GBR - Drivers of change in seagrasses of the Great Barrier Reef, 2008 - 2010 (MTSRF 1.1.3b, JCU)

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.).

本研究（MTSRF项目1.1.3b）的核心目标为明确光照与水温作为驱动因子，在北澳大利亚大堡礁(Great Barrier Reef)海草床群落变化中的作用。本研究采用实验方法与野外调查相结合的研究手段。

野外研究共设置4个采样点：磁岛(Magnetic Island)、邓肯岛(Dunk Island)、绿岛(Green Island)、低岛群(Low Isles)。在每个采样点的潮间带（高于最低天文潮位(LAT)）与邻近潮下带海草床（低于最低天文潮位1-3米）中，均持续监测海草冠层高度处的光照与水温参数。每间隔约3个月，同步测定海草响应变量，包括盖度百分比、生长状况、形态与生理特征。

本研究共设置3组实验，用以探究温度和/或光照对热带海草的影响：
1. 遮光实验：设置4个光照梯度——高光（66%地表光照）、中光（31%）、低光（14%）、极低光（1%），持续实验102天并测定海草响应变量。
2. 脉冲式温度实验：模拟大堡礁枯潮期间的水温波动，将水温短时提升至35℃、40℃与43℃，持续处理6天后恢复1天。
3. 温度-光照交互实验：设置27℃、30℃、33℃三个温度梯度（覆盖当前与未来预测的夏季水温范围），并分别设置饱和光强（400 μmol光子·m⁻²·s⁻¹）与限制光强（40 μmol光子·m⁻²·s⁻¹）两个光照水平。

监测结果显示，在野外研究周期内，低光照水平对采样点海草床造成了显著影响：潮下带海草床在季风季时常处于最低光照需求阈值以下。结合全年持续发生的低光照事件（大概率由潮汐与风浪导致沉积物再悬浮引发），磁岛海草盖度出现大幅下降（从2008年1月的45%降至2010年4月的1%），邓肯岛与低岛群同样出现海草流失现象。该类低光照的强度与影响时长，与实验中导致海草茎枝死亡的条件一致。截至2010年4月的野外监测数据中，未观测到显著的温度相关胁迫；但结合温度实验结果与未来水温预测，水温将逐步成为大堡礁海草床群落变化的关键驱动因子。

### 数据单元
#### 野外监测数据
* 光照：每30分钟记录一次，单位为μmol光子·m⁻²·d⁻¹
* 水温：每1.5小时记录一次，单位为℃
* 生境类型：潮下带/潮间带
* 海草物种：带形二药藻(Cymodocea serrulata)、圆头二药藻(Cymodocea rotundata)、单脉二药藻(Halodule uninervis)、汉氏泰来藻(Thalassia hemprichii)、穆勒氏大叶藻(Zostera muelleri)
* 海草响应变量：盖度百分比、生物量、形态特征（叶片尺寸、单茎叶片数、根状茎节间长度与分枝间距）、叶片生长速率、碳水化合物储存储备与光合色素（叶绿素a+b）

#### 实验数据
* 光照设置：遮光实验设置高光（66%地表光照）、中光（31%）、低光（14%）、极低光（1%）四个梯度；温度-光照交互实验设置饱和光强（400 μmol光子·m⁻²·s⁻¹）与限制光强（40 μmol光子·m⁻²·s⁻¹）两个水平
* 温度设置：脉冲式温度实验设置从环境温度提升至35℃、40℃、43℃；温度-光照交互实验设置27℃、30℃、33℃三个梯度
* 海草响应变量：茎枝密度、生物量、叶片生长速率、形态特征（叶片尺寸、单茎叶片数）、碳水化合物储存储备、光合色素（叶绿素a+b）、光合效率（光系统II(PSII)有效量子效率与Fv/Fm最大光化学效率）以及快速光曲线

### 已发表文献
- Collier CJ, Uthicke S, Waycott M (2011) 不同光照水平下两种海草物种的耐热性：对大堡礁未来分布的启示. 湖沼学与海洋学(Limnol Oceanogr), 56: 2200-2210, DOI: 10.4319/lo.2011.56.6.2200
- Collier CJ, Waycott M, Giraldo-Ospina A (已刊印) 印度洋-西太平洋四种海草物种对遮光的响应. 海洋污染公报(Marine Pollution Bulletin), DOI: 10.1016/j.marpolbul.2011.06.017

### 参考文献
- Collier C, Waycott M. (2009) 大堡礁海草分布与群落变化的驱动因子：文献综述与空白分析. 提交给海洋与热带科学研究基金(Marine and Tropical Sciences Research Facility)的报告. 凯恩斯珊瑚礁与雨林研究中心有限公司, 凯恩斯 (共55页)
- Waycott M, McKenzie L. (2008) 2008年12月进度报告. 提交给海洋与热带科学研究基金(Marine and Tropical Sciences Research Facility)的报告. 凯恩斯珊瑚礁与雨林研究中心有限公司, 凯恩斯 (共9页)