Unveiling thermal responses on macroalgae assemblages after a 10 years’ time span in a subtropical rocky shore from Southwestern Atlantic Ocean

The ocean is not a static backdrop; it is a living, breathing system that shifts with the seasons and the climate. For the plants that anchor the seafloor, particularly the diverse forests of seaweed known as macroalgae, temperature is a master switch. These plants do not just drift with the current; they are sensitive barometers of the water's health. When the ocean warms, especially during sudden, intense spikes in temperature known as marine heatwaves, these underwater gardens can change dramatically. Some species thrive in the heat, while others, accustomed to cooler waters, struggle or disappear entirely. Understanding how these communities react is vital because seaweeds form the foundation of coastal ecosystems, providing food and shelter for countless marine animals and helping to regulate the global climate. If the balance of these plant communities shifts, the entire web of life that depends on them can be altered.

In the subtropical waters off the coast of southern Brazil, a team of researchers set out to see how these underwater gardens had changed over a decade. They focused on a rocky island called Farol Island, a place where the ocean meets the land in a dynamic dance of tides and currents. The scientists returned to the exact same spots they had visited ten years prior, in 2014, to take a fresh inventory of the seaweed. They wanted to know if the warming ocean and the increasing frequency of extreme heat events had rewritten the list of who lives there. By comparing the plants they found in 2024 with the records from 2014, and by cross-referencing these findings with detailed temperature data from the past twenty years, they uncovered a surprising reversal in the rhythm of life on the shore.

The most striking discovery was a complete flip in the seasonal pattern of biodiversity. In 2014, the seaweed community was richest during the summer, when the water was warmest. This is a pattern often seen in these regions, where warmer months typically support a greater variety of life. However, by 2024, the situation had reversed. The researchers found that the winter of that year held a much higher number of species than the summer did. This was not a minor fluctuation; it was a fundamental shift in the community's structure. The change was driven primarily by two major groups of seaweed: the red algae and the brown algae. In the winter of 2024, these groups were far more diverse than they had been in the winter of 2014, while the summer richness for these same groups had declined.

The green algae, a third major group, behaved differently. They did not show the same seasonal flip. Instead, their overall numbers dropped in both seasons, and the community became dominated by just a few hardy, opportunistic species that can survive in harsh conditions. This suggests that while the red and brown algae are adjusting their timing to find the most comfortable temperatures, the green algae are struggling to keep up, with only the most resilient survivors remaining. The researchers noted that the red algae, in particular, seemed to be responding to the changing thermal landscape by becoming more abundant during the cooler months, effectively moving their peak activity to a time when the water is less stressful.

This shift coincides with a clear warming trend in the local ocean. Over the twenty-year period leading up to the study, the average sea surface temperature in this region rose by 0.3 degrees Celsius. More importantly, the ocean is experiencing more frequent and intense marine heatwaves. These are periods where the water stays unusually hot for days or even weeks, far exceeding the normal summer highs. The data showed that these extreme heat events have become significantly more common and longer-lasting since 2013. The researchers observed that the number of these heatwaves has doubled compared to the previous decade, and the intensity of the strongest events has increased.

The connection between the heat and the seaweed is not just a coincidence; the evidence suggests a direct link. The researchers propose that the seaweed communities are adapting to these new thermal realities. The red and brown algae appear to be shifting their seasonal peaks to avoid the most intense heat of the summer, finding a thermal sweet spot in the winter instead. This ability to adjust suggests a degree of resilience, a capacity to survive and even thrive in a changing environment. However, this resilience has limits. The decline in green algae and the loss of many species that were common in 2014 indicate that the stress is taking a toll. Some species that once thrived in the upper parts of the rocky shore have vanished, replaced by a different mix of organisms.

The study also revealed that the ocean is home to a greater variety of life than previously recorded in this specific area. The 2024 survey identified eight new species of red algae for the region, expanding the known diversity of the local ecosystem. Yet, this increase in the total number of species masks a deeper story of turnover. Many of the species found in 2014 were missing in 2024, and many new ones had appeared. This constant reshuffling, where some species leave and others arrive, is a sign of a community in flux, trying to find a new balance in a warming world.

The researchers are careful to note that while the temperature changes are the most likely driver of these shifts, they are not the only factor. The area around Farol Island is near a busy port and urban centers, which can introduce pollution and change the clarity of the water. These human impacts, combined with the rising heat, create a complex web of stressors. The study suggests that the seaweed communities are navigating a difficult path, balancing the need to survive the heat with the challenges of a changing coastal environment. The fact that the community has not collapsed entirely, but rather reorganized itself, offers a glimmer of hope. It shows that nature has a capacity to adapt, even when the rules of the game are changing rapidly.

Ultimately, this research provides a crucial snapshot of a subtropical ocean in transition. It moves beyond simple observations of warming to show how that warming is actively reshaping the living world beneath the waves. The seasonal inversion of seaweed richness is a clear signal that the old patterns are no longer reliable. As the ocean continues to warm and marine heatwaves become more frequent, these underwater communities will likely continue to change. The study serves as a baseline, a starting point for future monitoring that will help scientists understand how long these adaptations can last and what the future holds for the biodiversity of the Southwestern Atlantic. The story of Farol Island is a story of change, resilience, and the quiet, persistent ways in which life responds to a warming planet.