Nutrient and plankton dynamics in the Oyashio region of the western subarctic Pacific Ocean

Abstract

Abstract Nutrients and plankton dynamics were investigated in the Oyashio region of the western subarctic Pacific. Observations were carried out between 1990 and 1998 along A-line, which crosses the Oyashio, a cold western boundary current of the North Pacific. Nutrient concentrations were highest in March and decreased with the spring phytoplankton bloom, which was observed in April and/or May. Nitrate and silicic acid concentrations at the sea surface declined to near-depletion by August. Compared with the eastern subarctic Pacific, the Oyashio region is characterized by large seasonal variability in nutrients and plankton biomass. The magnitudes of seasonal changes in nitrate and silicic acid in the Oyashio region were 2.6 and 2.8 times those of Station P in the eastern subarctic Pacific, respectively. Phytoplankton and zooplankton were 53 and 7.9 times higher in the Oyashio than at Station P, respectively. These differences were suggested to be a result of higher new production and higher ecological efficiency of the Oyashio ecosystem than in the eastern subarctic Pacific. The spring bloom ended by June–July when both nitrate and silicic acid were still replete at half of the stations. Thus, high-nutrient low-chlorophyll water occurred in the Oyashio region during late spring to summer. This result indicated that factors other than macronutrient depletion were important in terminating the spring bloom. The concentration ratio of silicic acid to nitrate (Rsw) at the sea surface was 1.65 before the spring bloom. Rsw was almost constant at the early-bloom period. During the mid- to late-bloom period, the ratio first increased, then decreased. The net consumption ratio of silicic acid to nitrate (ΔSi(OH)4:ΔNO3) at the sea surface was 1.28 during March and April, and ΔSi(OH)4:ΔNO3 increased to 2.59 during April and May. The preferential uptake of silicic acid suggests diatoms were stressed during mid- to late-bloom period and the stress influenced nutrient and plankton dynamics in the Oyashio region. In order to understand the controlling factors of nutrient and plankton dynamics during the spring bloom, mathematical simulations with a silicon–nitrogen cycle model were carried out. The model showed that light and silicic acid limitation depressed phytoplankton growth, and cell sinking and zooplankton grazing decreased phytoplankton biomass. Changes in silicic acid and nitrate uptake by diatoms during the spring bloom was essential in reproducing the temporal change in nutrient dynamics in the Oyashio region. Although zooplankton grazing did not prevent the initiation of the spring bloom, grazing pressure was an important factor controlling the magnitude and the duration of the spring bloom. The observed large fluctuations in the biomass and timing of zooplankton recruitment suggested that zooplankton play an important role in determining the interannual variations of the spring bloom in the Oyashio region.