Study reveals decline in foundation species due to summertime Marine Heatwaves

Researchers have found compelling evidence that intense, summertime Marine Heatwaves play a significant role in the decline of vital marine species.
Coral bleach photo by AFP Great Barrier Reef Marine Park Authority
Photo by AFP/Great Barrier Reef Marine Park Authority

Marine heatwaves are periods of abnormally high seawater temperatures that are unusual for a particular season and region.

These extreme weather events can become stressful for marine life, affecting their ability to grow, reproduce and survive, and sometimes causing mass mortality of invertebrates, fish, birds and mammals.

As a consequence of global warming, researchers have found Marine Heatwaves have increased in intensity and duration over the past century and are expected to increase in the coming decades.

This latest study led by the Marine Biological Association (MBA), reveals that many coastal ecosystems are losing foundation species, which could cause greater harm to the biodiversity they support.

Foundation species such as corals, kelps and seagrasses are among the most important organisms in coastal ecosystems, as they alter the physical environment and provide complex living habitats that benefit other organisms.

Photo of kelp forest by Dr Dan Smale
Photo by Dr Dan Smale. Marine Biological Association

The loss of these foundation species can result in widespread ecosystem collapse and declines in biodiversity.

Using time-series datasets, researchers examined changes in seaweed and seagrass abundance, along with observations of coral bleaching and mass mortality events, in relation to Marine Heatwave activity.

The team collected 2,314 observations in total, spanning 85 marine ecoregions spanning the global ocean. They found that Marine Heatwaves have driven major changes in populations globally, with 79 of 85 ecoregions affected.

The occurrence of mass mortality events was greatest in the Western Mediterranean, where a single Marine Heatwave event led to an average of 44% mortality of gorgonian (soft branching) corals. The highest levels of coral bleaching was recorded in South India and Sri Lanka with 60% bleaching followed by the Maldives (51%) and the East African Coral Coast (40%) following individual Marine Heatwave events.

Dramatic declines in kelp forests were recorded in Northern California, with a 40% loss of density.

Photo of seagrass by Dr Dan Smale
Photo by Dr Dan Smale. Marine Biological Association

Lead author Dr Katie Smith, MBA Postdoctoral Research Assistant said:  

“Given their exceptional importance within coastal marine ecosystems, losses of foundation species will likely have far reaching implications for local biodiversity, ecological functioning and the provision of ecosystem services.”

Dr Smith added: “Identifying relationships between Marine Heatwave characteristics and biological responses of foundation species across ecoregions takes us a step further towards predicting the impacts of future extreme warming events. Additionally, gaining an understanding of the recovery of Marine Heatwave affected communities will shed light on the longer-term resilience of ecosystems.”

The study also highlighted the complexity of responses to Marine Heatwaves. Dr Dan Smale, Senior MBA Research Fellow and project lead commented:

“While our study overwhelmingly found that Marine Heatwaves had negative impacts on foundation species, we also found instances where they actually increased their extent or performance, and in other cases we observed no impacts at all. Unravelling this ecological complexity will allow us to improve predictions of the impacts of Marine Heatwaves, which will continue to intensify under anthropogenic climate change.”   

Read the full article in Nature Communications: