Exhaustion of nitrate terminates a phytoplankton bloom in Funka Bay, Japan: change in SiO4,: NO3 consumption rate during the bloom

Time series observations over 36 h were carried out in Funka Bay, Japan, to complement a 6 yr nutrient-dynamics study during a diatom spring bloom, es pecially focusing on nitrate and silicate as limiting nutrients. At the beg inning of the study, nutrient concentrations were high (NO3 = 10 mu M, SiO4 = 20 mu M and PO4 = 0.8 mu M) Nitrate was depleted first during the night while silicate and phosphate remained at 10 and 0.3 mu M, respectively. Hig h nitrate reductase activity was observed the following morning, indicating the assimilation of nitrate in situ by the dominant phytoplankter, Chaetoc eros socialis. The consumption ratio of SiO4:NO3 increased by 3 times from 0.83 before the peak to 2.5 after the peak of the bloom. This change was al so obvious in the elemental composition of phytoplankton, showing an increa se in Si:C and Si:N ratios by a factor of 2 to 4 after the peak of the bloo m, while C:N ratio remained constant at around 7. As a consequence of this change in the consumption rate of SiO4 and NO3, both silicate and nitrate w ere finally depleted in the photic zone.