By Catriona New, Freelance Writer

Published October 7th, 2023

There is increasing pressure on the search for effective mitigation strategies that may help reduce the impact that climate-induced stressors, such as marine heatwaves, have on marine ecosystems. A recent study shines a light on how Marine Protected Areas may provide a safety net for marine life in the wake of the climate crisis.

Marine heatwaves (MHWs) are abrupt disturbances in which abnormally high seawater temperatures persist for extended periods. These heatwaves are extreme weather and are increasing in frequency, length and intensity.

Every organism has a thermal niche, a temperature range determined by evolutionary conditions, within which it performs at an optimum rate. As a result of increased anthropogenic stressors, the thermal threshold that organisms can tolerate is being surpassed more regularly than ever before, desecrating ecosystems, and creating substantial, long-term ecological and socioeconomic disruption.

Local abundance shifts have been recorded in varied species, with those adapted to living in more temperate waters flourishing and cold-adapted species displaying a decline in abundance. Furthermore, the frequency of MHWs in our oceans is only set to increase, with the Arctic and tropical oceans predicted to be impacted most in the coming years.

‘Due to climate change, the number of days with MHWs has increased by 54% over the past century.’

Marine Protected Areas (MPAs) have been designated around the world to assist in reducing the impact that local anthropogenic stressors have on marine ecosystem health and biodiversity. These reserves work to restrict or prohibit fishing activity which helps to restore habitats and protect species against direct and indirect human interference.

However, currently, only around 6.6% of the global ocean is thought to have implemented MPAs and less than half of these reserves are believed to be fully protected against the impacts of fishing.

There is evidence to suggest that MPAs and no-take reserves can act as a buffer for some species against the effect of climate-related stressors, such as MHWs. A recent study focused on nearshore fish communities published in 2023, indicated that MPAs can strengthen the resilience of these communities long term and improve their ability to recover from and resist these disturbance events.

The result of the study showed that the MHW was responsible for a substantial change in the community structure within the MPAs monitored. Before the disturbance, mixed carnivores were the most common group both inside and outside of the MPA.

Following the MHW, there was a shift in trophic diversity, with the proportion of planktivores increasing by more than half and making up closer to 60% of the community structure in both sites. Meanwhile, the proportion of mixed carnivores declined, with the impact on these higher trophic level species being reduced within the MPA compared to the open reference areas.

Complete resistance against these extreme weather events was not shown within MPAs as the reserves still exhibited reduced diversity and a shift in trophic and community structure, with some species being more vulnerable to change than others.

For example, when comparing species-specific recovery rates inside and outside the MPAs, Gopher and Copper rockfish (Sebastes caurinus) and Olive and Vermilion rockfish (Sebastes miniatus) showed a more rapid increase in biomass within the MPA, whereas other species, such as Kelp rockfish (Sebastes atrovirens), displayed biomass declines in both the reference sites and reserves.

Certain species, like Lingcod (Ophiodon elongatus), even exhibited greater biomass declines within the MPA compared to open fishing areas following the MHW, likely due to this species’ very high pre-heatwave biomass in protected zones.

However, overall, the MPAs did seem to better resist the longer-term change caused by these events, with taxonomic diversity recovering 75% faster in these areas when compared to open fishing sites.

‘Funding agencies and governments must build research capacity to monitor MHWs, understand their impacts, and predict future heatwave events.’

There are several potential explanations as to why there is increased resilience against anthropogenic stressors within MPAs. One suggestion is that more diverse species within these protected zones may increase the likelihood of species with a higher tolerance to the changing thermal conditions.

The higher functional diversity and larger population sizes, a result of both the direct and indirect effects of the absence of fishing within MPAs, may also help to explain their enhanced resilience.

Although species recovery is demonstrated to be faster in MPAs, more research is needed to ascertain how long it would take, if at all, for habitats and marine communities to fully recover from the effects of the disturbance. It is possible that following such a climatic event, the area will never return to pre-heatwave conditions, and instead, there may be continued change.

One example of this change could be in the form of new, mobile, non-native species taking refuge in and colonizing the protected ecosystem as they migrate with the changing water temperatures.

To increase our understanding of how MPAs can be used as a tool to reduce some of the detrimental impacts of MHWs, there must be increased research efforts and long-term monitoring of these areas. Collecting data from more comprehensive studies can be used to modify reserves and work towards executing novel mitigation methods.

Modelling techniques can also be used to predict the impacts of future MHWs and assess the vulnerability that different species and habitats may have to this disruption. Management strategies can also be modelled to ascertain their effectiveness in reducing negative impacts.

By increasing the size, connectivity, and flexibility of protected areas, and considering the impacts that climate change has on different marine ecosystems when designing MPAs, we could increase marine protection against future climate disturbances.

‘MPA networks that are large, connected, have adaptable boundaries and are designed following systematic analysis of future climate projections can better support climate resilience.’

The formation of networks of MPAs, for example, may better facilitate the movement of species and improve the recovery of areas following MHWs. This may be due to increased species migration and the rise in genetic diversity that comes with this movement.

Furthermore, to strengthen results, MPAs can be used in tandem with other priorities. For example, the promotion of sustainable fishing, the increased investment in nature-based solutions to aid the reduction in greenhouse gas emissions, and the use of management strategies that better consider reliable MHW forecasts when implementing mitigation methods.

By combining methods that can be used to enhance the health of marine ecosystems, we can provide them with a better framework to help them withstand future anthropogenic stress.

It is important to note that while MPAs may help to buffer some of the impacts of marine heatwaves on a local level, they do not directly address the challenges that climate change poses to the marine ecosystem.

To reduce the impact of MHWs, more comprehensive strategies must be put into place to mitigate the cause and subsequently, the effects of climate change on a global level. Until then, we will continue to witness the growing impacts that MHWs have on fisheries, tourism, Indigenous Peoples and communities reliant on the marine biome.

Featured Image: Michael Herren | Unsplash

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