Tracing sewage dispersion using δ¹⁵N/δ¹³C in sediments and macrophytes in the coastal zone of Fernando de Noronha

Excessive nutrient input, while essential for life and initially boosting productivity, can disrupt the natural balance of marine ecosystems, triggering eutrophication, harmful algal blooms (HABs), and oxygen depletion, while favoring fast-growing opportunistic species. The primary objective of this research is to geographically map and track the dispersal of domestic effluents from Fernando de Noronha to delineate the invisible contamination plume and assess its ecological impacts on the island's coastal ecosystems. The methodology employed the analysis of stable isotopes δ¹⁵N (with values ​​ranging from 0‰ to +2‰ for fertilizers, +1‰ to +3.4‰ for raw sewage, and +7‰ to +13.8‰ for treated effluents) and δ¹³C (-18‰ to -24‰ for marine origin and -26‰ to -30‰ for terrestrial origin), as well as C/N ratios in surface marine sediments (long-term indicators) and macroalgae (short-term biomonitors). Samples were collected via diving at six sites, spanning the Environmental Protection Area (APA) and the Marine National Park (PARNAM), including the regions of Cagarras, Biboca, Porto, Laje Dois Irmãos, Baía dos Golfinhos and Ponta da Sapata. The results indicate that Fernando de Noronha's sediments exhibit δ¹³C and C/N signatures characteristic of a predominance of marine-derived, autochthonous organic matter. In contrast, the macroalgal genus Dictyota proved to be a more sensitive biological indicator than sediments for detecting recent contamination, showing strong δ¹⁵N enrichment and increased nitrogen levels near the Cachorro and Boldró wastewater treatment plants, confirming the dispersal and direct influence of nitrogen of fecal/anthropogenic origin. However, biogeochemical reactions within the N cycle (such as denitrification and ammonia volatilization) and ecosystem metabolism itself act as ecological filters that modify the original isotopic signature measured in the laboratory. Finally, the absence of distinct isotopic gradients on the seafloor suggests that high local hydrodynamic activity and rapid remineralization prevent the chronic, stable accumulation of sewage-derived organic matter in the island's sediments. These preliminary results indicate that stable isotope monitoring holds great potential for tropical oceanic systems, revealing that macrophytes clearly record recent pulses of coastal pollution, while regional tidal and current dynamics effectively disperse effluents and mitigate permanent ecological impacts.