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 resulting in shoot loss during experiments. There was little indication of temperature related stress during the field studies up to April 2010; however, the temperature experiments, in conjunction with future predictions of water temperature, indicate that water temperature will become a significant driver in GBR seagrass meadows.
Data Units:
Field:
* Light (recorded every 30mins), available as µmol photons m-² d-¹
* Temperature (°C recorded every 1.5hrs)
* Subtidal/intertidal habitat
* Seagrass species: Cymodocea serrulata, Cymodocea rotundata, Halodule uninervis, Thaalssia hemprichii and Zostera muelleri.
* Seagrass response variables: percent cover, biomass, morphology (leaf dimensions, leaves per shoot, rhizome internode length and branching distance), leaf growth rates, carbohydrate storage reserves and photosynthetic pigments (Chlorophyll a+b).
Experiments:
* Light: Shading experiment - high (66% surface light), moderate (31%), low (14%) and very low light (1%) and temperature x light experiment - high light (400 µmol photons m-² s-¹) and low light (40 µmol photons m-² s-¹)
* Temperature: Pulse experiment - ambient to 35, 40 and 43°C; and temperature x light experiment - 27°C, 30°C, 33°C
* Seagrass response variables: shoot density, biomass, leaf growth rates, morphology (leaf dimensions, leaves per shoot), carbohydrate storage reserves, photosynthetic pigments (Chlorophyll a+b), photosynthetic efficiency (PSII (effective PSII quantum efficiency) & Fv/Fm (maximal photochemical efficiency)) and rapid light curves.
Publications:
- Collier CJ, Uthicke S, Waycott M (2011) Thermal tolerance of two seagrass species at contrasting light levels: implications for future distribution in the Great Barrier Reef. Limnol Oceanogr. 56 2200-2210, DOI: 10.4319/lo.2011.56.6.2200
- Collier CJ, Waycott M, Giraldo-Ospina A (In Press) Responses of four Indo-West Pacific seagrass species to shading. Marine Pollution Bulletin, DOI: 10.1016/j.marpolbul.2011.06.017
References:
- Collier C and Waycott M. (2009) Drivers of change to seagrass distributions and communities on the Great Barrier Reef: Literature Review and Gaps Analysis. Report to the Marine and Tropical Sciences Research Facility. Reef and Rainforest Research Centre Limited, Cairns (55pp.).
- Waycott M and McKenzie L. (2008) December 2008 Milestone Report. Report to the Marine and Tropical Sciences Research Facility. Reef and Rainforest Research Centre Limited, Cairns (9pp.).
