Evidence suggests that rising ocean temperatures are limiting the ability of coral larvae to migrate between reefs, with the associated biodiversity losses reducing coral resilience to disease, overfishing, and the impacts of climate
change.
Climate change and global warming have been at the forefront of environmental news in recent years, and rightfully so—the facts keep stacking up, and the reality is that impacts are only going to worsen.
The effects of rising sea surface temperatures on coral reefs—an integral part of the marine ecosystem—are becoming increasingly severe. Tropical waters are estimated to rise by 4°C by the end of the century, which is predicted to cause
further extensive damage to reef communities. Aside from the broadly studied effect that is coral bleaching, scientists are discovering a multitude of other impacts caused by rising temperatures, which are further degrading and damaging
reefs in complex and worrying ways.
Fisherman in Fiji: Local communities across the tropics depend on healthy populations of reef fish to earn a living and provide sufficient protein in their diets. | Tom Vierus / Ocean Image Bank
Coral reef connectivity is vital in maintaining stable and healthy reef ecosystems and occurs through the movement of coral larvae. Larvae are released into the water column and passively drift in currents away from their natal area,
allowing different species to spread across their home reef. This dispersal reduces competition with parent corals and also helps establish new colonies in neighbouring reefs, increasing genetic variety and biodiversity.
These processes describe intra-reef connectivity and inter-reef connectivity respectively and improve the resilience and overall functioning of reef communities, benefitting all organisms within the reef.
‘Dispersal [larval] reduces competition with parent corals and helps establish new colonies in neighbouring reefs, increasing genetic variety and biodiversity.’
As sessile organisms in their mature life stage, corals are heavily reliant on the mobility of larvae to allow new habitats to be populated and to recolonize damaged or dead reefs, a key method of reef regeneration.
Researchers have found that increasing sea surface temperatures cause a significant decrease in the distance travelled by coral larvae; reducing settling time of larvae and increasing mortality rates. This ultimately means a higher
proportion of coral larvae are dying before they settle, and those which do survive are settling on, or very close to, their natal reef.
Consequently, the exchange of larvae between separate reefs is decreasing, leading to a loss of biodiversity across reefs, which can upset entire ecosystem processes. Decreased genetic diversity means reefs are becoming more homogenous,
reducing biological niche availability for other species, and increasing the disease susceptibility of the coral population.
Hawaiian coral colonies. | Kimberley Jeffries / Ocean Image Bank
Research led by a team from Macquarie University in Sydney, have found that highly connected reefs (those receiving a high influx of foreign coral larvae) contain double the biomass than their less connected counterparts. They are also home
to a greater variety of fish species and are more resilient to the pressures of climate change and other anthropogenic disturbances.
The team combined data on coral larvae sources, sinks, and movement corridors, with fish species diversity and ocean currents to model the dispersal of coral larvae around the world. Their work provides vital information for marine
conservationists and fisheries scientists, facilitating the strategic placing of marine protected areas (MPAs) to ensure important movement corridors and sources/sinks of coral larvae are protected.
Another research team, from the University of Plymouth, conducted similar research in the Red Sea, an area where reefs are typically homogenous. Previous theories have attributed this to the unique environmental conditions found there (high
saline), however, its semi-enclosed nature and restricted connectivity to the rest of the ocean may also play a part.
The researchers used a combination of genetic population data, dispersion model simulations, and satellite observations to map the connectivity of Red Sea reefs. As expected, the results showed ocean circulation features, such as surface
currents and eddies, form physical pathways which facilitate genetic flow between marine communities, mainly through the passive movement of larvae.
The highest rates of genetic homogeneity in its fish and coral species were found in the southern Red Sea, due to limited ocean circulation and minimal connection to the rest of the Indian Ocean.
Sunset on the Reef—a homogenous bed of branching coral in the Red Sea. | Alex Mustard / Ocean Image Bank
This novel use of satellite observation provides a cost-effective way of monitoring and mapping the biophysical connection corridors that link reef systems, providing essential data in the strategic placement of new MPAs and coastal
management plans.
Given that an estimated 70% of unprotected reefs are vital larvae sinks, sources, or part of movement corridors, protection must be strengthened to ensure a sustained genetic flow between reef communities.
Increasing the connectivity of reefs is an important conservation measure, as diverse reef communities are inhabited by a greater range of fish and coral species, which strengthens the resilience of a given reef to disease outbreaks, storm
damage, and elevated temperatures.
The value of reefs, not only to marine biodiversity but also to humans, is undeniably worth their conservation and protection. Globally, reefs are responsible for protecting 71,000 kilometres of coastline from erosion associated with
tropical storms, they provide a livelihood through tourism and fishing for over 500 million people, and represent a key source of protein and essential nutrients.
‘[Diverse reefs] strengthen the resilience of a given reef to disease outbreaks, storm damage, and elevated temperatures’
Whilst MPAs and sustainable marine ecosystem management practices can enhance connectivity and are essential in aiding reef recovery, they do not address the root cause of global coral reef decline—global warming.
Research into the predicted effects of climate change on reef connectivity will provide a basis for mitigation efforts, however, this will not prevent the oceans from warming. Ultimately, the human population must commit to mitigating
climate change if reefs are to avoid irreversible damage and continue to deliver critical ecosystem services.
Featured Image: Hannes Klostermann | Ocean Image Bank
Figueiredo J., Thomas C., Deleersnijder E. et al. (2021) Global warming decreases connectivity among coral populations. Nature Climate Change. Volume 12, issue 1, pages 83-87.
McLeod I., McCormick M., Munday P. et al. (2015). Latitudinal variation in larval development of coral reef fishes: implications of a warming ocean. Marine Ecology Progress Series. Volume 521,
pages 129-141.
Raitsos D., Brewin R., Zhan. et al (2017) Sensing coral reef connectivity pathways from space. Scientific Reports. Volume 7, issue 1.
