Environmental factors influencing Porphyra (Nori) farming in Tokyo Bay [Japan]: Long-term changes in inorganic nutrients and recent proliferation of diatoms
2008
Ishii, M.(Chiba-ken. Fisheries Research Center, Futtsu (Japan). Tokyo Bay Fisheries Research Lab.) | Hasegawa, K. | Matsuyama, Y.
Recently, fishery damage in Porphyra (Nori) farms due to discoloration has frequently occurred in Tokyo Bay. Diatom blooms cause depletion of the dissolved inorganic phosphorus (DIP) concentration in seawater and subsequently cause the discoloration of Porphyra. To clarify environmental factors underlying Porphyra degradation, we carried out intensive investigations on long-term changes in the nutrient concentration in Porphyra farming grounds, seasonal changes in the vertical distribution of DIP concentration from bottom to surface layer, and occurrences of diatom blooms. Since the 1980s, phosphorus loading to Tokyo Bay has markedly decreased due to administrative efforts to regulate total emissions, and these efforts have probably caused selective reduction of DIP concentration in Porphyra farming grounds. During the harvesting period of Porphyra (October to April), source of phosphorus for Porphyra growth depended on release of DIP from bottom sediments and collapse of the pycnocline since freshwater inflow is lowest. The DIP supply from bottom to surface layer gradually decreases in the late harvesting period. At this time, diatom blooms occur and subsequently reduced the DIP concentration in Porphyra farming grounds.
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