Biogeochemical cycles during the species succession from Skeletonema costatum to Alexandrium tamarense in northern Hiroshima Bay

Abstract

Temporal variations, budgets and cycling patterns of major biophilic elements [phosphorus (P), nitrogen (N) and silicon (Si)] in northern Hiroshima Bay during species succession from Skeletonema costatum to Alexandrium tamarense were documented using data collected on a weekly basis in 1996 and 1997. The budget calculations using a box model were made from the perspective of both particulate formation and decomposition of particulate matter. Thus, the overall circulation patterns of these three elements and their differences were examined in the present study. The turnover time of P was longer than that of N, indicating that the utilization efficiency of P by organisms is low. Along with this, absolute concentrations of dissolved inorganic phosphorus in this season and elemental ratios calculated from various forms of biophilic elements support the idea that DIP is too low for phytoplankton to utilize it. However, the utilization efficiency of P was higher in 1997 than in 1996, when A. tamarense formed a more intensive bloom, suggesting that these organisms could retain elements. When the bloom of A. tamarense formed in 1997, the residence time of water was extremely long, indicating that stagnation of water is a primary factor for bloom formation. In contrast to the large fluctuation in the residence time of water, residence times of biophilic elements were almost constant, indicating the importance of biological processes occurring in the system. On the other hand, in 1996, the residence times of biophilic elements were usually longer compared to that of water. This suggests that the particulate formation process by phytoplankton acts as a “nutrient trap” in combination with estuarine circulation.