Effects of hypoxia and organic enrichment on estuarine macrofauna in the inner part of Ariake Bay

Abstract

In the inner part of Ariake Bay, Japan, hypoxia frequently occurs in summer at the organically enriched bottom with salinity stratification caused by flooding after the rainy season. Sediment organic enrichment can work as a stressor for macrobenthos. To investigate the effects of both hypoxia and sediment organic enrichment on macrobenthos, samples were collected at 20 stations by grab sampling in May and August, representing the situation before and after hypoxia, respectively. Although sediment grain size did not change, sediment TOC increased significantly in August. Multivariate analyses showed that the community structure changed significantly in August. The variation in the community structure among stations also increased, which indicated disturbance by stressors during the study period. Similarly, the species richness and total abundance of macrobenthos decreased significantly after hypoxia even after the TOC effect was removed. In addition, the amount of TOC change and the abundance of the main species did not correlate in any case. These results suggest that the community changes during the study period are not caused by stress from the increased sediment TOC but are mainly from the subsequent hypoxic stress.

Community assembly by limited recruitment in a hypoxia-stressed soft bottom: A case study of macrobenthos in Ariake Bay

“…As the severity of hypoxia differs depending on the year, not all summer hypoxic events cause drastic changes in benthic communities. However, occasional severe hypoxia causes mass mortality of macrobenthos (Yoshino et al 2010). Hypoxia would surely contribute to the disappearance of other large species from the benthic community in the present study area over a long period.…”

“…Benthic communities are devastated during years of severe hypoxia, as observed in 2006 (Yoshino et al 2010). Small short-lived species can quickly recover during years of moderate hypoxia, which partly explains why small species dominate in hypoxia-stressed sites in general.…”

“…As a result, the community became characterized by small species, such as the semelid bivalve Theora fragilis (Adams 1855), the venerid bivalve Veremolpa micra (Plisbry 1904), the sternaspid polychaete Sternaspis scutata (Ranzani 1807), and the spionid polychaete Paraprionospio cordifolia (Yokoyama 2007), which were also abundant before deterioration (Yoshino et al 2007). Hypoxia repeatedly occurs during summer in this area, particularly around the neap tide when the tidal mixing force is weak (Tsutsumi 2006;Hayami 2007), which impacts the benthic ecosystem (Yoshino et al 2010). It is well known that small species dominate the community at hypoxia-stressed sites (Diaz and Rosenberg 1995).…”

Two-year temporal response of benthic macrofauna and sediments to hypoxia in a tropical semi-enclosed bay (Cienfuegos, Cuba)

“…damming, use of fertilizers, urban effluents) . Moreover, in semi-enclosed bays the reduced exchange with the open sea promotes water stagnation and reinforces hypoxic conditions (Yoshino et al, 2010) which may result in total absence of oxygen (i.e. anoxia).…”

Implications of changes in the benthic environment and decline of macro-benthic communities in the inner part of Ariake Bay in relation to seasonal hypoxia

“…These results indicate that the rapid decline in the macro-benthic communities was not simply caused by the progress of organic enrichment of the sediment with the development of reduced conditions, but occurred even in the oxidic sediment with much less oxygen consumption of the overlying water. Yoshino et al (2010) examined the distribution of the macro-benthic communities and the major environmental factors that controlled their distributions in the inner part of Ariake Bay, and found that the changes in the structure of the macro-benthic communities were not caused by stress as a result of the increased TOC content of the sediment, but that they mainly arose from physiological stress caused by the occurrence of hypoxic water. Our results suggest a possibility that the transportation of hypoxic water by the tidal currents may explain these phenomena, since Ariake Bay has the largest tidal amplitude (over 6 m at the spring tide) on the Japanese coast, and it often generates a fast tidal current along the longitudinal axis of the bay (Inoue 1980, Odamaki et al 2003.…”