Trophic ecology of Caribbean polychaetes: responses to environmental changes driven by massive Sargassum arrivals

Sargassum decomposition promotes ecosystem eutrophication and triggers trophic shifts in benthic organisms, including polychaetes. In this study we evaluated the trophic dynamics of polychaetes Eurythoe complanata, Lysidice caribensis, and Nicidion cariboea in response to the massive arrivals of pelagic Sargassum using stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N). The study was conducted across nine sites in the southern Mexican Caribbean during onset and ending periods of massive Sargassum arrivals in 2021. Results revealed significant variations in δ¹³C and δ¹⁵N values among sites in onset and ending periods, indicating shifts in carbon and nitrogen sources and possible changes in feeding behavior. Lysidice caribensis exhibited the highest trophic plasticity, with trophic position (TP) values ranging from 1.6 to 3, adapting to different trophic guilds depending on resource availability and site characteristics. In contrast, E. complanata, traditionally considered a higher consumer, exhibited a consistently low trophic position (1.3-2.6), suggesting a dietary shift towards algae and detritus under Sargassum-influenced conditions. Nicidion cariboea also displayed lower trophic position and narrower isotopic niches, particularly in sites impacted by Sargassum leachates (1.8 to 3). Standard ellipse area ranged from 0.07 to 5.91 across species, sites and periods, with L. caribensis showing the broadest values in sites with Sargassum influence during ending period. Periodical fluctuations also influenced niche overlap, with greater overlap observed during the ending period, suggesting increased interspecific competition under resource-limited conditions. These findings highlight the adaptive strategies of polychaetes to environmental disturbances and their potential role in a changing reef environment.