Seasonal variation in the relative importance of assembly processes in marine fish communities as determined by environmental DNA analyses

Compositional variation among local communities is a result of environmental (e.g., environmental filtering) and spatial (e.g., dispersal limitation) processes. Growing evidence suggests that their relative importance varies temporally, but little is known about the short-time scale dynamics, that is, seasonality. Using marine fish communities in a Japanese bay as a model system, we tested the hypothesis that seasonal changes in the environment induce a shift in the relative importance of environmental and spatial processes. We used one-year monthly monitoring data obtained using environmental DNA and conducted a variation partitioning analysis to decompose the two processes. The relative importance of environmental and spatial processes was comparable averaged over the year but changed seasonally. During summer, when lower dissolved oxygen concentrations may adversely affect organisms, species composition was more explained by space despite larger environmental heterogeneity than in other seasons. This suggests that environmental processes weakened during the season with extremely severe environments, likely due to the random loss of individuals. We conclude that the assembly processes of communities of mobile organisms, such as fishes, can shift even within a year in response to seasonal changes in environmental severity. The results also indicate the applicability of eDNA techniques for community assembly studies.