What is a marine heatwave?

Discover what a marine heatwave is and why they are a problem to marine life

4 min read

Drone footage of Plymouth Sound.

What is a marine heatwave?

Marine heatwaves (MHWs) can be defined as periods of abnormally high sea temperatures above the seasonal average which can last days to months [1]. 
The frequency of marine heatwaves has approximately doubled since the 1980s and there is considerable evidence that this increase is being driven by human activities [2]. 
The negative impacts of marine heatwaves have received widespread attention in tropical regions where they can result in mass mortality of coral reefs. However, we are also seeing increased marine heatwaves around the United Kingdom. Recently in June 2023, the UK saw abnormally high sea temperatures 4-5 °C above the norm, with sea temperatures as high as 18°C documented in Plymouth Sound compared to the typical June average of 13-14 °C [3]. 
The predicted increases in the frequency, duration and severity of marine heatwaves over the next century may have the potential to disrupt marine life from individual animals to whole ecosystems, as well as the important services that they provide such as fisheries and aquaculture [1].
Categories of marine heatwaves (Hobday et al. 2018). The solid, black line represents typical, local temperatures and the purple, dashed line represents the high temperatures occurring over time during a heatwave. Heatwaves can differ in intensity where increases in temperature can range from moderate to extreme. Creative Commons

Categories of marine heatwaves (Hobday et al. 2018). The solid, black line represents typical, local temperatures and the purple, dashed line represents the high temperatures occurring over time during a heatwave. Heatwaves can differ in intensity where increases in temperature can range from moderate to extreme. Creative Commons

Why are heatwaves a problem to marine life?

The majority of marine invertebrates and fish are what we call ectothermic where their body temperatures rely upon the temperature of the surrounding environment. These animals have a certain window of temperatures in which they can survive. 
However, animals need to do more than just simply exist – they also have to perform key activities such as movement, feeding, growth and reproduction. These activities are all essential for sustaining both healthy populations and diverse marine ecosystems. Therefore, within the temperature window for survival, animals may actually have a much narrower range of temperatures that are optimal for their performance. 
The high temperatures occurring during heatwave events can cause stress [4] and may push animals past their optimum temperature range resulting in reduced fitness and mortality in more extreme cases. 
There may be some ability of organisms to partially increase their tolerance of temperature if given sufficient time to acclimatise. For example, we know that in many species thermal tolerance can increase over the months from winter to summer. However, much less work has been conducted on whether the ability of animals to adjust to temperature can keep up with the rapid, extreme temperatures typical of a heatwave. 
Species that are immobile, such as many benthic invertebrates, will not have the ability to escape high temperatures and will essentially have to try to 'weather the storm'. For highly mobile species, e.g. some fish, we may see their loss from warmer areas, as they outwardly migrate seeking cooler temperatures [5]. 
This redistribution of species in our seas may also bring the increased risk of invasion of non-native species from warmer climates [1]. Heatwaves can also cause a suite of other problems such as triggering the growth of harmful algal blooms, affecting predator-prey interactions and also reducing the oxygen available in the water for marine animals to breathe [1].
The effect of environmental temperature and heatwaves on animal performance.

The effect of environmental temperature and heatwaves on animal performance.

Dealing with rising temperature in an ecologically-important coastal invertebrate

A case study from the Plym Estuary
Water temperatures in the upper regions of the Plym Estuary, Plymouth, can be highly variable and currently reach a maximum temperature of approximately 20-21°C in late summer [6]. 
The estuary is teeming with amphipods, a type of shrimp-like crustacean, which are important in the food chain by shredding decaying plant material and acting as prey for bigger invertebrates and fish.
Here, we looked at the challenge that increased temperatures pose to a local amphipod species called Gammarus chevreuxi. We looked at how a range of temperatures from 15 - 30 °C affects swimming speed of both adult males and females. The ability to swim is highly important for movement and foraging, so any decline in swimming speed with temperature could have negative effects on the fitness of organisms.
Males were able to maintain swimming speed between 15 and 20 °C but speed then declined above this temperature. Females were less sensitive and able to maintain swimming speed up to a temperature of at least 25 °C [7].
Both males and females can perform well under their current habitat temperatures up to 20-21 °C. However, any future increases in maximum summer temperatures may disproportionately affect the performance of males in the population and reduce their fitness.
 
The absolute treat of a field site that is the Plym estuary by Marsh Mills.
Results from the experiment
The amphipod Gammarus chevreuxi inhabits the Plym Estuary near Marsh Mills. Males and females display different abilities to maintain swimming performance under increased temperatures, with the males being most sensitive.
 
 
 
 
 
The Marine Biology and Ecology Research Centre contributes substantially to the understanding of how marine biodiversity is responding to environmental change and has informed management decisions at regional, national and international levels. 
Our three research groups aims to address the:
  • influences of environmental change, fisheries and pollution on marine biodiversity
  • use of large-scale temporal and spatial data for the understanding and management of marine biodiversity
  • development and application of new approaches for biological assessment.
Getty images 589120498 fish

References
1. Smith et al. (2023) Biological Impacts of Marine Heatwaves, Annu. Rev. Mar. Sci., 15, 12.1–12.27
4. Collins et al. (2023) The environmental cellular stress response: the intertidal as a multistressor model, Cell. Stress and Chap., 1-9
5. Sunday et al (2012) Thermal tolerance and the global redistribution of animals, Nat. Clim. Change., 2, 686-690
6. Collins et al. (2022) Consequences of thermal plasticity for hypoxic performance in coastal amphipods, Mar. Env. Res., 177, 105624
7. Clarke, M. and Collins (2023) unpub. data.