Seasonal change in spatial variability of eelgrass epifaunal community in relation to gradients of abiotic and biotic factors

Abstract Seagrass‐associated macroinvertebrates (epifauna) play important roles as mediators between primary producers and higher consumers in seagrass beds. However, many aspects of the variation in the epifaunal community in seagrass beds remain to be investigated. In this study, we examined how the epifaunal community structure varied seasonally and spatially with abiotic and biotic factors in an eelgrass bed of Akkeshi, northeastern Japan. We predict that abiotic factors are more definitive of the variation in the epifaunal community than biotic factors when the eelgrass bed is less productive in early spring and late autumn, whereas biotic factors including macro‐epibiotic organisms (epiphytic macroalgae and sessile animals on eelgrass blades) are more influential when the eelgrass bed is more productive from late spring to early autumn. The epifaunal community composition varied greatly with season, and its spatial variation was correlated with different sets of abiotic and biotic factors in different seasons. Epibiotic organisms, in addition to other abiotic and biotic factors, explained some of the variation, especially in the productive summer seasons, despite the relatively low biomass of epibiotic organisms compared to that of eelgrass and epiphytic microalgae. Inconsistent with our prediction, abiotic factors were more definitive of the spatial patterns of the epifaunal communities from spring to summer when the productivity of the eelgrass bed was increasing, whereas biotic factors were found to be more important in the autumn when the productivity declined. Our result highlights the importance of epibiotic organisms on eelgrass blades to the epifaunal community, which was underappreciated in previous studies.