The effects of climate change on marine coastal ecosystems – what do we know and what can we do?
Climate models predict substantial transformations in the state of our oceans over the next century, bringing about significant impacts on the entire Earth system. These changes will have far-reaching consequences for marine biodiversity and human well-being. Globally, coral reefs, kelp and mangrove forests, salt marshes, seagrass beds and other bio-engineers rank among the most vulnerable marine and coastal ecosystems to climate change. In addition to global changes, the impact of local human activities on these ecosystems is profound. These local stressors can significantly reduce the resilience of these ecosystems and influence their ability to withstand the effects of climate change, i.e., their capacity to cope with heavy perturbations, such as a heat wave or a hurricane.
While climate change cannot be locally managed or contained, local stressors can be. It is therefore crucial to understand which of these stressors need to be managed to increase the resilience of marine and coastal ecosystems to climate change’s impacts. Furthermore, did you know that marine and coastal ecosystems are some of the bests at sequestering atmospheric carbon or protecting our coastline from erosion and sea level rise? As such, taking local actions to foster the good health of these natural environments not only favours their functioning and resilience but also plays a vital role in mitigating and adapting to climate change.
We brought together a team of 20 scientists and engaged in a momentous collaborative effort to identify and synthesise current information on how climate change and local stressors together affect marine and coastal ecosystems. The review focuses on six of these ecosystems that are predominant in Europe and its Overseas Territories, covering three ecoregions of focus in the project: Northern European kelp forests, salt marshes and seagrass beds; Mediterranean corals Cladocora caespitosa, maërl beds and seagrass beds; and Caribbean coral reefs, mangrove forests and seagrass beds.
Environmental tipping points and safe operating spaces
Tipping points are understood as the point where, following a perturbation, a self-propagated change can eventually cause a system to shift to a qualitatively different state. A safe operating space, then, is understood as the range of environmental conditions that lies between tipping points for which the functioning of the ecosystem remains stable.
The review we conducted presents the climate change stressors, such as warming, extreme weather events, ocean acidification, or sea level rise and how they affect our focus ecosystems at different levels, from their metabolism and reproduction to their survival and distribution. More particularly we were interested in identifying potential thresholds above which the ecosystems are severely impacted, such as for instance, the water temperature above which kelp forest dies (23°C for Laminaria digitata and L. hyperborea) or stop their growth and reproduction (18°C) depending on the development stage. However, experiment size, costs, and project time constraints are among the main limitations to investigating these thresholds, making it difficult to establish when a tipping point for an environmental factor is precisely reached for many ecosystems.
What about the combined effects of direct local human pressures?
The complex interactions between climate change drivers and local stressors further affects where these environmental thresholds are located, making it increasingly challenging to identify them. However, our review allowed us to highlight local stressors that further increase the vulnerability of marine and coastal ecosystems to the effects of climate change, as well as some overall cumulative effects of local and climate change stressors on these ecosystems. For instance, studies on the interactive effects of climate change and local stressors on coral reefs have intensely focused on warming, terrestrial run-off, and grazing. More particularly, nutrient enrichment resulting from coastal run-off acts in synergy with increased temperatures to favour shifts to algal-dominated communities and promote coral diseases.
Disentangling the combined effects of multiple stressors is extremely difficult, especially when considering that the response of the ecosystems studied is also often specific to a species, and for the same species, the response can further vary between populations and individuals, depending on environmental factors, genetics, etc. As a result, of all these variability factors, there are still many unknowns when it comes to identifying thresholds.
Where do we go from there?
Without devaluing the urgency to drastically reduce greenhouse gas emissions into the atmosphere (cf. COP1 to 28), we need to prioritise local actions to relieve some of the anthropogenic pressures acting on marine and coastal ecosystems to help them thrive under climate change. Going back to the coral reefs example above, having a watershed approach to addressing nutrient enrichment through, for instance, better wastewater treatment and reduced use of fertilizer in agriculture, would alleviate this pressure on coral reefs and allow them to better cope with temperature increases. Furthermore, as coral reefs, seagrass beds and mangrove forests are often connected, supporting each other, any conservation action implemented for one ecosystem will most likely benefit the other ecosystems as well.
There will always be knowledge gaps and uncertainty around the potential cumulative impacts from climate change-related and local stressors. One thing we know for sure is that we must act now and effectively to reduce the local pressures that are manageable, whilst climate change should be addressed both locally and globally by cutting the emissions of greenhouse gases.
The full paper can be found here: https://doi.org/10.1016/j.biocon.2023.110394
Text by Ewan Trégarot, Silvia de Juan, Kieran Deane and Cindy Cornet
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