Seagrass meadows are globally important coastal habitats support high biodiversity and underpin key ecological functions, yet they are increasingly threatened by eutrophication. While the impacts of nutrient enrichment on seagrass condition and productivity are well established, its influence on the structure of associated epifaunal communities remains less clearly understood, particularly across broad spatial scales. In this study, we examined epifaunal assemblages associated with Zostera marina meadows at 16 sites spanning lagoon, estuarine, coastal, and island environments around the British Isles. Using standardised field sampling in combination with mixed-effects modelling, we investigated how variation in environmental setting within the seascape, seagrass morphological traits, and leaf tissue nutrient concentrations (nitrogen and phosphorus) shaped patterns of epifaunal abundance, richness, and community composition. Epifaunal assemblages exhibited strong spatial heterogeneity, with local site-level environmental conditions explaining a large proportion of variation in community structure. While broader habitat categories provided some explanatory value, seascape-level fine-scale environmental context consistently emerged as an additional driver. Seagrass morphological traits, including leaf length, width, biomass, and epiphyte cover, had limited influence on epifaunal richness and abundance, suggesting that small-scale habitat complexity was not the primary determinant of biodiversity within these meadows. In contrast, nutrient availability, particularly nitrogen enrichment, had a marked effect on epifaunal diversity. Higher nitrogen concentrations were generally associated with reduced epifaunal abundance and richness when standardised by leaf area, consistent with detrimental ecological effects of eutrophication. However, these relationships varied among habitat types: moderate nitrogen levels corresponded with increased diversity in estuarine and island sites, whereas coastal and lagoon meadows showed stronger declines under enhanced enrichment. Phosphorus exhibited especially negative effects in lagoon environments. These findings demonstrate that elevated nutrient concentrations alter seagrass-associated biodiversity in context-dependent ways. Effective management and restoration will therefore require site-specific nutrient reduction and monitoring strategies that reflect local ecological conditions rather than uniform regional targets.
Hope et al. (Wed,) studied this question.