Spatial and temporal reorganization of Polychaeta assemblages and trophic structure across the main habitats of the Mar Menor lagoon, Spain

Coastal lagoons are heterogeneous and highly dynamic ecosystems that are particularly sensitive to anthropogenic pressures. This study analyses how polychaete assemblages in the Mar Menor lagoon (Spain) reorganized their taxonomic and trophic structure across main habitats between 1980s and 2017-2018 periods. We tested whether species composition, assemblage structure and trophic-group structure differed among habitats, and habitat-specific temporal changes. Polychaeta data from soft and hard bottom habitats and their biocenoses were analysed using PERMANOVA, PCO and SIMPER analyses. Species richness decreased from 1980s (77 species) to 2017 (54 species) and 2018 (67 species), whereas mean density increased significantly. Dissimilarities among periods were very high, indicating strong species composition turnover. For its part, trophic diversity declined up to the most recent periods, with predator richness decreasing and detritivore richness increase. Spatially, soft bottom habitats were mainly dominated by detritivore species of Capitellidae and Spionidae families, whereas hard bottoms assemblages were characterized by the non-indigenous species Timarete caribous and Hydroides elegans. Temporal changes were strongly habitat-dependent: whereas Caulerpa prolifera dominated biocenosis, unvegetated muddy bottoms and infralittoral rocky bottoms showed marked shifts towards detritivore and filter-feeder dominance; shallower soft bottoms dominated by Cymodocea nodosa, sandy bottoms and docks exhibited comparatively limited temporal variation in taxonomic and trophic structure. These findings demonstrate that polychaete assemblages experienced great reorganization either in taxonomic and trophic structure, and highlight the need to integrate taxonomic composition, diversity and trophic structure when monitoring ecological changes in coastal lagoons.