Critical events in the Ariake Bay ecosystem: Clam population collapse, red tides, and hypoxic bottom water

Small species often dominate benthic communities in hypoxia-stressed sites, but the organizing process has rarely been investigated. Community assembly was investigated in northwestern Ariake Bay, which is exposed to hypoxia during summer. Sampling was performed 11 times between May 2010 and May 2012. The timing and duration of hypoxia differed between 2010 and 2011, which caused a different pattern of seasonal change in the community structure between the 2 years. The community was mainly composed of small species such as Theora fragilis, Veremolpa micra, and Sternaspis scutata at all times. The exception was the large arcid bivalve Scapharca kagoshimensis. This bivalve newly settled after hypoxia in 2010 and became a dominant species of the subsequent community, whereas its density decreased greatly after hypoxia in 2011. Interestingly, new settlement of arc shells did not occur in 2011 when hypoxia was relatively moderate compared with that in 2010. These results suggest that small species dominance at a hypoxia-stressed site cannot be attributable only to repeated hypoxic devastation and quick recovery of short-lived small species but also to the variability in natural recruitment of long-lived large species. Large species cannot easily recover if low recruitment years continue, even if hypoxic conditions are moderate. Therefore, the need for more attention to recruitment variability of large species is emphasized in striving to understand community assembly and efficiently restore coastal soft bottom communities that have deteriorated due to hypoxia.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Yoshino

Katano

Ito

et al. 2014

“…The macrobenthic community consists of various mollusks and other taxa including two and eight endangered species registered in Korea and Japan, respectively, while the molluscan community declined drastically in Ariake Bay in Japan, where approximately 40% of Japanese tidal flats are concentrated, in the 1980s to 1990s (Kikuchi 2000, Tsutsumi 2006). In Korea, the registration of endangered species has been done by two different govern- In this paper, we have described the minimum species of macrobenthic animals that we could identify precisely.…”

mentioning

confidence: 99%

Concluding remarks on the joint survey of macrobenthic fauna on Suncheon Tidal Flats by the participants of “Korea and Japan Joint Symposium on Biology of Tidal Flats 2009”

Choi

Tsutsumi

2010

Self Cite

As a social activity in "Korea and Japan Joint Symposium on Biology of Tidal Flats 2009" held in Sucheon, Jeollanamdo, the southern part of South Korea, most of the participants to the symposium joined the surveys with the purpose of making a list of macrobenthic fauna that occur on Suncheon Bay Tidal Flats with approximately 1,200 ha in total area between June 21 and 23, 2009. In this survey, sixteen species of bivalves, twenty species of gastropods, one species of chiton, fourteen species of polychaetes, eight species of crabs, and three species of other animals were collected, and identified by the participants. Among these sixty-one species, two species of macrobenthic invertebrate animals including gastropod, Ellobium chinense, and crab, Sesarmops intermedius, are listed in the latest Red List of the Ministry of Environment of the Republic of Korea, and eight species of them including five species of potamidid gastropods are listed in that of the Ministry of Environment, Japan. The results of this survey indicate that the ecosystem on the tidal flats in Suncheon Bay are still healthy, and preserved almost in their original conditions, due to the serious efforts of the people in Suncheon City and the municipal authorities.

“…However, in Ariake Bay, the frequency and the total duration of red tides per year have increased significantly since the second half of the 1990s, despite the fact that nutrient loading from the land to the bay has not increased, and may have even decreased slightly since the 1960s (Tsutsumi 2005, 2006, 2011, Ministry of the Environment 2006. As a result of an analysis of the relationship between the scale of red tides and precipitation in the coastal areas of the bay during the 40 days preceding the occurrence of each large-scale red tide, which occurred in autumn and early winter (October to December), Tsutsumi et al (2005) clearly found a further increase in the scale of red tides in the bay since 1998.…”

Section: Introductionmentioning

confidence: 99%

“…The water of these areas is extremely muddy due to the continual resuspension of mud from the sea floor. There were very few reports of the occurrence of large-scale red tide events in the bay until the first half of the 1990s (Shirota 1980, Tsutsumi 2005, 2006, and hypoxic water occurred on a limited basis in the western part of the innermost area of the bay until the same time frame (Kyushu Regional Agricultural Administration Office 2014). In those days, the inner part of Ariake Bay, including the mega-tidal flats, was praised as a fertile sea , due to the occurrence of high densities of various organisms as well as extremely large catches of fish and shellfish (Inoue 1980, Sugano 1981, Kikuchi 2000.…”

Section: Introductionmentioning

confidence: 99%

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

Tsutsumi

Takamatsu

Nagata

et al. 2015

Self Cite

Abstract:In the inner part of Ariake Bay, located on the west coast of Kyushu, in western Japan, red tides have occurred with more frequency since the second half of the 1990s. Hypoxic waters have occurred during the summer months since the 2000s, despite the fact that nutrient loading from the land to the bay has not increased over the last five decades. We monitored water conditions at nine stations in the inner part of the bay, conducted benthic environmental surveys, and quantitative samplings of macro-benthic communities at the innermost four stations between 2002 and 2008. Each summer, the water was well-stratified due to the development of a halocline and a thermocline. The DO of the water below the pycnocline fell to hypoxic conditions. At the innermost three stations in the bay, the mud content and organic matter content of the sediment increased significantly, and the carbon stable isotope ratios of the organic matter contained in the sediment ranged between −21.3±0.5 and −20.7±0.5 of δ 13 C. These facts indicated that the organic matter was derived photosynthetically from marine phytoplankton. The increase in the mud content of the sediment indicates a deceleration in the tidal current. This may be a key event that induces a series of environmental changes and disturbances, including the stratification of the water, the more frequent occurrence of red tides, the progress of the organic enrichment of the sediment, and the occurrence of hypoxic water during the summer.