Machiko Yamada

Standing stock and production rate of phytoplankton and a red tide outbreak in a heavily eutrophic embayment, Dokai Bay, Japan.

The seasonal variation of phytoplankton biomass and primary productivity in a heavily eutrophic embayment, Dokai Bay, Japan, was determined. Dokai Bay was characterized by high phytoplankton biomass and productivity during summer and low phytoplankton biomass and productivity during other seasons. The results suggested that phytoplankton growth was limited by only irradiance and water temperature under the high nutrient concentrations available for phytoplankton growth in the entire year. Moreover, in spite of sufficient nutrient for phytoplankton growth in the entire year, a red tide occurred only in the summer period in this bay. Our results suggested that a red tide occurred by the high phytoplankton growth rate in the summer season, but in other periods surface phytoplankton was flushed out of the bay before forming the red tide, because phytoplankton growth rate was low and could not form the red tide due to low irradiance and low water temperature.

Effects of temperature and irradiance on growth of strains belonging to seven Skeletonema species isolated from Dokai Bay, southern Japan

The effect of temperature on the growth of Skeletonema ardens, S. costatum sensu stricto, the S. marinoi–dohrnii complex, S. japonicum, S. menzelii, S. pseudocostatum and S. tropicum isolated from Dokai Bay in southern Japan were examined under five to seven different temperatures and an irradiance of 150 µmol m−2 s−1. The effect of irradiance on the growth of the seven Skeletonema species was also examined under a wide range of irradiances ranging from 7–700 µmol m−2 s−1 at 20°C. All Skeletonema species were able to grow at temperatures ranging from 15 to 25°C. Intra-species differences in specific growth rates of four strains for S. menzelii, and five strains for S. ardens, the S. marinoi–dohrnii complex, S. japonicum and S. tropicum were not significant (Kruskal–Wallis test, P > 0.05). Significant inter-species differences in specific growth rates were observed at 10, 15, 25 and 30°C (Kruskal–Wallis test, P < 0.01; Steel–Dwass test, P < 0.01). The S. marinoi–dohrnii complex and S. japonicum grew faster than other species at the lower temperatures of 10 and 15°C, and S. ardens and S. menzelii grew at the highest temperature of 35°C. The maximum specific growth rates (μ max) from growth–irradiance curves ranged from 1.50 to 3.44 d−1. Threshold values of irradiance (I 0) and saturation irradiance (S) for growth ranged from 3.9 to 7.6 µmol m−2 s−1, and from 250 to 740 µmol m−2 s−1, respectively. In Dokai Bay, our results suggested that the occurrence of Skeletonema species should be affected mainly by temperature and less by irradiance. In particular, only two species, the S. marinoi–dohrnii complex and S. japonicum could maintain their populations in the surface water during the cold season, whereas during other periods, all seven species could maintain their population under the strong influence of estuarine circulation, which rapidly flushed the surface water out of Dokai Bay. Temperature and irradiance dependent growth values were in good agreement with their geographical distributions. The S. marinoi–dohrnii complex and S. japonicum were capable of growing in cold regions, but our results suggested that S. ardens and S. menzelii will tend to prefer tropical regions.

Impacts of Oxygen-Deficient Water on the Macrobenthic Fauna of Dokai Bay and on Adjacent Intertidal Flats, in Kitakyushu, Japan

Abstract Dokai Bay is surrounded by the city of Kitakyushu, Japan. We assessed the chemical conditions of its waters and sediments, and carried out quantitative samplings of the macrobenthic fauna of the bay and on adjacent intertidal flats from 1992 to 1993. Large amounts of organic matter were deposited on the floor of the bay and intertidal flats due to eutrophication, and resulted in the occurrence of oxygen-deficient water from the bottom to the middle layer during the summer. On the tidal flats, the sediments were kept oxygenated and accessible to benthic animals throughout the year. However, the macrobenthos declined markedly in summer. The oxygen-deficient water of the bay seemed to temporarily upwell onto the intertidal flats causing a catastrophic environmental disturbance. Following this, only a few polychaete species survived, in extremely low densities.

