Sub-habitat classification of temperate salt marshes in Japan based on aquatic fauna

Salt marshes are being lost worldwide even though they provide essential ecosystem services. Habitat classification of salt marshes based on fauna, which is an effective indicator of their ecosystems, is expected to be useful to effectively assess the baseline for conservation and restoration; however, it has not been well studied. This study aimed to identify the diversity of salt marsh sub-habitats for fauna, where biota was collected and the physicochemical environments were measured in 101 small patches of salt marsh (i.e., survey sites) across 32 temperate tidal rivers and inlets in Fukuoka Prefecture, Kyushu, Japan. A total of 78 aquatic animal species (decapod crustaceans, gastropod mollusks, bivalves, nereidid polychaetes, and fish) were observed, 63 of which were used as indicators for sub-habitat identification. Although reed plants covered 90 of the 101 survey sites, the survey sites were classified into six clusters based on fauna. Representative aquatic animals were identified in each cluster, and these six clusters had different physicochemical environments, such as salinity and sediment conditions. The results indicated that the six sub-habitats had different biotic and abiotic characteristics. On the other hand, the results of the ordination analysis indicated that the fauna of the six sub-habitats do not occur independently of each other but are composed of continuous and gradually changing fauna along environmental gradients. Although no significant differences in the areas of salt marsh patches among the clusters, species richness differed significantly, suggesting that physicochemical environments, such as salinity and sediment conditions, rather than spatial factors, may affect species diversity. The findings emphasize that each sub-habitat supports species diversity in temperate salt marshes and are expected to contribute to planning targets for future salt marsh conservation/restoration and further improvements in artificial marshes.