edaphic scientific blog
Many tropical marine species are starting to appear in temperate waters. Around Sydney, several tropical fishes, such as surgeonfish, have been observed. Corals have also been found to be overtaking algal forests off the coast of New South Wales. In New Zealand, tropical fishes have been observed in marine waters where they have never been seen before. Given these developments, surely it won’t be long before the Great Barrier Reef migrates south to tropical Tasmania.
Temperature dictates where marine species live
The distribution of many marine species is restricted by sea surface temperature (SST). Species survive, reproduce and grow over an optimal range of temperatures. The species habitat, then, is locked into, or bound, by that range of temperature. So marine species do not necessarily know they are moving towards the equator or towards the poles, they simply move where the optimal SST takes them. This is known as “thermally bound habitats” or the region between two isotherms (a maximum and a minimum) that defines a species thermal range.
The thermal range of lobsters in Maine, USA, is between 0C and 25C (Cowan 2004). The larvae of the southern rock lobster, which lives off the coast of Tasmania, cannot survive beyond 21.5C (Pecl et al 2009). This maximum temperature for this species thermal range means its distribution does not extend far beyond southern Victoria. At the tropical end, clownfish, of Finding Nemo fame, has a narrow thermal range between 25.9C and 29.2C.
How fast are tropical waters moving?
Scientists know that SST has shifted towards the poles off the east coast of Australia. It is estimated that since 1944 this shift has been approximately 350km southwards (Ridgway 2007; Hill et al. 2008).
Australian scientists recently calculated how fast marine isotherms, or regions of maximum and minimum temperatures, will move in the future. An average across all ocean regions in the southern hemisphere found isotherms will move at approximately 57 km per decade whereas in the northern hemisphere they will move at approximately 111 km per decade (Sen Gupta et al 2015). These values are not definite and vary across regions, however it does give an approximate indication how rapidly ocean temperatures are expected to change over the coming decades.
So how long will it take for the Great Barrier Reef to move to Tasmania? The southernmost tip of the Great Barrier Reef is around Fraser Island in Queensland. The distance between Fraser Island and Cape Portland, the north-eastern tip of Tasmania, is about 1800 km. Therefore, based on these crude calculations, it will take approximately 31 decades, or 310 years, for the Great Barrier Reef to move to Tasmania.
This is not to say that this will certainly occur. This calculation is determined by simplifying complex models and assuming numerous variables will hold, remain constant, or not have additional ramifications. Most importantly, it assumes sea surface temperature is the only important variable in driving the distribution of the Great Barrier Reef. Temperature is important but other variables in the environment can wreak havoc on the distribution of marine species.
So is the Great Barrier Reef on the move south?
Yes. As mentioned above, many observations have been made of tropical species of fish being found in previously temperate waters of central and southern New South Wales. But the corals which make the Great Barrier Reef a famous icon are also on the move. Four species of tropical reef-building coral have been found near Solitary Island, northern New South Wales, off the coast of Coffs Harbour. Such species are pioneer species, or early colonists, of new habitat.
An example of a reef-building coral species (Acropora monticulosa) off the coast of Coffs Harbour, moving the Great Barrier Reef southwards. Image source: https://theconversation.com/on-the-move-corals-migrate-south-into-nsws-warming-waters-8238
CO2 maybe more critical to survival than temperature
The Great Barrier Reef is on the march south but it may still never make it to Tasmania. Even if the temperature conditions remain optimal for the growth and survival of the reef in Tasmania, there are a number of other environmental parameters that will cause problems. One significant factor is carbon dioxide (CO2).
Increasing atmospheric concentrations of CO2 is one of the primary drivers behind warming temperatures and an increase in SST. Not only is CO2 released into the atmosphere, but it is also absorbed by the oceans. Since the industrial revolution, about 30% of the CO2 released by human activities has been absorbed by the oceans (Sabine et al 2004). Dissolved CO2 (CO2 in water) forms carbonic acid which, in turn, makes seawater more acidic. The process is known as ocean acidification and it will be a significant problem for the Great Barrier Reef.
Ocean acidification can cause a number of issues for marine life. In terms of the Great Barrier Reef, one major issue with increasing acidity is a net decrease in the amount of carbonate ions available for corals. Today, the ocean is saturated with calcium carbonate. But as the ocean absorbs more CO2 and pH decreases (becomes more acidic), the level of calcium carbonate saturation decreases. The decrease in carbonate ions increases the difficulty for corals to form viable skeletons and the skeletons that do form are vulnerable to dissolution (Orr et al 2005).
Increasing atmospheric CO2 and ocean acidification is already impacting the Great Barrier Reef. Since 1990, coral growth rates on the Reef have decreased by 14%. When atmospheric CO2 levels reach 450ppm (in 2015 levels are 400ppm), calcification in reef building corals will be reduced by 50%. If CO2 levels reach 800ppm, then all calcification will cease (Veron et al 2009).
Therefore, even if ocean temperatures reach a level suitable for the Great Barrier Reef to survive in Tasmania, other processes, such as ocean acidification, may prevent corals from growing anywhere at all.
- Hill KL, et al. 2008. Wind forced low frequency variability of the East Australia Current. Geophysical Research Letters, 35 L08602, doi:10.1029/2007GL032912
- Orr, James C. et al. (2005). “Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms” Nature 437 (7059): 681–686
- Pecl G, et al. 2009. The east coast Tasmanian rock lobster fishery – vulnerability to climate change impacts and adaptation response options. Report to the Department of Climate Change, Australia. http://eprints.utas.edu.au/9641/1/rock-lobser-full.pdf
- Ridgway KR. 2007. Long-term trend and decadal variability of the southward penetration of the East Australian Current. Geophysical Research Letters, 34, L13613, doi:10.1029/2007GL030393.
- Sen Gupta, A. et al. 2015. Episodic and non-uniform shifts of thermal habitats in a warming ocean. Deep Sea Research II, 113, 59-72. http://dx.doi.org/10.1016/j.dsr2.2013.12.002
- Veron JEN, et al. 2009. The coral reef crisis: The critical importance of <350 ppm CO2. Marine Pollution Bulletin, 58, 1428-1436.