Climate Change Impacts on Calcifying Marine Algae
Two central challenges facing marine systems today are climate change and urbanisation. These important national problems threaten the productivity and survival of coralline algae along the south-eastern Australian coast. Dr Ross Hill, a Vice-Chancellor’s Postdoctoral Research Fellow at UNSW Australia, is studying these coralline algae, which are ubiquitous on rocky reefs worldwide and play critical ecological roles. They are frequently the dominant bioengineers in coastal environments, with their calcified structure providing protection from grazers and enhancing resistance to wave action. The biogenic production of complex three-dimensional habitats by coralline algae supports a diverse range of fauna and acts as a vital source of food. Coralline algae also contribute significantly to carbonate sediment production, form the scaffolding of natural coastal structures, and produce settlement cues, making them central to the recruitment of many invertebrate species.
Threats to coralline algae can therefore have major impacts on community structure. Climate change will greatly impact the coastal waters of south-eastern Australia. Sea temperatures are warming as a result of the anthropogenic release of carbon dioxide (CO2) and the strengthening of the East Australian Current which is bringing warmer water further south along eastern temperate coast. Rising CO2 is also reducing seawater pH and the availability of carbonate ions (ocean acidification), which are essential for building the CaCO3 skeletons of coralline algae. Ocean acidification, along with increasing storm intensity, will render coralline algae more susceptible to wave action.
Within the coastal marine environment, calcifying algae experience pressure from biotic stressors – natural enemies, such as herbivores and disease. A successful mechanism utilised by calcifying algal species to reduce grazing, maintain rigid structural support and provide strong substrate attachment, is the deposition of a hard skeleton. It is this very defence mechanism which is threatened by ocean acidification, thus potentially increasing herbivore consumption. Temperate macroalgae, including coralline algae, are increasingly at risk from disease through bacterial bleaching, where individuals experience severe tissue degradation. Rising ocean temperatures are known to increase pathogen virulence and/or reduce host resistance, which interact to control disease outbreaks. However, in temperate Australia, coralline algae diseases have not been studied, despite the ecological importance of these habitat-forming algae.
The coastal marine environment in temperate Australia is also under increasing pressure from urbanisation. The release of nutrients and toxic pollutants into waterways on a local scale may lower the resistance of calcifying algae to global climate change. As such, there is an urgent need to characterise the vulnerability of coralline algae to environmental change, determine how climate change will affect the interaction of coralline algae with biotic (herbivory and disease) and abiotic (urbanisation and wave action) stressors, and identify mechanisms to improve coralline algae resilience to current and future environmental changes.