Changes in Fishery Resources in Relation to Water Environments in Osaka Bay

March 15, 2015An attempt was made to analyze the relationship between the change in the quantity of fishery resources and the change of phosphorus load deposited into Osaka Bay. The phosphorus load had steadily increased from 1955 to 1975, then decreased slowly towards the 2000s. The major benthic fishery organisms, such as octopus, shrimp and crab, mantis shrimp, tongue-fish and right-eye flounder, were employed to examine the characteristics of change in fishery resources in relation to water environments of Osaka Bay. Excluding shrimp and crab, catch amounts showed a largely equivalent response to phosphorus load in the bay. The results presented here demonstrate that a close relationship exists between phosphorus emissions and benthic fishery resources in Osaka Bay

Phosphate Uptake and Growth Kinetics of Skeletonema costatum isolated from Hiroshima Bay

広島湾より分離したSkeletonema cestatumのリン酸塩取り込みと増殖に関する動力学的解析を行った。リン枯渇細胞を用いて解析したリン酸塩取り込みの半飽和定数は0.68μM、最大取り込み速度は38.4fmol cell-1 h-1、最大比取り込み速度は11.6h-1であった。また、半連続培獲を用いて測定したところ、最小細胞内リン含量は2.80fmol cell-1、最大比増殖率は1.25d-1が得られた。以上の結果から、S. costatumは低濃度のリン条件下で他種よりも有利に増殖することが可能であるが、広島湾表層海水中のリン酸塩濃度においては、本種でさえも、その増殖をリンによって制限されているものと推察された。Phosphate uptake and growth kinetics were investigated in a diatom Skeletonema costatum isolated from Hiroshima Bay. Phosphate uptake kinetic parameters, i.e. half saturation constant, maximum uptake rate and maximum specific uptake rate, examined using phosphorus-starved cells were 0.68 μM, 38.4 fmol cell-1 h-1 and 11.6 h-1, respectively. Growth kinetic parameters, i.e. minimum cell quota for phosphorus and maximum specific growth rate, examined by semi-continuous culture method were 2.80 fmol cell-1 and 1.25 d-1, respectively. These results show that, even S. costatum, the growth might be limited by the low phosphorus concentration in the surface seawater of Hiroshima Bay, although S. costatum has an advantage in growth under phosphorus-limited conditions compared to other phytoplankton species

Possibility of the deterioration of Coastal and Shelf Ecosystem due to the Change in the Nutrient Input Ratio

We evaluate the possibility of the deterioration of the coastal marine ecosystem based on the following "silica deficiency hypothesis". While the loadings of nitrogen (N) and phosphorus (P) are enhanced by the human activities, dissolved silicate (DSi) supplied naturally tends to be trapped in the eutrophicated still waters such as dammed reservoirs, which are also increasing globally. The consequent change in the N: P: Si stoichiometric ratio of the river water flowing into the coastal sea may be advantageous to flagellates (nonsiliceous and potentially harmful) but not to diatom (siliceous and mostly benign). We develop it further to the "extended silica deficiency hypothesis": while the diatoms effectively draw down the substances from the upper layer with their sinking after the spring bloom (biological pump), other algae do not. Such a change will cause the retention of the surplus nutrients and biogenic substances in the upper layer and will be significant in the coastal and shelf seas of the east Asia

Utilization and Excretion of Dissolved Organic Phosphorus by Atexandrium tamarense (Hiroshima Bay Strain)

有毒渦鞭毛藻 Alexandrium tamarense 広島湾株による溶有態有機リン(DOP)の取り込みと排出に関して、広島湾汲み置き海水を用いたパッチ培養と半連続培養による実験を行った。バッチ培養では、培養液中のリン酸塩濃度約2μMを境に、それ以下で有意な正味のDOP取り込み、それ以上で正味の排出が見られた。半連続培養では、A. tamarense は希釈率が大きく、培地中の十分なリン酸塩を利用して活発に増殖している場合にDOPの正味の排出を示した。以上のことから、予備的実験結果ではあるが、A. tamarense 広島湾株はリン酸塩濃度が低い場合にはDOPを利用し、リン酸塩濃度が高くて活発に増殖している場合にはDOPを排出することが示唆された。Capability of utilization and excretion of dissolved organic phosphorus (DOP) by the toxic dinoflagellate Alexandrium tamarense (Hiroshima Bay strain) was investigated with both batch and semi-continuous cultures using natural aged-seawater from Hiroshima Bay. In the batch cultures, significant net DOP uptake was detected in phosphate concentrations below 2μM, while net DOP excretion was observed above this concentration. The semi-continuous culture showed that A. tamarense excretes DOP at high dilution rates. At these rates, this organism grows fast by utilizing sufficient phosphate in the medium. These experiments, although preliminary, suggest that A. tamareme (Hiroshima Bay strain) can utilize DOP of natural seawaters under a condition of low phosphate concentration and excretes DOP under a condition of high phosphate concentration

Spatial and Temporal Variations of Biophilic Elements in Hiroshima Bay, Japan, during 1991-2000, with Special Reference to the Deviation of Phytoplanktonic C : N : P Ratio from the Redfield Ratio

1991～2000年の広島湾海水中の親生物元素(C, N, P, Si)の時空間的変動をまとめた. Ch1α濃度は北部海域表層でブルームに相当する高い値がしばしば観測された. 表層溶存態無機リン(DIP)濃度は1990年代前半の春季～夏季にしばしば検出限界以下となり, 同時にDIN:DIP比およびDSi:DIP比が高い値を示した. 広島湾のTN:TP比は太田川水中のTN:TP比に比べて低く, 広島湾において脱窒が盛んであることが示唆された. 植物プランクトン態のC:N比が10年間横這いであったのに対して, N:P比およびC:P比は1990年代前半に低い値を示し, 次第に上昇して1998年にピークとなり, 2000年にはレッドフィールド比に戻った. これらの親生物元素の時間的・空間的変動に関し, リン・窒素の負荷削減対策との関連について考察した.We described the spatial and temporal variations of biophilic elements (C, N, P and Si) in Hiroshima Bay, the Seto Inland Sea, Japan, during 1991-2000. There was no year to year trend in chlorophyll a concentration. However, depletion of surface dissolved inorganic phosphorus (DIP) in spring and summer in early 1990s was obvious, often at concentration less than the detection limit, giving extremely high DIN : DIP and DSi : DIP ratios. The TN : TP ratio in seawater of Hiroshima Bay was low compared to that in the Ohta River water which empties into the bay, suggesting denitrification process occurs in Hiroshima Bay. In contrast to stable phytoplanktonic-C: N ratio during this decade, phytoplanktonic-N : P and C: P showed large deviation from the Redfield ratio, low in the early 1990s, high in 1998, and then equivalent to the Redfield ratios in 2000. The spatial and temporal variations in these biophilic elements were discussed in relation to the reduction measure of phosphorus and nitrogen