Hydrogen sulfide is generated through sulfate reduction under anaerobic condition in enclosed coastal seas. It is highly toxic, depletes oxygen and generates blue tide. To evaluate the sulfide reduction effect of steelmaking slag, we carried out field experiments in Fukuyama inner harbor, where the people there have suffered from odor caused by gasses including hydrogen sulfide generated from the sediments. We placed the steelmaking slag on the sediments, and monitored the water quality of interstitial water in the sediments and the overlying water of the sediments as well. Hydrogen sulfide gas was also collected and measured.The results showed that dissolved sulfide concentrations in interstitial water in the control plots ranged from 100 to 350 mgS/L, on the other hand, those in the steelmaking slag construction area were suppressed, being below 5 mgS/L. The reduction effect of dissolved sulfide by steelmaking slag has lasted for about two years. It was supposed that Fe ions eluted from steelmaking slag may have reacted with sulfide. Species number and individual numbers of macro benthos increased in the experimental area.The results imply that capping the deteriorated sediments with steelmaking slag can effectively improve the water and sediment quality of coastal areas.
In the present study, a numerical model that simulates species competition between the toxic dinoflagellate Alexandrium tamarense and the non-toxic diatom Skeletonema costatum was constructed using data from a number of experiments and field observations. In the model, not only vertical migration of swimming cells but also the encystment and excystment processes of A. tamarense were taken into consideration. Sinking of S. costatum cells was also considered. Both zooplankton and oysters, which are intensively cultured in this bay, feed on the phytoplankton but the contribution of grazing to the decrease in the cell densities of both species was small. The model predicts that while sinking is one of the major processes that reduce the cell density of S. costatum, encystment of A. tamarense is a crucial process that determines the cessation of blooms of this dinoflagellate. Advection and diffusion are also important physical processes that affect, in positive and negative ways, the fluctuation of cell densities.
Artificial reefs (ARs) have been implemented as a tool to increase fish catch by gathering fish and creating new habitat. In Japan, fisheries production has been decreasing in the last several decades. Research on ARs to date has mainly focused on fish biomass and community structure. The present study describes the community structure of marine organisms attached to the ARs deployed in Mitsu Bay, Hiroshima, Japan. We compare those communities on three types of ARs, which are made of timbers (ATRs), ATRs with oyster shells (ATRsOS) and ATRs with leaves and branches (ATRsLB). Attached organisms were collected seasonally from 2016 to 2018. Overall, 272 taxa were identified from two deployment sites. Arthropods were the dominant group identified, followed by molluscs and annelids. Seasonal variation of individuals collected was observed, with numbers being high in summer and low in winter. The number of individuals was high in the first year after deployment of ATRs, decreasing in the second year and beyond, suggesting the animal community may have matured by balancing the growth and feeding by fish. All three types of ATRs were commonly large in individual number and small in species number, characterized by a low diversity index. The highest individual number and highest species number were observed in ATRsLB and ATRsOS, respectively. More specifically, the diversity index for simple ATRs was lower than those for ATRsOS and ATRsLB. This result suggests that ATRs with additional materials can provide a wide range of feed animals which may attract more fish. The deployment of ATRs made of materials like timbers and oyster shells is also good practice for promoting a recycling-oriented society.
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