播磨灘南部おょび備讃瀬戸における香川県ノリ養殖漁場の珪藻類の出現動向と栄養塩環境

摘要

Considering the consumption of dissolved inorganic nitrogen (DIN) by diatom blooms causing the bleaching of nori (Pyropia yezoensis), we investigated the changes in the trend of diatom appearance and nutrient salt condition at Nori cultures of Kagawa Prefecture in the southern part of Harima-Nada and Bisan-Seto, the Seto Inland Sea, during the season of nori culture (October-March) from fiscal year 2000 to 2014. Diatoms were the most abundant phytoplankton, and we confirmed at least 44 genera of diatoms during the 15-year investigation period. The major diatoms found were Chaetoceros spp.,Eucampia spp.,Thalassiosira spp., Skeletonema spp. and others. They appeared in large quantities at slow current culture grounds and cultures that influenced fresh water from rivers. On the other hand, diatoms appeared in small quantities at rapid current culture grounds. With regard to the overall trend of changes in the appearance ratio of diatoms, many types of diatoms appeared from October to December when the water temperatures were high, whereas a few types of diatoms appeared from January to March when the water temperatures were low; the main species during this period was the large-sized diatom E. zodiacus. Chaetoceros spp. had a tendency to appear throughout the season of nori culture. The DIN concentration was high from October to December, following which it began to decrease in various patterns. It was confirmed 8 times during the 15-year period that the DIN concentration mainly decreased because of diatom blooms in less than 3 μM,which began the bleaching of nori. This phenomenon occurred 5 times because of E. zodiacus, 4 times because of Chaetoceros spp. and 1 time because of eacn T. diporocyclus and Rhizosolenia imbricata. The bleaching of nori by multi-species diatom blooms occurred 3 times. The DIN concentration suddenly decreased from December to January when dried sheet production of nori was in the peak season, and it had a large influence on the growth of nori. E. zodiacus is a main diatom causing the bleaching of nori in Kagawa Prefecture. However, it has been confirmed that many other diatoms also consume DIN. Therefore, it is considered that a diatom species increases adapting weather and hydrographic conditions on each occasion. It required that investigation organisms do monitoring the whole tendency of diatoms with care and send informations. The producers of nori are required attention on all diatoms appearance as well as specific species for proper management.