Response of nematode community structure to hypoxia in an enclosed coastal sea, Omura Bay, for three consecutive years

Abstract

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.

Salinity drives meiofaunal community structure dynamics across the Baltic ecosystem

“…Bottom water oxygen also correlated with the difference in meiofaunal community composition, especially in the SBP. The role of oxygen is not surprising considering that oxygen is essential for the majority of meiofaunal organisms (Braeckman et al, ), and oxygen availability is known to cause shifts in the community composition of for example nematodes (Nguyen et al, ). The local regions as defined in this study (Figure and Table ), also harboured significantly different communities of meiofauna (Figures and ).…”

Impact of seasonal hypoxia on benthic copepod communities in Omura Bay, a highly enclosed coastal sea in southwestern Japan

“…The present study indicates that the densities of benthic copepods were significantly lower during hypoxia than during normoxia. In contrast, there was no significant difference in nematode (the most abundant meiofaunal taxon) abundance during the same period at the same study site (Nguyen et al 2018). We are unsure why the copepod densities at the study site did not recover after the decrease in copepod densities during hypoxia in 2014, even though they showed some recovery trend after hypoxia in 2013.…”

“…The dissolved oxygen (DO) concentration was measured using a multi-parameter monitoring device (AAR, JFE-Alec) placed 1 m above the sediment surface. Details regarding these processes and measurements of DO values have been provided in our previous study (Nguyen et al 2018). The sampling dates of this study are shown in Table S1.…”

“…As most benthic copepods are known to be distributed in the sediment s top layer (0-1 cm depth, Shimanaga Copepods (adults and juvenile copepodites) and nauplii, most of which were considered as those of copepods, were extracted from sediment by centrifugation with a 32 µm mesh and colloidal silica (Ludox HS40, Sigma-Aldrich). The details of these processes are also provided by Nguyen et al (2018), who focused on the composition of nematode communities extracted from the same samples. Extracted copepods and nauplii in each supernatant were sorted and counted under a binocular stereo microscope.…”

“…Among the metazoan meiofauna, nematodes, the most abundant metazoan meiofaunal taxa, usually prevail in suboxic or even sulfidic environments (Giere 2009). Recently, Nguyen et al (2018Nguyen et al ( , 2020 reported that the community composition of nematodes temporally shifted between normoxic and hypoxic conditions in Omura Bay. However, these temporal patterns were also affected by spatial heterogeneities in the degree of seasonal hypoxia and other environmental factors in the surface sediment of the bay.…”