Fernando Lima, CIBIO/BIOPOLIS, University of Porto, Portugal, discusses his article: Simulated intertidal heat stress on the brown seaweed Ascophyllum nodosum demonstrates differential population sensitivity to future climate
Climate change is often thought to threaten only vulnerable species, but what about the tough ones? New research on a resilient seaweed suggests that even the hardiest species may struggle to keep up with the pace of warming. In fact, how well a species copes with climate change can depend on where it lives and how adapted it is to local conditions. Ascophyllum nodosum, a brown seaweed also known as knotted wrack, thrives on rocky shores from chilly to warmer waters. It forms dense mats on the shoreline, providing shelter for many small coastal animals. If any species could handle a bit of extra heat, you might think this tough seaweed could. After all, it regularly survives being dunked in cold ocean water at high tide and then baked in the sun at low tide. But as climate change brings hotter days and more extreme temperature swings, can this seaweed keep coping?
Simulating Tides in the Lab
In a recent study, we set out to test the heat limits of Ascophyllum nodosum from different parts of its range. We collected seaweed from ten spots across Europe, from Norway to Portugal. In the lab, we built a special tank system like a mini tide pool that would simulate natural cycles by flooding the seaweed with water at high tide and draining it at low tide, while adjusting light and temperature to mirror real conditions. Using actual temperature data from the field, we created realistic low-tide heat waves. We also tried different baseline water temperatures – one cold (~15°C) and one warm (~20.5°C) – to determine whether warming from a cooler or warmer starting point changed how the seaweed handled the heat.
It’s the Temperature Swing that Stings
After running these simulated tidal cycles, we found that what stressed the seaweed most wasn’t just the hottest temperature reached – it was the change in temperature between high tide and low tide. In other words, a rapid jump from cool water to hot air did more harm than a smaller, steadier rise. Interestingly, seaweed in warmer water coped better with heat stress than seaweed in colder water. It sounds counterintuitive, but when the water was warm to start, the jump to hot low-tide air was smaller – a gentler transition. In the cold 15°C water, the leap to a hot low tide was a bigger shock.
Local Differences and a Race Against Warming
A key takeaway from this research is that not all populations of this seaweed respond the same way. Ascophyllum from Portugal might handle heat differently than Ascophyllum from Norway because each population has adapted to its local climate. Indeed, we saw differences in how well seaweed from various locations survived the heat. Some were more heat-tolerant than others simply because they were accustomed to warmer conditions. However, even the most heat-tolerant groups have their limits. When we plugged their results into climate models, we found warming could soon outrun the seaweed’s ability to adapt. In simulations of future conditions, some populations that fared well in the lab would struggle to survive the higher temperatures expected in coming decades. In fact, these projections line up with what’s already happening: in some warming areas, local populations of A. nodosum have started to disappear. Going forward, the impacts won’t be uniform – some locations will be hit harder than others, depending on how sensitive each population is and how fast its home waters heat up.
Bottom Line
Climate change impacts are not “one size fits all” – even for a tough species. Ascophyllum nodosum may be resilient, but it’s not invincible. Each local population has its breaking point, and rapid warming can push even hardy organisms over the edge. We shouldn’t assume “strong” species will be fine in a warming world. Rocky shore ecosystems, home to resilient and adaptable life, might actually be more vulnerable than they appear. As the climate continues to heat up, even the toughest seaweed might not keep up – a reminder that resilience has its limits.
