播磨灘北部海域におけるノリ色落ちと漁場環境の変遷

抄録

In order to elucidate the relationship between the discoloration of cultivated “Nori” (Porphyra thalli) and the change of environmental factors, we examined changes in dissolved inorganic nitrogen (DIN) and dissolved oxygen saturation of the bottom layer from 1973 to 2005, and compared with the changes of total nitrogen (TN) in the sea, loaded TN from land, reclaimed area, total number of incidents of red tides observed, and catch of benthic organisms in the northern part of Harima-Nada. Although DIN concentration in the sea water and input of TN from land decreased or remained on the same level, TN concentration in the sea water increased. Catch of large bivalves decreased in response to the decrease of the shallow area because of reclamation, but small bivalves like Manila clam increased. However, the catch of Manila clam showed a marked decrease because of extreme hypoxia that occurred in 1980 and 1998. The recovery of the catch in 1980 was rapid, but was slow in 1998. These facts imply that the decrease of shallow area has led to a decrease of suspension feeders like large bivalves, and to a loss of functions of water-purification in this area. These losses led to an oxygen deficiency and by a spiral of deterioration, small bivalves like Manila clam decreased suddenly, and thus allowed the occurrence of large-size phytoplankton like Eucampia zodiacus which is not edible for other organisms except for suspension feeders like bivalves and cause the discoloration of Porphyra through competitive utilizing of nutrients (DIN) in the sea. Therefore the restoration of the shallow waters is needed as a fundamental countermeasure for the recovery of the reproducible condition of this area.