摘要
The present water quality conditions in the Seto Inland Sea of Japan are described along with an historical background before and after the measures taken to reduce eutrophication. The directive to reduce phosphorus discharge into this area was very effective, reducing the number of red tides from about 300 cases per year at their peak in 1976 to the recent level of about 100 cases per year, indicating the improvement of seawater quality. However, the hastiness of the measures taken to reduce phosphorous seems to have led to a depletion of dissolved inorganic phosphorus that is an essential nutrient for the growth of phytoplankton. Fishery production has also decreased with the reduction of phosphorus, showing a time lag, and the relationship between them shows a hysteresis-like pattern indicating that the condition of fishery production is currently in a critical stage of collapse. This implies that the phosphorus reduction could have lowered the phytoplankton primary production and also caused a detrimental effect on the fishery production. Noteworthy is the change in the phytoplankton species composition. The dominant species that form red tides have changed from non-harmful diatoms to harmful raphidophytes in the eutrophication process and then finally to harmful/toxic dinoflagellates in the oligotrophication process. This indicates that the measures to reduce phosphorus have caused a change in phytoplankton species composition, thereby altering the food web structure, suggesting that this might be the major cause of the reduction of fishery production. In conclusion, the Seto Inland Sea of Japan is apparently in an oligotrophic condition, and it could be said to be in the state of “cultural oligotrophication” caused by the hasty reduction of phosphorus loading. Dam construction, as another possible cause of the cultural oligotrophication, is also discussed, and evidence relating to the existence of numerous dams in this area is also shown. Stress is placed on the regulation of the level of nutrients and their elemental ratio in the seawater, because these factors are considered to be effective in preventing the generation of harmful algae and in sustaining the fishery production through preservation of the natural environment. A proposal is also described regarding how these factors could be controlled.
摘要译文
描述日本濑户内海目前的水质状况以及为减少富营养化而采取的措施前后的历史背景。减少磷排放到这个地区的指示是非常有效的，赤潮数量从1976年的峰值每年约300例减少到近年来的每年约100例，表明海水水质的改善。然而，为减少磷而采取的措施似乎已经导致溶解无机磷的消耗，这是浮游植物生长所必需的营养物质。渔业生产也随着磷的减少而下降，呈现时间滞后，它们之间的关系表现出一种滞后性的模式，表明渔业生产状况目前处于崩溃的关键阶段。这意味着磷的减少可能会降低浮游植物的初级生产量，并对渔业生产造成不利影响。值得注意的是浮游植物物种组成的变化。在富营养化过程中形成赤潮的环境由无害硅藻转变为有害的桡足类，最终转变为有害/有毒的鞭毛藻。这表明减少磷的措施已经引起浮游植物物种组成的变化，从而改变了食物网结构，这表明这可能是渔业生产减少的主要原因。总之，日本的濑户内海显然处于寡营养状态，可以说是由于急剧降低磷负荷而造成的“文化寡养”状态。大坝建设，作为文化寡养的另一个可能的原因，也进行了讨论，也显示了在这个地区存在的大量水坝的证据。重视海水养分水平及其元素比例的调节，因为这些因素被认为是有效的防止有害藻类的产生，并通过保护自然环境维持渔业生产。还提出了一个关于如何控制这些因素的建议。
T.Yamamoto;. The Seto Inland Sea––eutrophic or oligotrophic?[J]. Marine Pollution Bulletin, 2003,47(1-6): 37-42