Aim Identifying drivers that shape biodiversity across biogeographical regions is important to predict ecosystem responses to environmental changes. While beta diversity has been widely used to describe biodiversity patterns across space, the dynamic assembly of species over time has been comparatively overlooked. Insights from terrestrial and marine studies on temporal beta diversity has mostly considered environmental drivers, while the role of biotic mechanisms has been largely ignored. Here, we investigated patterns of temporal variation in beta diversity of seagrass-associated animals (amphipods, as model organisms). Location We conducted a study in three biogeographical regions across a temperate to subtropical latitudinal gradient (approximately 2,000 km, 13º of latitude). In each region, we randomly selected three C. nodosa meadows, totalling nine meadows sampled seasonally (i.e., four times per year) from 2016 to 2018. Methods We partitioned temporal beta diversity into its turnover (i.e. species replacement) and nestedness (i.e. differences in species composition caused by species losses) components and addressed the relative influence of both temporal variation in habitat structure (i.e., biotic driver) and environmental conditions on such patterns. Results Our study revealed high temporal beta diversity of amphipod assemblages across the three biogeographical regions, denoting significant fluctuations in species composition over time. We identified species turnover as the primary driver of temporal beta diversity, strongly linked to temporal variability in local habitat structure rather than regional climatic drivers. Subtropical Atlantic meadows with high structural stability over time exhibited the largest turnover rates compared with temperate Mediterranean meadows, under lower structural stability, where nestedness was a more relevant component of temporal beta diversity. Main conclusions Our results highlight the crucial role of habitat stability in modulating temporal beta diversity patterns on animals associated with seagrasses, stressing its importance for developing management and restoration actions in the context of diversity loss and fragmentation of ecosystems.

The European sardine (Sardina pilchardus, Walbaum 1792) is indisputably a commercially important species. Previous studies using uneven sampling or a limited number of makers have presented sometimes conflicting evidence for the genetic structure of S. pilchardus populations.  Here we show that whole genome data from 108 individuals from 16 sampling areas across 5,000 Km of the species’ distribution range (from the Eastern Mediterranean to the archipelago of Azores) supports at least three genetic clusters. One includes individuals from Azores and Madeira, with evidence of substructure separating these two archipelagos in the Atlantic. Another cluster broadly corresponds to the center of the distribution including the sampling sites around Iberia, separated by the Almeria-Oran front from the third cluster that includes all of the Mediterranean samples, except those from the Alboran Sea. Individuals from the Canary Islands appear as belonging to the same ancestral group as those from the Mediterranean. This suggests at least two important geographical barriers to gene flow, even though these do not seem complete, with many individuals from around Iberia and the Mediterranean showing some patterns compatible with admixture with other genetic clusters. Genomic regions corresponding to the top outliers of genetic differentiation are located in areas of low recombination indicative that genetic architecture also has a role in shaping population structure. These regions include genes related to otolith formation, a calcium carbonate structure in the inner ear previously used to distinguish S. pilchardus populations.  Our results provide a baseline for further characterization of physical and genetic barriers that divide European sardine populations, and information for transnational stock management of this highly exploited species towards sustainable fisheries.