Chemical and microbiological parameters were combined to characterize vertical and horizontal distribution of sediments in Kagoshima Bay. Sediments from four contrasting environmental settings were analyzed to determine their texture, total organic matter (IL), acid volatile sulfide (AVS), free sulfide and sulfate-reducing bacteria (SRB) during the summer and autumn of 2004. Sediments taken at the inner and central areas contained significant silt and clay contents, while the outer area had sand and gravel as the main components. IL concentrations were ranged from 2.1% to 13% and higher IL contents were observed at the stations located in central area and station near Amori River (inner area). AVS were ranged from 0.01 to 0.9 mg/g of sediment (dry weight) and showed significant difference between sites. Free sulfide were observed only in the stations belong to the inner area, with the range from 0.01 to 0.2 mg/g of sediment (dry weight). SRB were ranged from 1.7×102 to 5.0×105 cell/g of sediment (dry weight). The results give a clear picture of the horizontal and vertical characterization of the sediments and also make clear the differences at four stations belong to the inner, central and outer areas. It also showed that, coupled with chemical indicators, sulfate-reducing bacteria can be used as a useful indicator to evaluate the sediment condition of Kagoshima Bay.
The distribution and abundance of photoautotrophic picoplankton (PPP. Synechococcus group) in the aphotic bottom sediments of Lake Biwa were investigated by direct counting and viable counting (most probable number, MPN) methods. In the surface layer of bottom sediments (0–1 cm). where large PPP blooms occurred in the past 5 years, >105 cells cm−3 of PPP were found to be viable throughout the year. Furthermore, the density of PPP deposited on the sediment surface (0–0.1 cm) was one order of magnitude higher (MPN = 1.3 × 106 cells cm−3. direct count = 9.9 × 106 cells cm−3) than that of bulked surface sediments (0–1 cm). Even in the deeper layer (13–14 cm) of bottom mud, viable PPP were still found (101 cells cm−1. In winter, viable PPP in the aphotic bottom sediments were 104–105 times greater per Unit volume than those in the euphotic lake water. Since the aphotic bottom sediments have high levels of PPP, as well as high growth potential (high ratio of viable count/total direct count), they are likely to seed PPP blooms in the North Basin of Lake Biwa.
Phytoplankton growth can cause red tide, lowering water quality and damaging fisheries. The simple Lotka–Volterra model (LV model), estimating the population changes in different species, is often used to analyze the growth and competition of causative species. However, the LV model has not been sufficiently applied to examining the actual occurrence and development of red tides in the field. Therefore, the mechanism of seasonal dominant species replacement and applicability of the LV model were studied to analyze the competition between the harmful raphidophytes Chattonella marina and Heterosigma akashiwo. Sensitivity analysis was performed, and optimization method of competition coefficient (α) was discussed by comparison with measured data from culture experiments. We found that it is crucial to evaluate in advance the initial cell density conditions under which the population size changes concerning α (the order of root mean squared logarithmic error ≥ 10−2). Using this as a reference, we pointed that the initial cell density difference between the two species in co-culture experiments should be as large as possible. In addition, simulation analysis using the model revealed that growth rate (r) and carrying capacity (K) determine H. akashiwo dominance in spring, and the increase in α due to rising temperatures causes C. marina dominance in summer. Comparing α of C. marina with α of C. antiqua cited from a previous study found that the formation of red tides of C. marina less than that of C. antiqua can be explained by α that is 0.42 times smaller than α of C. antiqua. Moreover, the increase in α with increasing temperature is related to the physiological and ecological characteristics of C. marina, which is more likely to dominate the space suitable for growth with temperature rise. We consider that investigations of α, in particular, contributes to modeling various competition and more improving accuracy in analyses. These results provide valuable scientific findings on the occurrence and development of red tide flagellates and their analytical method.
The effect of an oxygen-releasing compound (ORC) magnesium peroxide (MgO(2)) on the changes in the bacterial community in organically polluted sediment of aquaculture farms was tested in a microcosm experiment. The sediment, to which fish feed was added, was treated with 1% or 5% MgO(2). The addition of fish feed induced a highly reduced environment with low redox potential, high total sulfides, and abundance of sulfate-reducing bacteria (SRB) . Although the sediment remained highly reduced at 1% MgO(2), there was a significant reduction of total sulfides, increase of redox potential, and resultant reduction of SRB. The bacterial community clearly changed with the treatments according to denaturing gradient gel electrophoresis (DGGE) analysis of 16S ribosomal RNA gene (16S rDNA) . Aerobes disappeared in the fish feed-added sediment, and some SRB emerged in place of these aerobes. On the other hand, the SRB disappeared in the ORC-amended sediment due to its highly oxic condition. This study revealed the bacterial community in the sediments was affected mainly by the redox potential and resultant sulfides produced by SRB, but total organic carbon and nitrogen were not determinants of the microbial population.
Seasonal changes in the photosynthetic activity of pico phytoplankton and its contribution to total primary productivity in surface waters at eleven pelagic stations in the north and south basin were examined. Chlorophyll-α concentration of the pico phytoplankton was 0.3 to 5.7 mg chl.a m-3, and was two times higher in the south basin than in the north basin. High standing crops were observed from late July through August. The photosynthetic rate of the pico phytoplankton was 0.9 to 36 mg C m-3 hr-1 and changed seasonally according to a pattern. High values were obtained in the south basin and throughout the lake during summer. The seasonal changing pattern in photosynthetic rates of the larger phytoplankton showed quite a difference from that of the pico phytoplankton. The assimilation number of the pico phytoplankton was 1.8 to 7.3 mg C mg chl.α-1 hr-1 and changed seasonally with high values in summer. The percentage of the standing crop of the pico phytoplankton in the total phytoplankton was generally low, but an appreciable increase was observed from late July to early October. However, the contribution of the pico phytoplankton to primary productivity was lower. In summer, when a high standing crop and high assimilation number of pico phytoplankton were observed, however, the pico phytoplankton made a major contribution to the total primary productivity. The present results suggest that the photoautotrophic picoplankton, Synechococcus, is one of the principal primary producers and plays a significant role in the biogeochemical carbon cycle in the pelagic area of Lake Biwa.
To understand the pattern of sedimentation rates as fundamental physical parameter of coastal environment, the 210Pb dating method was applied to core samples collected from Kagoshima Bay, Southwestern Japan. The sedimentation rate varied at each location within the bay (0.08–0.30 g·cm−2·y−1), and the rate at the bay-head area was less than that at the centre of the bay. The inventory of ex210Pb has a lower value in the bay-head area. The low ex210Pb inventory at Stn.5' is considered to be due to physical, and chemical conditions in the bay-head area.
