Seagrass meadows in decline
Project Leader: Assoc Prof Michelle Waycott, JCU
While the condition of seagrasses in the Great Barrier Reef region in general is currently considered to be fair, MTSRF-funded research into trends in condition indicate that the region’s coastal seagrass meadows are declining. Reef-wide surveys show that 38% of sites exhibit shrinking meadow area, a large number of sites have ongoing trends of reduced seagrass abundance, and many sites have limited or no sexual reproduction (ie no seeds are being produced that would enable rapid recovery). There is evidence that light limitation is the primary driver of seagrass abundance at most sites, and that improved water quality (= increased light availability) would improve their status. While the declines observed at some sites will naturally reverse due to climatic fluctuation, others appear to be on a consistent trajectory towards decreased seagrass diversity and abundance. Increasing temperatures, sea-level rise and increasing frequency of disturbance regimes - projected to occur due to climate change - will add to the stressors on coastal seagrass meadows in the Great Barrier Reef region. While no single indicator of seagrass meadow condition appears sufficient to interpret causes of change, ongoing measurement of the following parameters is recommended:
- Long-term increases in seagrass tissue nutrient content (as integration of environment nutrient loads)
- Elemental ratios of tissue nutrients (indicative of the relationship between nutrient and light availability at that site)
- Plant morphology (alters as a result of light stress)
- Chlorophyll concentration (declines under heat and light stress)
- Variation in rate of production of sexual reproductive structures (decreases under stress).
The Reef Rescue Marine Monitoring Program (incorporating Seagrass-Watch) presently supports the ongoing collection of information on changing meadow size, seagrass abundance, morphology, nutrient and reproductive status, providing direct evidence of coastal meadow status and resilience for more than 20 seagrass meadows in the Great Barrier Reef.
Risk analysis and reporting for Great Barrier Reef seagrass meadows
Project Leader: Assoc Prof Michelle Waycott, JCU, & Len McKenzie, DEEDI
While most seagrass habitats in the remote Cape York region of the Great Barrier Reef are classified as low risk, almost two-thirds of coastal seagrass habitats along Queensland’s urbanised coastline are at high or medium risk from multiple anthropogenic threats. Twelve ‘hot spots’ were identified as priority for conservation actions. Reducing the risk to coastal seagrass habitats in these ‘hot spots’ will require: (1) improving the quality of terrestrial water that enters the Great Barrier Reef lagoon; (2) mitigating the impacts of urban and port infrastructure development and dredging; and (3) addressing the hazards of shipping accidents and recreational boat damage. Risk analysis outputs include the development of a spatially-explicit predictive model of coastal seagrass distribution, estimation of risk to deepwater seagrass habitats from the Queensland East Coast Otter Trawl Fishery, and the estimation of the risk of coastal seagrass habitats from multiple anthropogenic threats. Community and agency monitoring of intertidal seagrass meadows (Seagrass-Watch) along the Queensland coast has established a dataset indicating that intertidal seagrasses have declined in abundance at more sites than expected during the last 10 years. Seagrass-Watch newsletters (magazines) containing updates on monitoring results were produced throughout the year and have been distributed to participants (also available for general public download via the Seagrass-Watch website).
New information about the likely impacts of climate change on seagrasses
Project Leaders: Assoc Prof Michelle Waycott, JCU, & Len McKenzie, DEEDI
Experiments demonstrated that seagrasses are strongly negatively affected by low light levels (which occur under conditions of poor water quality) and short-term spikes in water temperature to 43°C (temperatures that have already been recorded occurring in the field). In addition, experiments investigating the interaction between light intensity and water temperature indicated that seagrasses experiencing low-light conditions - as occurs with poor water quality - will be more negatively impacted by high temperatures than those in high-light conditions. This is good news, because it indicates that improvements in water quality could reduce and delay some of the impacts of climate change on coastal seagrasses. Different seagrass species are likely to respond differently to climate change, leading to shifts in species composition of seagrass meadows. Interestingly, experiments showed that the northern Australian population of Halodule uninervis appears to exist well within its optimal temperature range, meaning that this species could continue to thrive in the Great Barrier Reef region and potentially even expand its southward range as mean sea temperatures increase. In contrast, Zostera muelleri currently exists near its upper thermal threshold, and a temperature hike of 3°C by 2100 is likely to result in the contraction of this species from the tropical Australian coastline. This could have ecologically significant ramifications, as Z. muelleri is often the only seagrass species able to inhabit muddy habitats.
Joint report by AIMS and QDPIF researchers for Project 1.1.3.
Report prepared in December 2008 outlining the results of a spatial risk assessment for coastal seagrass habitats of the Great Barrier Reef from various anthropogenic factors. This study includes habitats in both the dry and wet tropical areas of the World Heritage Area.
This report by researchers from the Northern Fisheries Centre in Cairns details the results of a workshop involving regional seagrass experts and end users of seagrass monitoring information. The report identifies key coastal seagrass risk areas in the Great Barrier Reef World Heritage Area and examines how well the current research program addresses information requirements in these areas. Also included are recommendations for developing the seagrass status and trends program in the future.
Report by Queensland Department of Primary Industries and Fisheries that provides a summary of the state of knowledge of Queensland seagrasses and which outlines the gaps in knowledge that still remain and issues confronting management agencies responsible for protection and development of coastal waters.