Climate change impacts on the ocean habitat

The ocean of the Great Barrier Reef is home to many open water plants and animals such as plankton, jellyfish, fish and whales. The open ocean (pelagic) system is vital for delivering nutrients and larvae throughout the Great Barrier Reef.

Changes in ocean circulation patterns, increasing water temperature and ocean acidification will affect the productivity of phytoplankton (microscopic algae). This will result in changes throughout the open-ocean ecosystem as plankton forms the basis of the pelagic food web.

The distribution of many fish may also change, as well as larval supply to reef habitats.

Changes to ocean currents

Climate change will alter ocean circulation patterns at a range of scales in both space and time. Over time, these changes will have far reaching consequences throughout the Great Barrier Reef. Changes to ocean circulation will affect the supply and distribution of nutrients to open-ocean food webs.

For example, reduced upwelling limits nutrient supply to plankton, which is essential food for bait fish. Aggregations of bait fish are in turn a critical resource for many animals including seabirds, fish, squid, and marine mammals. Altered ocean circulation patterns will also affect the transport of eggs and larvae within and between coral reefs and other Great Barrier Reef habitats.

Rising sea temperature

Pelagic food webs are highly complex and the specific impacts of rising sea temperature are difficult to predict. Variation in water temperature affects numerous aspects of the biology of pelagic organisms. Many of these organisms survive in a narrow range of environmental conditions – although migrations through significant temperature gradients are common.

As a result, rising sea temperatures are expected to alter the movements and geographic ranges of pelagic organisms. For example, temperature limits can affect the distribution and abundance of bait fish aggregations. Some species are likely to expand their geographic ranges southward (or contract their migrations northward) as waters warm.

Sea temperature rise will not be uniform across the Great Barrier Reef. Rather, the number, size and duration of warm pools (or hotspots) are all expected to increase. Increased exposure to these hotspots is likely to affect the growth, development and survival of plankton in patches over coming decades.

Ocean acidification

Many plankton species depend on favourable ocean chemistry to build calcium carbonate (limestone) shells, plates and skeletons. Ocean acidification will make it more difficult for many plankton to build these structures. Experimental results suggest that the growth and reproduction of a range of species could be impacted.

Ocean acidification could also affect the pelagic eggs and larval stages of various fish and other marine organisms. Although difficult to predict, major changes are likely to occur to the composition of the open-ocean plankton. Such changes would have flow-on effects up the pelagic food chain.

Other habitats throughout the Great Barrier Reef could also be affected if acidification alters the survival or distribution of eggs and larvae.