Heat waves pose problems for human health and infrastructure, ravage crops and livestock, and have wide-ranging impacts on the natural world. What is less obvious, however, is that heat waves in the sea can impact marine organisms and alter the structure of entire ecosystems.

In early 2011, the coastal waters off Western Australia warmed to unprecedented levels, with temperatures soaring to up to 5°C above the long-term summer average (Figure 1). Sea surface temperature records dating back over 140 years indicated that the warming was the highest-magnitude on record. Temperature anomalies of 2–5°C persisted for more than 2 months along over 2000 km of coastline; the heat wave was both spatially and temporally extensive (Figure 1). Oceanographers have shown that the heat wave was caused by unusually strong La Niña conditions, which enhanced the poleward flow of warm, tropical water into cooler temperate regions.

The impacted region off south-west Australia is a known global biodiversity hotspot, with very high levels of diversity and endemism for seaweeds, reefassociated fish and some invertebrate taxa (Figure 2).

The extensive coastline is characterized by a large-scale ocean temperature gradient, which corresponds to a shift from hard coral to kelp-dominated ecosystems on submerged reefs. At some locations within the temperate–tropical transition zone, kelp beds and coral reefs co-exist to form mosaics of highly diverse benthic habitat (Figure 2). A team of researchers led by Dr Thomas Wernberg (University of Western Australia) and Dr Dan Smale (Marine Biological Association) have investigated the ecological effects of the extreme warming event. Using SCUBA divers and remotely operated technology (e.g. autonomous underwater vehicles) the team resurveyed historical sites to examine how key populations and assemblages of seafloor organisms were impacted by the warming. During the event, abnormally high levels of coral bleaching were observed, as certain coral taxa became physiologically stressed by the extreme temperatures (Figure 3). Subsequent surveys by other research groups have indicated that coral mortality rates were very high along much of the Western Australia coastline.

Fig. 1. The 2011 heat wave in the southeast Indian Ocean. a, Blended sea surface temperature anomaly map for March 2011 (relative to a 1971–2000 baseline), indicating a warming anomaly of >2.5°C along the southwest coast of Australia. The key study locations are also shown (Jurien Bay, ‘JB’, and Hamelin Bay, ‘HB’). Plots show weekly temperature anomalies during 2011 (relative to means of the preceding five years) generated from in situ measurements at 10m depth at the study locations, Jurien Bay (b) and Hamelin Bay (c).

Fig. 2. Benthic marine biodiversity along the West Australian coastline. a; Kelp beds and hard corals co-exist in the temperate-tropical transition zone, forming highly diverse seafloor habitats. b; submerged rocky reefs off the southwest coast of Australia support dense kelp forests, which provide food and habitat for a wealth of marine plants and animals. Image: Dan Smale.

Crucially, cool-water seaweeds were severely impacted by the warming, which resulted in widespread mortality, local extirpations and geographical range contractions of several key habitat-forming species (Figure 3). In the most impacted locations, the spatial coverage of canopyforming seaweeds plummeted by more than 60%. Loss of seaweed canopies has led to large patches of bare, open reef habitat which has been partially colonized by turfforming ‘weedy’ algae. The loss of dense seaweed canopies, which provide complex structural habitat and food for a myriad of other plants and animals, will likely have far-reaching ramifications for the structure and functioning of the seafloor ecosystem.

Dr Smale said; “We’ve been overwhelmed by the magnitude and extent of the ecological impacts. Species have retracted their geographical ranges by more than 100 km in a matter of months, whole kelp beds have been restructured, and sensitive coral reef habitats were hard hit too”. Understanding whether kelp beds will eventually recover towards a pre-impacted state remains a research priority. “Kelp ecosystems in other regions, such as California, have undergone rapid deforestation in recent decades due to climate variability, but these systems often recover. However, the oceanography of the region, combined with the life histories of the most impacted seaweeds, suggest that this might not happen in Western Australia. Instead, we could be seeing a step-wise shift towards a different ecosystem state, but only time will tell”, Dr Smale remarked.

Fig. 3. Marine organisms impacted by the warming event. a; High levels of bleaching of hard corals, such as Montipora spp., were recorded during the marine heat wave. The warming event caused local extirpations and range contractions of key habitat-forming seaweeds, including the kelp Ecklonia radiata (b) and the fucoid Scytothalia dorycarpa. High mortality rates led to a decreased seaweed canopy cover and large patches of bare reef (c). The abundances of some warm-water fish species, such as Labracinus lineatus (lined dottyback, d), increased immediately after the warming event and caused a ‘tropicalization’ of fish assemblages. Images: Dan Smale (a), Thomas Wernberg (b, c) and Timothy Langlois (d).

As well as changes in seaweed populations, the team also observed a proliferation of several warm-water fish species after the warming event (Figure 3), but the mechanisms underpinning this response remain unclear. What is clear, however, is that extreme events can impact a variety of marine life, from seaweeds to fish to birds. There is mounting evidence to suggest that the frequency and magnitude of discrete warming events may increase as a direct consequence of anthropogenic climate change. In fact, the number of days characterized by extremely high sea temperatures has increased along 38% of the world’s coastlines in the last 30 years. These events will subject organisms to acute thermal stress, which may interact with chronic stressors such as gradual warming, eutrophication and pollution. In consequence, organisms, populations and communities may find themselves pushed ‘over the edge’ as they struggle to deal with rapid environmental change. Thomas Wernberg at the University of Western Australia concluded: “It is well known that marine life is responding to longterm gradual climate change, in Australia and elsewhere, through shifting distributions or changing behaviours. But this marine heat wave has demonstrated how quickly species and entire ecosystems can be impacted by increased temperature. We’ve been studying this system intensively for over 10 years, and we thought we were beginning to understand its ecology. But we now need to divide our data and understanding into two parts; before the heat wave and after the heat wave”.

Dan Smale is a research Fellow at the Marine Biological Association. dansma@mba.ac.uk


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Dan Smale