northern Great Barrier Reef. Experimental approaches as well as field investigations were undertaken. Field studies occurred at four locations (Magnetic Island, Dunk Island, Green Island, Low Isles). There was continuous monitoring of light and temperature at seagrass canopy height in both intertidal (above lowest astronomical tide, LAT) and near-by subtidal meadows (1-3m below LAT) at each of the four locations. Approximately every 3 months, seagrass response variables were also measured, including, percent cover, growth, seagrass morphology and physiology. There were three experiments testing the effects of temperature and/or light on tropical seagrasses. 1. A shading experiment where seagrass response variables were measured for 102 days at four light levels: high light (66% surface light), moderate (31%), low (14%) and very low light (1%). 2. A pulsed temperature experiment where short-term spikes of water temperature to 35, 40 and 43°C, which mimic current temperature ranges during low tide in the GBR, were applied for 6 days followed by one day of recovery. 3. A temperature and light interaction experiment at 27, 30 and 33°C, which spans current and future predicted summer water temperature, at saturating (400 µmol photons m-² s-¹) and limiting light levels (40 µmol photons m-² s-¹). Results indicated that low light levels had a significant impact at field study sites during the monitoring period with subtidal meadows often pushed below minimum light requirements during the monsoon season. This combined with ongoing low light events throughout the year (probably due to re-suspension of sediments by tides and wind) was associated with substantial loss of seagrass at Magnetic Island (from 45% cover to 1% cover from January 2008 to April 2010) and loss also occurred at Dunk Island and Low Isles. The light levels and duration of impact are similar to those ...
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