Climate Change in Our Oceans – Part 2

In my last article, I presented the physical changes that are taking place as a result of human-driven climate change. These include warming, acidification, sea level rise and increasing severe storm events. These will all have a clear and direct impact on humans; our weather, where we live, our ability to produce food and access to fresh water.

No doubt there are tough times ahead for humans, but we’re not the only ones. Plants and animals will struggle too. Unlike us, they can’t see what’s coming, and they have no ability to change the outcome. Only humans can do that. Life on land and in the oceans will just have to deal with what planet earth throws at it. So how will our marine life cope?

Most marine life has a degree of mobility. Large animals can typically swim large distances. Small animals, algae and plants may spend most of their lives in one place, but they often have a life stage which is mobile. For example, algae produce spores, and corals produce larvae, which travel on the currents before settling. So, one response for our marine life is to simply move towards the poles, to cooler waters.

Yet the consequences of such a mass migration are both obvious and obscure. Most marine organisms live in a particular depth range, or require a particular substrate or habitat, and these are often already crowded. Just look closely at the intertidal zone – it’s hard to find a patch of rock not occupied by something living. Organisms moving into new areas, or “range extensions”, therefore will have to displace other local species. This can result in changes to community structure with far-reaching consequences1. Urchins replace kelp, tropical fish replace subtropical, species which are not a problem in their home range become invasive.

Organisms may also move deeper, where the water is cooler. This has some potential, but is also limited. Deeper waters are often quite different substrate; soft sediment rather than reef. Algae, which need light and rocky reef, can only go so far. Similarly, organisms which rely indirectly on photosynthesis, like most reef-building corals, have a depth limit. Other factors, such as availability of food, exposure to new predators and competition will also constrain this escape route.

Of course, life on earth has evolved since it first emerged from the primordial slime, but the rate of change is the problem. Atmospheric CO2 levels are believed to be higher and rising faster than any time in the last 20 million years2. Many organisms simply can’t cope with this rate of change and if there is no-where to move to, they will simply die out.

Going going gone? The giant kelp that once graced many shores in Tasmania is disappearing. Warm, nutrient-poor waters from the north, thanks to climate change, together with hungry black urchins arriving on those same currents are thought to be the cause. Scientists estimate that up to 95% of giant kelp has been lost from some areas.

Another major yet indirect impact is the changing availability of habitat. We are still struggling to understand the magnitude of the tragedy of the recent coral bleaching event in northern Australia. Two thirds of the corals in the northern part of the Great Barrier Reef have died3. On some reefs in the north, nearly all the corals have died. Not only does this represent loss of millions of corals, but also will impact on the many and diverse fish and invertebrate species which rely on them. Coral provides important habitat and food for a large proportion of life on the reef – and that habitat base is now gone.

Similarly, our Great Southern Reef is changing. Spanning 71,000 square kilometres from NSW to southern WA, it comprises rocky reefs covered in a range of macroalgae including beautiful kelp forests. Like our coral reefs in the north, these sub-tropical and temperate reefs provide habitat and primary production that sustain our marine life4. As a cold-water species that needs nutrient-rich waters, kelp cannot withstand the increasing warm, nutrient-poor flows coming down on the East Australian and Leeuwin currents. Algal communities are retreating south, to the point where they may have no-where to go. They are also becoming increasingly tropical in their community structure5.

Changing marine communities and range extensions have been occurring for some time, and the Redmap (Range Extension Database Mapping) citizen science project was created in 2009 to track them6. It’s no coincidence that it was created by scientists in Tasmania. As the East Australian Current strengthens and warms, and the Leeuwin current on the west coast brings increasingly warmer waters south as well, our southernmost waters are copping the brunt of the tropical invaders7 8.

We are all aware of the need to reduce our use of fossil fuels, but this will not be enough. We are already seeing changes in our oceans and are committed to even higher temperatures whatever we do today. But we are not without options. Scientists and managers are working hard at three levels:

  • Monitoring – allowing us to understand what is going on, so we can respond more quickly to changes as they occur.
  • Adaptation – if marine organisms are unable to adapt quickly enough, we may be able to intervene to assist them, for example by building resilience through a network of well-designed and managed marine sanctuaries.
  • Mitigation – intervening to reduce CO2 levels, for example by re-establishing macroalgae beds9, seagrass beds and mangrove forests as carbon sinks.

All of these strategies required resources; we are now beyond a passive approach where the ocean can just fix itself. Resources need funding, and that’s where advocacy groups such as AMCS come in. It’s up to us to make it clear to governments at all levels that we have one planet, covered mostly by ocean, and this has to be a priority for the future.

Related Article:

This article is originally published in AMCS Turning the Tide

2 Impacts of Climate Change on Marine Organisms and Ecosystems. Brierley, Andrew S. et al. Current Biology , Volume 19 , Issue 14 , R602 – R614
7 Hobday and Pecl (2014) Identification of global marine hotspots: sentinels for change and vanguards for adaptation action. Reviews in Fish Biology and Fisheries, June 2014, Volume 24, Issue 2, pp 415-425