I wanted to add a little to Ove’s continued defence against ‘The Australian’ on going war against science. Whilst most people see the Great Barrier Reef as being one large coral reef, it also contains an array of other habitats including seagrass meadows that are critical to the overall ecosystem. Seagrasses, amongst there many roles in the GBR, are critical in supporting biodiversity and fisheries productivity. These seagrass meadows, like coral reefs, are also under threat from increasing seawater temperatures.

The potential 4°C increase in global temperature by the end of the century, that the leader of the opposition recently described as “not a big moral challenge”, would have an enormous detrimental impact upon seagrass meadows, particularly the abundant intertidal meadows present throughout the GBR. Research published back in 2006 found how seagrasses of the GBR suffer irreparable effects from short-term or episodic changes in seawater temperatures as high as 40–45 °C. Although these temperatures sound high, intertidal pools can commonly approach and exceed these temperatures for short periods throughout the GBR, and seagrasses are observed to ‘burn’. If temperatures were to increase by 4°C, such ranges would be exceeded too regularly to allow for recovery, and seagrass meadows are likely to deteriorate with huge detrimental impacts upon fisheries and coastal productivity.

The Great Barrier Reef described to be “blue again” by ‘The Australian’ is under continued stress. Seagrasses although important in their own right make excellent ‘coastal canaries’ and their tissues are good time integrated indicators of the coastal nutrient environment. Monitoring throughout the GBR continues to find coastal seagrasses containing highly elevated C:N:P ratios, indicating rich and potentially eutrophic environments that are continuing to be enriched. Increasing nutrients onto the reef and into seagrass will continue to promote algae and reduce the resilience of coral and seagrass to future climate change and increasing temperatures.  The combination of elevated nutrients and increased temperatures are of concern as greater temperatures increase metabolic rate, resulting in increased light requirements for seagrass. Such light requirements are not possible when increasing nutrients reduce light availability due to increased epiphytes and phytoplankton, resulting in eventual loss of the seagrass.

As Ove said previously, there exists no evidence to suggest that the GBR is “blue again”, and to the contrary, seagrass biomonitoring suggests nutrient conditions are continuing to deteriorate, with many coastal locations becoming increasingly eutrophic (see Figure 1 taken from the latest Seagrass-Watch magazine). The available evidence suggests that seagrasses and the coastal environment of the GBR are under increasing nutrient stress, reducing future resilience to climate change.

 

2 Responses to Reducing resilience of the Great Barrier Reef to increased temperature stress

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  2. Catherine Collier says:

    Richard, interesting post, it’s nice to see seagrasses get a mention here as they are all too often overlooked! As you point out there are various other habitats on the Great Barrier Reef that are vulnerable to the impacts of climate change.

    Seagrasses are indeed a highly important feature of coastal ecosystems, including the Great Barrier Reef. For example, seagrasses rate amongst the most valuable of the world’s ecosystems in terms of their contribution to the global economy (Costanza et al 1987) due largely to their role in nutrient cycling in coastal environments. As well as supporting biodiversity and fisheries productivity, tropical seagrasses are the sole food source for dugongs and a vital food source for marine turtles.

    As you rightly point out, one of the impacts of climate change on seagrasses will be increased frequency and severity of exposure to short periods of very high temperatures. To expand slightly: In shallow coastal habitats, water is generally more turbid than it is further away from the shore. Seagrass meadows are restricted to fairly shallow-water habitats where they receive enough light for photosynthesis (up to 70m (Short et al 2007 JEMBE 350:3-20). High turbidity in coastal areas forces seagrasses into even shallower water (Dennison et al 1993 BioScience 43: 86-94). It is these shallow water habitats that are the most susceptible to high water temperature. At low tide, in particular, they may be exposed to the air, or remain sitting in very shallow pools of water that reach 40-45°C.

    Improving water quality is critically important for the future health of seagrasses on the Great Barrier Reef. As you describe, the combined effect of higher nutrients and climate change can increase the light requirements of seagrasses, potentially forcing them into even shallower waters. Conversely, any improvements to water quality that increase light penetration will extend the habitable depth range of seagrasses, allowing them to move into deeper waters. Seagrasses growing in deeper water will be less susceptible to damaging shallow-water temperatures expected under future climate change.

    Dr Catherine Collier
    School of Marine and Tropical Biology
    James Cook University

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