MEPS 193:45-51 (2000)  -  doi:10.3354/meps193045

ABSTRACT: 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, especially focusing on nitrate and silicate as limiting nutrients. At the beginning of the study, nutrient concentrations were high (NO₃ = 10 µM, SiO₄ = 20 µM and PO₄ = 0.8 µM). Nitrate was depleted first during the night while silicate and phosphate remained at 10 and 0.3 µM, respectively. High nitrate reductase activity was observed the following morning, indicating the assimilation of nitrate in situ by the dominant phytoplankter, Chaetoceros 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 also obvious in the elemental composition of phytoplankton, showing an increase in Si:C and Si:N ratios by a factor of 2 to 4 after the peak of the bloom, 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 were finally depleted in the photic zone.

KEY WORDS: Diatom bloom · Nitrate · Silicate · Limiting factor · Consumption rate · Phytoplankton