Floristic surveys of the rocky intertidal zones of the Santana Archipelago, Macaé (RJ, Brazil)

The rocky shores where the ocean meets the land are not merely empty stone; they are bustling cities built by plants that live underwater. These plants, known as seaweeds, form the foundation of coastal ecosystems, creating the food and shelter that countless fish, crabs, and other marine animals depend on to survive. Because they are so sensitive to changes in their environment, the health of these seaweed communities often serves as a warning system for the overall condition of the coast. When the water is clean and stable, a rich variety of these plants thrives. When pollution, temperature shifts, or physical disturbance occur, the community changes, often losing its most delicate members. Understanding exactly which species live in a specific place is the first step in protecting them, yet for many remote or industrialized coastlines, this basic knowledge remains missing. Without a clear inventory of what is there, it is impossible to know what is being lost or how to manage the area effectively.

In the northern part of Rio de Janeiro state, Brazil, a small group of islands known as the Santana Archipelago sits eight kilometers off the coast of the city of Macaé. This area is a protected zone, but it faces constant pressure from two directions: the intense tourism that visits its beaches and the nearby oil industry that operates in the deep waters of the Campos Basin. Despite its importance, the underwater plant life of these islands had never been properly cataloged. To fill this gap, a team of researchers from the Federal University of Rio de Janeiro traveled to the archipelago to conduct a detailed survey of the seaweeds growing on the rocky shores of French Island, one of the main islands in the group. They visited the site twice, once in the winter of July 2024 and again in the summer of February 2025, scraping samples from the rocks at low tide and bringing them back to the lab for identification.

The researchers found a vibrant and diverse collection of life, identifying a total of 74 different species on French Island alone. The most abundant group was the red seaweeds, which made up nearly 60 percent of the species found, followed by green seaweeds and brown seaweeds. Among the red seaweeds, a specific group known for its delicate, branching structures was the most common, with many of these plants growing as tiny hitchhikers on other larger seaweeds. The green seaweeds were also well-represented, with some species forming thick, lush mats that covered large areas of the rock. One discovery stood out as particularly significant: the researchers found a species of green seaweed called Caulerpa ambigua that had never before been recorded on the northern coast of Rio de Janeiro. Previously, this plant was only known from the southern part of the state, meaning this finding expands the known range of the species by hundreds of kilometers.

The survey revealed that the type of seaweed found depended heavily on where the researchers looked and when they looked. One site, a beach with coarse sand and strong waves, hosted a much richer variety of plants, with 69 different species recorded, compared to 48 species at a nearby site with fine, red clay. The summer visit yielded far more species than the winter visit, suggesting that the warmer, wetter conditions of the summer months encourage the growth of many different types of seaweed. The team also compared their new findings with old records stored in a university herbarium, which contained samples collected from the archipelago over the last two decades. By combining their fresh collections with these historical records, the total number of known seaweed species for the entire Santana Archipelago rose to 115.

Despite the success of the survey, the researchers noted that their work was not without limitations. Some of the samples they collected were too small or damaged to be identified down to the exact species, leaving them classified only by their genus. This is particularly true for certain types of calcified red seaweeds, which look very similar to one another and are difficult to distinguish without genetic testing. The team suspects that some of these unidentified specimens might actually be species that have never been described by science before. Furthermore, the study was limited to just one of the islands in the archipelago, and the researchers emphasized that the full diversity of the region's marine flora remains unknown until the other islands are also surveyed.

The presence of these seaweeds in an area so close to oil ports and heavy tourism highlights the delicate balance of the ecosystem. The researchers observed that the site with the most human activity and wave disturbance had a different mix of plants, dominated by fast-growing species that can quickly colonize open spaces. In contrast, the more stable areas supported a wider variety of slower-growing, complex species. This study provides the essential baseline data needed to monitor these changes over time. By knowing exactly what lives there now, scientists and managers can detect future shifts caused by pollution or climate change, ensuring that this unique and threatened corner of the Brazilian coast is protected for the future.