Megabenthic bivalves in the subtidal area of the Gulf of Cádiz (SW Spain): Diversity and assemblages patterns

The subtidal area of the Gulf of Cádiz (GoC, SW Spain) is influenced by high sediment and seasonal nutrient inputs from rivers (Guadalquivir and Guadiana) that favor large bivalve beds and associated ecosystem services. However, and despite its ecological and social importance, research on bivalve diversity and distribution remains limited. This study aimed to analyze megabenthic bivalve diversity and distribution in spring, linking them to environmental and density-dependent factors. The bivalve populations in GoC were densely distributed, particularly in shallow waters (6–7 m depth), with higher densities near river outflows, except the Guadiana River. Geostatistical analyses showed slight differences in density between the dominant Chamelea gallina and total bivalves. This species significantly influenced ecological indices, though its dominance reduced community evenness. Species richness peaks west of the Gulf (> 8), while lower diversity (< 4) occurs between the Tinto-Odiel and Guadalquivir rivers. Environmental factors (depth, temperature, salinity, and sediment organic content) shape bivalve diversity. Two types of assemblages were detected: 1) A patched distribution along the whole subtidal area, and 2) closely associated with plumes of river mouths. Sediment organic content and depth played a differential role for this assemblage differentiation. This study provides a baseline of bivalve ecology and highlights the need for improving biodiversity knowledge to support sustainable ecosystem-based management in areas affected by anthropogenic pressure and environmental variability such as the GoC. • The highest densities of megabenthic bivalves were detected at 5–7 m depth and near river outflows. • The dominance of Chamelea gallina significantly influenced ecological indices. • Species richness peaked west of the Gulf of Cádiz, lower diversity was observed between Tinto-Odiel and Guadalquivir rivers. • Depth, temperature, salinity, and sediment organic content were key factors shaping bivalve diversity. • Two types of assemblages were identified: 1) patchy in the entire subtidal zone, 2) associated with river plumes.