The understanding of long-term variability in population abundance is an important goal in marine conservation, but the availability of data series allowing assessing abundance trends is a major research gap restricting this trend. This study aims to describe and understand annual and monthly trends in seabird community composition and abundance in south Portugal (from a barrier island in Ria Formosa, Algarve), using monthly monitoring data collected from 2014 to 2020. These observations were carried under the framework of a Seabirds and Marine Monitoring Network, RAM, which integrates citizen science. Northern gannet Morus bassanus was the most abundant species, both in the winter and summer seasons. To detect trends in species abundance, we used the TRIM log-linear model. Our results showed a steep increase in the vulnerable, locally breeding, Audouin’s gull Larus audouinii; a moderate increase in the migratory Cory’s/Scopolis's shearwaters Calonectris diomedea/C. borealis; a moderate decrease for the local breeder Little tern Sternula albifrons, and a steep decrease in the wintering Mediterranean gull Larus melanocephalus. The Generalized additive model (GAM) was used to analyze the influence of environmental variables on species abundance. Primary production, chlorophyll-a concentration and wind speed were three of the most important variables impacting seabird abundance.
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