Ocean Newsletter

No.474 May 5, 2020

  • What Is The Impact of Marine Heatwaves on Marine Ecosystems? Ben P. Harvey
    Assistant Professor, Shimoda Marine Research Center, University of Tsukuba
  • The Real Image of Nami no Ihachi, as Seen from an Ocean Perspective ISHIKAWA Takeo
    Director, Kamogawa City Folk Museum, Chiba Prefecture
  • Introducing Ocean Education to Landlocked Areas SATO Hiroaki
    Project Lecturer, Science & Education Center, Ochanomizu University

What Is The Impact of Marine Heatwaves on Marine Ecosystems?

Marine heatwaves are getting more frequent, hotter, and longer

Global warming is continually heating the world’s oceans. This is due to the oceans absorbing around 90% of the heat trapped by excess greenhouse gases since the mid-century. This has culminated in last year (2019) reaching an all-time record-level high, and the past five years being the top five warmest years recorded in the ocean. All indications suggest that this human-caused heating of our planet will continue unabated.

As well as raising the baseline temperature of the oceans, this excess heat can also increase the frequency, severity and duration of marine heatwaves. Marine heatwaves are when oceans reach extreme temperatures for at least five consecutive days. Globally, marine heatwaves have become 34 percent more frequent, 17 percent longer, and the number of heatwave days per year was more than 50% greater between 1987 and 2016 than in 1925-1954.

Some of the largest marine heatwave events have struck at the same time as the El Niño-Southern Oscillation. The El Niño-Southern Oscillation system, with its positive El Niño and negative La Niña phases, is a natural ocean cycle that contributes to some parts of the ocean having their temperature pushed up. It is not certain how future global warming will influence El Niño-Southern Oscillation, but it has been indicated that they may get more severe. What is certain, however, is that marine heatwaves are much worse when they strike at the same time. This is because the starting temperature is higher, and so the absolute temperature reached by the marine heatwave is also higher, and therefore more impactful.

How do we assess the impact of marine heatwaves?

Globally, not all regions have demonstrated the same trends in increasing marine heatwaves. It was therefore necessary to derive global time series and regional trends based on a combination of satellite-based, remotely sensed SSTs and in situ-based seawater temperatures. By highlighting the areas where heatwaves have increased most, we can establish whether those regions will be particularly sensitive. An area might be sensitive because it is: harbouring high levels of biodiversity, possessing large numbers of species that are already near their thermal limit, or already being exposed to addition stressors such as overfishing. This global assessment highlighted that a number of ‘hotspots’ existed, which would be particularly impacted by marine heatwaves. As well as places like southern Australia, the Caribbean Sea and the coastline bounding the mid-eastern Pacific; the waters around the coasts of southern and western Japan also showed a marked increase in the marine heatwave occurrences. All of these areas have been shown to be regions of high biodiversity. Similarly, the north-west Pacific in general is exposed to a range of non-climate human-induced stressors, which means that marine organisms around Japan may be sensitive to marine heatwaves.

In order to assess the impact that marine heatwaves could have into the future, we carried out a study to analyse those well-studied marine heatwaves in the past. These included the effects of eight marine heatwaves on ecosystems from around the world, including four El Niño events (1982-83, 1986-87, 1991-92, 1997-98), three extreme heat events in the Mediterranean Sea (1999, 2003, 2006) and one in Western Australia in 2011. We first quantified the physical attributes of each marine heatwave (including duration and intensity). Then, we examined 1,049 ecological observations across 116 research papers, and identified how different organisms are likely to respond (by comparing the biological performance of organisms before and after the heatwaves). We found that despite the magnitude of past marine heatwaves having varied in terms of their maximum intensity and duration, they all had adverse impacts on a wide range of organisms, from plankton to invertebrates, to fish, mammals and seabirds.

Marine heat waves affect the foundations of ecosystems

The increasing number of marine heatwaves has been shown to cause large-scale damage to the health of foundation species, including coral reefs, kelp forests and seagrass meadows. For example, a particularly extreme marine heatwave struck the west coast of Australia in 2011, and caused widespread loss of kelp and seagrass meadows. An estimated 900 square kilometres of seagrass (36% of the total coverage) was lost, and some of the kelp species became regionally extinct for hundreds of kilometres. Similarly, consecutive marine heatwaves across the Great Barrier Reef in 2016 and 2017 caused more than 80% of the individual coral reefs to experience severe coral bleaching, with the loss of many branching coral species. This is concerning, because foundation species are hugely important since they support thousands of other species which are dependent on the habitat, food and shelter that they provide. When the foundation of an ecosystem is lost, then that ecosystem will cease to exist.

Most of these heatwave-induced losses of kelp in Australia were directly due to the temperature itself being stressful, however, their recovery was then hindered because tropical herbivorous fish (including rabbitfish) followed those warmer conditions and began to heavily graze on any remaining kelp. Five years after the heatwave, these kelp forests still have not shown any signs of recovery. Instead, those temperate kelp species are disappearing and starting to be replaced by subtropical and tropical reef communities. This phenomenon is termed tropicalisation. A similar tropicalisation is occurring in Japan due to gradually warming waters. Within both west Kyushu and south Shikoku, tropical fish (rabbitfish, parrotfish and sea chubs) have become permanent residents and now dominate the temperate reef fish communities, causing an overgrazing of the local kelp species (termed isoyake) resulting in their loss.

These types of shifts in the community, where the dominant species is being replaced (e.g. loss of kelp forests), are becoming increasingly more common as the oceans warm with climate change. The main concern is that such changes were expected to occur over the long-term, whereas instead marine heatwaves can rapidly drive these changes in the ecosystem and also hinder any subsequent recovery following the event. Consequently, the changes in ecosystems that we project with climate change, may occur sooner than expected.

Increasing frequency of ocean heat waves will affect human society in the future

The loss of foundation species due to marine heatwaves is also harmful for humanity. This is because as well as supporting other marine life, foundation species and their associated biodiversity also supply humans with a number of ecological goods and services. For example, many marine ecosystems support commercial and recreational fisheries, contribute to carbon storage and nutrient cycling, offer venues for tourism and recreation, or are culturally or scientifically significant. All of these services have substantial benefits to society, and their reduction (or loss) will have deleterious effects for the hundreds of millions of people that rely on coastal marine ecosystems.

The impacts of marine heatwaves and climate change can only be reduced by rapid and meaningful reductions in the emissions of greenhouse gases. It then remains a major challenge for marine conservation and management approaches to establish a pragmatic approach for maintaining the integrity of marine ecosystems to both long-term climate change and marine heatwaves.

Increase in the number of annual heat wave days between 1987 and 2016 compared to 1925 and 1954. The figure shows data for every 1 ° square. (Adapted from a figure in Smale et al. 2019). The colour bar indicates the number of days of increase in the occurrence of marine heatwaves from 1987 to 2016, with darker red indicating a greater number of days of increase.

Example of the type of diverse kelp ecosystem, supporting other marine organisms, which could be threatened by marine heatwaves. Photo by Daisuke Shibata

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