Competition for Nutrient Uptake during Winter Season among Cultivated Nori Laver and Phytoplanktons Causing Red Tide in Ariake Bay

Recently, the frequency of red tide occurrence has been increasing with the advancement of Nori laver (Porphyara) cultivation, while the production of bivalves has been decreasing particularly at fisheries in Ariake Bay. That is, a great number of planktons causing red tide consume large amounts of nutrients such as dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), which are essential for the growth of Nori laver, even under a condition of decreasing nutrient supplies. Moreover, the decrease in the production of bivalves might accelerate photosynthesis because the rate of plankton feeding by bivalves decreased. This competition for nutrient uptake among Nori laver and phytoplanktons such as Dinoflagellates, Diatoms and Raphidophyceae is considered to be the cause of the recent deterioration of Nori cultivation qualities in Ariake Bay. After examining the process of competition for nutrient uptake with a box-model simulation, we found that phytoplanktons can grow at water temperatures higher than before. Moreover, it was considered that DIN is taken up for the primary production of phytoplanktons, and is reduced by the increase in particulate organic nitrogen (PON) by the decrease in bivalves, as evidenced by the increasing ΔCOD.