Climate change impacts on marine plants

While evidence suggests macroalgae may thrive in the changed conditions, seagrass may not fare so well. This brings good and bad news for the Great Barrier Reef ecosystem.

There are over 600 species of macroalgae (visible algae such as seaweeds) and over 15 species of seagrass on the Great Barrier Reef.

Macroalgae

Macroalgae are a key source of carbon in the Reef ecosystem and they are involved in other important processes including construction of reef frameworks, coral settlement and creation of habitats.

They are a direct food source for herbivorous fish, crabs and sea urchins. The carbon they produce in photosynthesis enters the food chain via the microbes.

However, they have also been identified as both a cause and consequence of coral reef degradation. They have a negative impact on coral health and thrive when corals are unable to compete with them for space.

Sea temperature

Rising sea temperatures will increase the production of some species of macroalgae. Changes in temperatures could also lead to changes in these species' life cycles and although there is limited available evidence on this topic, the consequences of these changes could impact food webs.

Ocean acidification
Assuming nutrient concentrations in the Reef ecosystem increase due to coastal run-off, the increase in available carbon in the ocean will mean macroalgae can photosynthesise and grow at a faster rate. Larger populations of macroalgae and improved conditions for their growth will have detrimental impacts on coral reefs.

Sea level rise
Sea level rise may create more available habitat space for macroalgae to grow as more land area will be inundated with water. While the increase could impact some species that live in shallow habitats by reducing their exposure to sunlight (the more water will mean more distance for sunlight to travel to reach the macroalgae), as a group macroalgae is not vulnerable to negative impacts of sea level rise.

Storms and rainfall
The predicted increase in the frequency of severe weather events such as cyclones, storms and floods will bring an influx of nutrients into the Reef ecosystem, which will increase macroalgae growth and reproduction. Cyclones and storms can also destroy coral reef structures, increasing habitat areas for macroalgae to grow (as was reported following previous cyclones).

Seagrass

An important food source for dugong and marine turtles, seagrass is highly vulnerable to the impacts of climate change, particularly the species that grow in shallow and inshore habitats.

Sea temperature
Seagrass photosynthesis rates are determined in part by water temperature. Increases in temperature can decrease the efficiency of photosynthesis; however the extent of this impact may be dependent on the species' reliance on light. For example, a species of seagrass that requires less light to grow will be less vulnerable to increased water temperature. The reverse is also true.

Temperature also plays a role in seagrass flowering (and thus reproductive) patterns. Available information for this topic is limited; though it is expected temperature changes will have significant effects on the reproduction of most seagrass species.

Sea level rise
As the sea level rises, the depth of seagrass habitat will increase, reducing the amount of light reaching the plants. This impact could be compounded by the water quality of this habitat, as murky water also impedes the exposure of seagrass to sunlight.

The inundation of water onto coastal habitats will erode the land and destroy the substrate seagrass grows from. Scientists believe it is unlikely species will migrate away from the coast.

Storms and rainfall

Storms, including cyclones, and huge rainfall that lead to flooding will have impacts on both the habitat of seagrass and their physiological processes. Winds from storms and cyclones can uproot seagrass plantations, cause erosion and bring loads of sediments from inland areas. Seagrass recovery from physical disturbances can take some time and this is being investigated following recent cyclones.