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On the edge: The influence of multiple stressors on thermal tolerance in poleward edge populations in a climate change era

Periodic Reporting for period 2 - EdgeStress (On the edge: The influence of multiple stressors on thermal tolerance inpoleward edge populations in a climate change era)

Reporting period: 2022-02-01 to 2023-01-31

Global climate change has a profound effect on species and ecosystems across the globe. Temperatures at the equator are becoming too hot for many species to survive, and in response species are re-distributing to avoid deadly heat. They do so in a mainly poleward direction, and they are moving fast - more than 50-70 km per decade on average. The Arctic is warming 3-4 times faster than the global average, and non-Arctic species have started to expand into the Arctic seeking a ‘thermal refuge’ away from the heat. However, recent measurements indicate that temperatures in some Arctic regions already exceeds the temperature limit of many species, thus the Arctic may already too warm to provide shelter from climate change. Moreover, climate change is not only affecting species by warming their environment. In the Arctic, climate change is also causing ocean acidification (a decrease in the pH level of the water), and an unpreceded melting of the Greenland Ice Sheet, which decrease ocean salinity though a process called ‘freshening’. The project EdgeStress improves knowledge and perspectives on the effects of multiple climate change stressors on species and ecosystems at high latitudes.

Specifically, the overall objectives of the project are:

O1. Investigate if high summer temperatures lead to lethal heat stress in populations at their poleward limits.
O2. Investigate if the combination of several interacting stressors in a natural environment decreases the thermal tolerance of habitat-forming species, hereby changing ecosystem structure.
O3. Investigate if freshening and increased warming allow the expanding blue mussel Mytilus edulis to outcompete a native sister species in the Arctic
Edgestress has provided fundamentally new information on the effects of climate change on species physiology and ecosystem structure and functioning. The intertidal ecosystem at high latitudes is poorly described, and global patterns in biodiversity isn’t well understood. I have shown how global patterns on intertidal biodiversity are, to a high extend, controlled by local processes, and not latitudinal changes in the climate. Moreover, I placed biomimetic temperature sensors for measuring body temperatures of key species, and this data is being used to calculate how the body temperature of mussels actually response to heat at high latitudes.

My results show a remarkable resilience in blue mussels to global warming. Physiological measurements have been done on all biological levels from genes, protein expression, to whole-animal responses. Furthermore, have I been exploring the effects of multiple stressors on sensitivity to warming. For instance have I looked at how parasites affects thermal tolerance, and the preliminary results clearly show that parasite infections interfere with thermal tolerance of the host organisms. I have also investigated the interaction of ocean acidification and thermal stress on several blue mussel species

In West Greenland, two species on blue mussels are present - the southern Mytilus edulis and the northern Mytilsu trossulus. In North Greenland, there is a hybrid zone in which the two species overlap, and to increase knowledge on how the two series response to climate change, laboratory experiments have been conducted on thermal tolerance. The Northern species seems to struggle compared to the southern species under warmer conditions, indicating a potential shift in species dominance and distribution.

When conducting experiments on Mytilus mussels, a key challenge is to identify the species. Mytilus are cryptic species making it impossible to distinquish them by visual inspection. The classic approach to this issue has been to confirm the species ID after the experiment, using of DNA extraction. However, for laboratory experiment, knowing the ID before the experiment would optimise the experimental design and reduce the number of animals needed. I have worked on a designed a code in the R program that via machine learning use Elliptic Fourier analyses of shells outliners to distinguish individual Mytilus species. For now, the results indicate that this method can distinguish the species with a 73% success rate, but I am continuously finetuning the method, and the hope is to reach an 85-90% rate.

Results from EdgeStress includes:
- Public presentations at science events
- Presentations at scientific conferences
- Supervision and talent development of students at all level
- 8 published peer-reviewed papers, with multiple more in preparation
- outreach at social media
The data and results have already progressed the current state of the art. Little is known about how species cope with the climate change at higher latitudes, but I have shown that blue mussels at their northern edge are resilient to climate warming. For example, we found no evidence for cellular stress response in animals collected on a warm summer day in the Arctic, and for the first time identified a massive expansion of the HSPA12 heat shock protein in the blue mussel Mytilus edulis. This novel discovery indicates an unknown evolutionary adaptation to the intertidal life. Thus, blue mussels appear to have considerable capacity to withstand the current rates of Arctic warming, but I have also produced data showing that increased freshwater runoff from the Greenland Ice Sheet in response to warming render key intertidal species more sensitive to heat stress. From another of my studies, it seems that parasites increase thermal tolerance in some species, which was until now unknown, so the effects of multiple stressors work in different directions and their internal interactions are a fruitful topic for future research.

I have described how intertidal communities are controlled by local- rather than large-scale environmental drivers, and the Greenlands intertidal ecosystems seems resilient to climate change. This indicates that no major changes in community structure is expected in a near future. These results on the resilience of species and ecosystems at high latitude are novel, and the papers produced so far has moved beyond the current state of the art. Therefore they are receiving significant attention and is already being widely cited. The intraspecific dynamics among M. edulis and M. trossulus in North Greenland has never been studied before, and these data will pave for way for our understanding of how climate change affects closely related species.

The results obtained in EdgeStress are important for scientists and societies as improved knowledge and perspectives on the effects of multiple climate stressors on individuals to ecosystems is crucial for understanding the effects on species survival, biodiversity changes and ecosystem stability. New information on multiple climate stressors on coastal systems is important worldwide. Coastal systems provide nursery grounds for commercially important fish and shellfish species, and ecosystem services such as, carbon sequestration and shoreline protection with an estimated value in the trillons of euro. The results from this project will therefore have implications for global and local communities and economies as it will be used to inform stakeholders, organisations, researchers, and the public about the future effects of climate change on one of the most sensitive ecosystems on earth.
Intertidal blue mussels at high latitudes