Recovery of the marine bottom environment of a Japanese bay

Abstract Dokai Bay lies adjacent to Kitakyushu city, one of Japans major cities with a population of more than 1 000 000. In this city, various heavy chemical-industrial plants have been established since the 1900s. Waste water from the factories and untreated sewage effluent from the city have heavily polluted the water and marine bottom environment of the bay. In the 1960s, this bay was called the ‘dead sea’ owing to the apparent absence of aquatic organisms. Since 1970, environmental recovery projects have been carried out by the local government. The effluent and waste water have been severely regulated. The polluted bottom sediments of the bay have been dredged. Consequently, in the past two decades, the environmental conditions of the bay have dramatically recovered. Since 1989, we have assessed the water and benthic conditions of the bay to describe the recovery of the benthic ecosystems, and to monitor the effects of environmental recovery projects on the bottom environment of the bay since 1970. The results of these studies indicate a drastic decrease in the levels of heavy metals in the bottom sediments and the recolonization of various benthic organisms, although the innermost areas of the bay remain seriously organically polluted. In these areas, in the summer, the benthic communities are seriously damaged owing to the occurrence of anoxic water masses. Prior to 1993, there was no regulation of the amount of nutrient salt loading present in factory waste water and city sewage. Overloading of nutrient salts generates large amounts of primary production by phytoplankton, and results in the overloading of organic matter at the bottom environment in the innermost areas of the bay. For further recovery of the ecosystem it is necessary to control the total amount of loading of nutrient salts to the bay system.

Decrease in anthropogenic nutrients and its effect on the C/N/P molar ratio of suspended particulate matter in hypertrophic Dokai Bay (Japan) in summer

Biologically important nutrient concentrations in Dokai Bay have declined as a result of reductions in anthropogenic inputs of total nitrogen and total phosphorus. A decrease in nutrient concentrations affects phytoplankton growth, thereby changing the biochemical characteristics of autochthonous particulate matter. We therefore investigated changes in the C/N/P molar ratio of suspended particulate matter (SPM) in the summer, when phytoplankton growth is vigorous, before environmental quality standards (EQSs) were attained (1995–1998) and afterward (2006–2009). We found that the ratio of particulate organic nitrogen (PON) to particulate phosphorus (PP) changed in conjunction with changes in the ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus (DIP) that resulted from reductions in nutrient loading. Furthermore, we suggest that because the DIP concentration in seawater was high before EQSs were attained, inorganic phosphorus was possibly adsorbed onto SPM. After the attainment of EQSs, however, the DIP concentration fell, and PON/PP was high. Phosphorus limitation of phytoplankton growth in the mouth of the bay may explain the high PON/PP ratios after EQS attainment.

A Challenge of Water Purification in Dokai Bay, Japan

A history of water pollution and countermeasures for water purification in Dokai Bay, Japan are reviewed. Now, Dokai Bay suffers from the occurrences of red tides and oxygen-deficiency in summer. In order to prevent the occurrences of red tides and oxygen-deficiency, an ecological numerical model has been developed. The model experiments forecast that when the phosphorus or nitrogen load from land will be cut by 90% or 95%, respectively, red tides and oxygen-deficiency will not occur in Dokai Bay. If the industrial and sewage facilities are not sufficient to cut the nutrients load to the necessary degree, we have to consider other countermeasures such as cultivating bivalves and algae in order to decrease the nutrients concentration in the bay. International co-operation related to coastal zone management is also discussed.

Utility of mitochondrial-encoded cytochrome c oxidase I gene for phylogenetic analysis and species identification of the planktonic diatom genus Skeletonema

Small subunit (SSU) and large subunit (LSU) rDNA sequences have been commonly used to delineate the taxonomy and biogeography of the planktonic diatom genus Skeletonema, but the genes occur as multiple copies and are therefore not suitable for barcoding purposes. Here, we analyzed phylogenetic relationships of Skeletonema using the mitochondrial‐encoded cytochrome c oxidase I gene (cox1), as well as partial LSU rDNA (D1–D3) and SSU rDNA, to identify the factors that define species and to evaluate the utility of these three markers for this taxon. Twelve Skeletonema species were divided into six clades, I–VI, each of which comprised the same species by the three markers: clades I (S. japonicum, S. grethae, S. pseudocostatum, and S. tropicum), II (S. menzelii), III (S. dohrnii and S. marinoi), IV (S. costatum, S. potamos, and S. subsalsum), V (S. grevillei), and VI (S. ardens). However, the branching order among these clades was incongruent among the markers. In clade III, six S. marinoi strains had identical cox1 sequences. These S. marinoi strains branched along with S. dohrnii, except for strains from the Gulf of Naples, with high support in cox1. Species delimitation between S. dohrnii and S. marinoi was therefore not supported. In clade IV, S. costatum and S. subsalsum were robustly clustered, with S. potamos as a sister clade in the cox1 tree, not in the LSU and SSU trees. In clade II, cox1 also confirmed that S. menzelii includes three subclades potentially distinguishable from each other by morphological features. Cox1 proved to be the most useful marker for the identification of Skeletonema species because it gave a tree with highly supported clades, has sufficient variation within and among species, encodes a protein in a single copy, and requires relatively few primers.