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The utilization of dissolved organic phosphorus (DOP) by the two toxic dinoflagellates Alexandrium tamarense (Lebour) Balech and Gymnodinium catenatum Grahamm which were isolated from Hiroshima Bay, Japan, was studied. Alexandrium tamarense grew poorly on fructose‐6‐phophate, glucose‐1‐phosphate, glycerophosphate, and ribose‐5‐phosphate with a phosphomonoester bond, although it grew well on the nucleotides adenosine‐5‐diphosphate (ADP) and adenosine triphosphate (ATP), as well as on dissolved inorganic phosphorus (DIP; as metaphosphate, pyrophosphate, tripolyposphate and orthophosphate). The results imply that A. tamarense was able to utilize DOP and DIP from ambient water using nucleotidase, pyrophosphatase and poly‐phosphatase, which hydrolyze phosphodiesters. In contrast, G. catenatum was able to utilize DOP compounds of various molecular weights and structures as well as DIP. In time‐course experiments, alkaline phosphatase activity (APA) was induced at orthophosphate concentrations of 0.43 mmol/L and 3.3 mmol/L for A. tamarense and G. catenatum, respectively, and APA increased with orthophosphate depletion. The experiments also demonstrated that APA was maximum at the optimum temperatures for the growth of A. tamarense and G. catenatum; that is, 15°C and 25°C, respectively. These results suggest that the DIP‐depleted conditions in Hiroshima Bay might have led to the outbreaks of noxious dinoflagellates in recent years.

Hydrogen sulfide is highly toxic and fatal to benthic organisms as well as causing depletion of dissolved oxygen and generating blue tide in eutrophic coastal seas. The purposes of this study are to reveal adsorption characteristics of hydrogen sulfide onto crushed oyster shell, and to evaluate removal efficiency of hydrogen sulfide from pore water in organically enriched sediments using container experiment in order to develop a coastal sediment amendment. The crushed oyster shell was mainly composed of CaCO(3) with calcite and CaO crystal phase. The batch experiment showed removal kinetics of hydrogen sulfide can be expressed as the first order equation and Langmuir plot fitted well in describing the adsorption behavior with the adsorption maximum at 12 mg-S g(-1). The container experiments suggested the oyster shell adsorbs hydrogen sulfide in pore water effectively and reduces oxygen consumption in the overlying water. Furthermore, oxidation-reduction potential of the sediment was higher with addition of crushed oyster shell than the control without oyster shell. Thus, it is concluded that crushed oyster shell can be an effective amendment to remediate organically enriched sediments in eutrophic coastal seas.

SUMMARY Shellfish poisoning by the toxic dinoflagellate Alexandrium tamarense (Lebour) Balech occurred for the first time in Hiroshima Bay, Japan, in 1992. Oyster culture in the bay produces as much as 60% of the total production in Japan, and it suffered severe damage. In the present study, we experimentally investigated the growth rate and phosphate uptake kinetics of A. tamarense, Hiroshima Bay strain. A short‐term phosphate uptake experiment revealed that the maximum uptake rate was 1.4 pmol P cell‐1 per h and the half‐saturation constant was 2.6 umol L‐1. In semicontin‐uous culture, the maximum specific growth rate and the minimum phosphorus cell quota were 0.54 day‐1 and 0.56 pmol P cell‐1, respectively. These uptake rates suggest that A. tamarense is a poor phosphorus competitor compared with other species. However, the large phosphorus storage capacity (Qpmax/qo= 36), the surge phosphorus uptake ability (Vs/Vi= 4.1) and the low growth rate would be advantageous for surviving brief periods of phosphorus limitation which frequently occur in Hiroshima Bay.

Fish cage farming generates large amounts of organic waste in the form of unconsumed feed and fecal matter, resulting in sediment deterioration and a threat to its own sustainability. Field studies analysed the scale of the impact of fish cage farming on sediment quality, variability of impact and spread of sediment enrichment in the vicinity of a fish farm. Two fish culture sites near the Fukuyama area of the Seto Inland Sea of Japan were chosen for the study. There was a significant difference between the quality of the sediment in aquaculture and non‐aquaculture areas. The sediment underlying the fish cage farms was found to be extremely reducing, acidic and sulfidic. Sediment ignition loss did not vary markedly with the season. However, the redox potential and acid volatile sulfide content of aquaculture areas did show some seasonal variation. Sediment encircling a 165 m radius from the center of a farm in Yokota was judged as organically enriched by fish farming. A variation in sediment quality within the two sites and also within the stations was observed. Based on sediment quality, the stations could be categorized as less affected, highly affected or moderately affected.