Disturbance of benthic macrofauna in relation to hypoxia and organic enrichment in a eutrophic coastal bay

This study demonstrated the spatiotemporal patterns of the environmental conditions and benthic macrofauna in Tokyo Bay, Japan, and investigated the factors causing disturbances in the assemblage structure. In the north-central areas, the density and species diversity of the macrobenthos was low. Although hypoxia appeared in July, defaunation occurred in August. The delayed defaunation and recolonization soon after the abatement of hypoxia were attributed to several polychaete and bivalve species that were tolerant to the hypoxic environment. In the southeastern areas, however, the density and species diversity of the macrobenthos was high throughout the year, and no defaunation was recorded. Multivariate analyses showed that the disturbance in the macrofauna correlated with organic enrichment in the sediment and bottom-water hypoxia. There is a concern about further impairment of the macrofauna in the bay due to the expansion of sediment with high levels of organic matter towards the southern regions that could cause hypoxia and subsequent defaunation.

► We elucidated the factors causing a disturbance of the macrofauna in Tokyo Bay. ► Several months of hypoxia exposure caused defaunation in the northern regions. ► Recolonization was attributed to the recruitment of several hypoxia-tolerant species. ► No disturbance was observed in the macrofauna in the normoxic southern regions. ► Future impairment of biota may occur in the southern areas due to organic enrichment.