Tohru Seiki

Sediment oxygen demand in Hiroshima Bay

Abstract Effects of environmental factors, depth and layer thickness of sediment, organic matter, reducing substance, oxygen concentration and temperature, on sediment oxygen demand (SOD) were examined by laboratory experiments. Apparent effects on the SOD rates were recognized from each factor, except for the layer thickness and organic matter content in sediment. The SOD rates varied with sediment depths, which was attributed to the reducing substances in sediments. Temperature ( T ) revealed the most marked effect among these factors, and SOD rates increased linearly with temperature in the range of 10–30°C. Oxygen concentration in overlying water did not influence the SOD rate when it was over 2–3 mg/l, but influenced below this concentration. The SOD rates in Hiroshima Bay, measured by both an in situ chamber (bell jar) and a laboratory method with intact cores, agreed well with each other and ranged from 0.10 to 0.61 g O 2 /m 2 /d, with a mean of 0.3–0.4 g O 2 /m 2 /d on an annual basis. The rates in summer and autumn were twice those in winter. It also became evident in this study that SOD rates in Hiroshima Bay were mainly controlled by biological consumptions from summer to autumn and by chemical consumption of the reducing substances during winter and spring.

Benthic nutrient remineralization and oxygen consumption in the coastal area of Hiroshima Bay

In situ and laboratory measurements of nutrient fluxes across benthic sediments and the sediment oxygen demand (SOD) rates were made and compared in a shallow, eutrophic coastal area of Hiroshima Bay. The difference in the SOD rates was sarcely noticed between the two methods, but the nutrient fluxes from in situ sediments were about 2 times higher than laboratory measurements. The SOD rates in the coastal area of Hiroshima Bay were in the range of 0.20–0.61 g/m2/day. On the other hand, the in situ values of the benthic fluxes of nitrogen and phosphorus across the surface sediments were in the ranges of 3.7–53.7 g N/m2/day and −0.9 to 17.2 g P/m2/day, respectively, and showed higher value, 10 times greater in summer than in winter or spring. It became clear from comparing the sedimentation rates of particulate matter suspended in the water with the benthic fluxes that 60–70% of the particulate nitrogen and phosphorus which settled on the sediments might return again to the water column.

Limiting nutrients of phytoplankton community in Hiroshima Bay, Japan

Abstract Ratios of DIN:DIP, indigenous algal assay and nitrogen fixation by phytoplankton were determined to estimate the limiting nutrient for phytoplankton growth in Hiroshima Bay, Seto Inland Sea, Japan. The algal assay was performed using indigenous pico-, nano- and microphytoplankton assemblages fractionated by filtration. Nitrogen fixation was determined using an acetylene reduction method. The DIN:DIP ratio in the study area was above 16:1 (Redfield ratio) in August 1993 and May 1994. However, in other months, the ratio was below 16:1. These values suggest that the growth of phytoplankton community is limited by nitrogen, with an exceptional phosphorus limitation in August 1993 and May 1994. Nutrient enrichment algal assay of size-fractionated phytoplankton community also supported these findings. The growth of picophytoplankton fraction was stimulated by phosphorus addition in August 1993 and May 1994, while nano- and microphytoplankton fractions were stimulated by nitrogen addition in almost all months during the study period. No nitrogen fixation by phytoplankton was detected during the study period. The results suggest that nitrogen rather than phosphorus is the limiting nutrient for the growth of the whole phytoplankton community, but picophytoplankton by itself, tends to be limited by phosphorus compared to other large-size phytoplankton when high DIN:DIP ratio was observed.

Decomposition characteristics of particulate organic matter in hiroshima bay

The decomposition characteristics of particulate organic matter (POM) sampled with plankton nets in Hiroshima Bay were investigated under aerobic conditions in a laboratory experiment.The POM derived from plankton consisted of both a labile fraction (70–80 % of the whole) and a refractory fraction (20–30%). The labile fraction was completely decomposed within 40 days at 20°C. Although the concentrations of particulate organic carbon (POC) decreased gradually with time, an apparent lag phase was recognized in the decomposition of particulate phosphorus (PP) at an early stage, which might result from a specific uptake of dissolved inorganic phosphorus (DIP) by bacteria. A comparison of the metabolic activity between dissolved organic matter (DOM) and POM by measuring ATP contents showed that the former was one order of magnitude larger than the latter.On the other hand, there was no significant difference among the decomposition rates of POM collected at various depths. The change of the first-order rate constant (k) for the POM decomposition by temperature was expressed ask=0.0329 exp(0.0644T), and the Q10 value was 1.94. There were fairly large variances ink values obtained from the various plankton species. Thek values averaged 0.144 day−1 and ranged from 0.078 to 0.20 day−1 at 20°C.