Impaired megabenthic community structure caused by summer hypoxia in a eutrophic coastal bay

Abstract

Eutrophication and hypoxia are major problems affecting the health of coastal ecosystems throughout the world. Tokyo Bay, Japan, is a eutrophic coastal area where the abundance of the megabenthic community has been decreasing. To assess factors associated with the impaired biota, seasonal surveys of the megabenthic community and water and sediment quality were conducted in the bay. Cluster analysis showed a difference in the community structure between the northern and southern parts of the bay. The density of species and species diversity were high throughout the year in the southern part of the bay, whereas in the northern part of the bay species diversity was low and defaunation occurred in August. At this time, bottom hypoxia due to temperature and salinity stratification, and high concentrations of nutrients, chlorophyll a, and organic matter in the water column and/or sediment, dominated the northern part of the bay. In October, bottom hypoxia was less severe but was still present in the northern part of the bay, and recolonization by mobile fishes and sessile mussels occurred. Multivariate analyses of the megabenthic community and environmental parameters in August showed the spatial pattern of the community could be explained by concentrations of dissolved oxygen and particulate organic carbon in the bottom water, and total sulfide and total organic carbon in the sediments. In particular, impairment of the biota in the northern area could be explained by the threshold concentrations of dissolved oxygen < 1.7 mL L(-1) and total organic carbon > 20.3 mg g(-1).

High spatial resolution analysis of the distribution of sulfate reduction and sulfide oxidation in hypoxic sediment in a eutrophic estuary

“…Moreover, the fine particles comprising the sediment at Site 3 may reduce the diffusive transport of oxygenated species (e.g., DO, nitrate and ferric oxide) and eventually stimulate sulfate reduction. Kodama et al (2010) also reported severe bottom hypoxia (<45 μM of DO) in a northern area of Tokyo Bay, including all sites in this study in August 2005, and found H 2 S concentrations were higher in silt and clay sediments than in sand.…”

“…In particular, urbanized and industrialized watersheds can result in large inputs of organic matter and nutrients into estuaries. Degradation of organic matter in an estuary results in reduction or complete depletion of dissolved oxygen (DO) in the bottom water, which are termed hypoxia and anoxia, respectively (Kodama et al 2010;Sato et al 2012). …”

“…Because H 2 S is both toxic to benthic biota and consumes DO, H 2 S production is harmful to estuarine ecosystems and should be prevented (Kodama et al 2010). However, the mechanism of H 2 S production is complicated and depends on sediment physicochemical properties including DO, oxidation-reduction potential (ORP), pH, nitrate, sulfate and organic matter concentrations, as well as sediment grain size and porosity (Valdemarsen et al 2009); therefore, it is difficult to control.…”

Diatom-derived oxylipins induce cell death in sea urchin embryos activating caspase-8 and caspase 3/7

“…The transparent embryo enables easy observation of malformation, making it possible to detect sub-lethal effects of pollutants on multicellular body formation at an early stage in development. To date, the stressors that have been examined using the sea urchin as a model include physico-chemical changes in the water, such as acidic pH (Dupont et al, 2010), hypoxia (Kodama et al, 2010), UV Schröder et al, 2005;Bonaventura et al, 2005Bonaventura et al, , 2006Lister et al, 2010aLister et al, , 2010bRusso et al, 2010), X-rays Bonaventura et al, 2011), and chemicals such as antifouling agents/pesticides (Garaventa et al, 2010;Aluigi et al, 2010), endocrine disruptors (Sugni et al, 2010;Horiguchi et al, 2010) and metals (Pinsino et al, 2014;Kiyomoto et al, 2010). Natural toxins represent a major source of stress for marine organisms.…”

Impacts of shifts in spawning seasonality and size at maturation on the population growth of mantis shrimp in Tokyo Bay

“…Settlement inhibition due to seasonal hypoxia (when dissolved oxygen reaches levels at which most animals suffocate; < 2 mg l -1 ; Diaz 2001) is the only explanation provided so far. In the bottom waters at the head of Tokyo Bay, hypoxic conditions occur annually in the summer (Ando et al 2005), causing high faunal mortality (Kodama et al 2010). The disproportionate survival observed for 1 of the 2 seasonal spawnings in a benthic fish population in Tokyo Bay (dragonet Callionymus valenciennei) also might have been caused by seasonal hypoxia (Ikejima & Shimizu 1999).…”