Hypoxia in bottom environments of coastal marine ecosystems is a serious problem adversely affecting both benthic life and local fisheries. In this study, we monitored abundance, composition, and feeding types of nematode communities under pre-, mid-, and post-hypoxic conditions in Omura Bay, Nagasaki, Japan, for three consecutive years (2013)(2014)(2015). The bay is almost completely enclosed, and experiences hypoxia at the bottom every summer. A positive correlation was found between dissolved oxygen (DO) concentration and nematode abundance over the entire sampling period (p<0.05, r=0.61). The nematode community compositions among the pre-, mid-, and post-hypoxic conditions were significantly different (one-way analysis of similarities (ANOSIM), p<0.05), which suggests that DO in the bottom water acts as a major driver for the community shift. The increases in abundance of nematodes with toothless feeding apparatus in hypoxic periods, relative to normoxic periods, further suggested that the transfer of organic matter from bacteria through nematodes became more important in the bay under hypoxia than normoxia. It was also demonstrated that full recovery of nematode populations from hypoxic to normoxic conditions would require more than two weeks of continuous normoxic DO levels (>3 mg L −1 ). These findings will help us to understand how global trends of ocean deoxygenation could shape the meiobenthic community and alter benthic ecosystem functioning in coastal areas.

No abstract

Declining oxygen concentrations in aquatic habitats represent extreme conditions that threaten benthic life. Hypoxia has recently become an important research topic, as areas affected by these phenomena are spreading globally. Omura Bay is one of the most highly enclosed seas in Japan, and severely hypoxic conditions occur at the bottom every summer. We conducted a preliminary study in the center of the bay to evaluate how seasonal hypoxia affects the abundance and community composition of benthic meiofauna, with particular reference to copepods. The copepod densities and their nauplii differed significantly among seasonal categories (before, during, and after hypoxia). Furthermore, the degree of the seasonal decline in copepods during hypoxia seemed much more severe than that in nematodes, the most abundant meiofauna. The assemblages of adult copepods had the simplest composition during hypoxia, when harpacticoid copepods in the family Cletodidae, which have smaller and slender bodies, occurred at significantly higher frequencies (a contribution of 84% to the mean similarity among seasons). After hypoxia, the relative abundance of copepods in the families Ectinosomatidae and Longipediidae increased, which may likely be attributed to their higher swimming abilities and rapid recruitment via specific planktonic nauplius stages, respectively. High frequencies of copepods in the Cletodidae family have also been observed under hypoxic conditions in the Mediterranean Sea, suggesting that similar processes affect benthic copepod communities, which work to the advantage of cletodid species with small and slender forms in the subtidal sediment bottom under severe hypoxia in Omura Bay and other regions.