Influence of light intensity on diatom physiology and nutrient dynamics in the Oyashio region

Abstract

The spring diatom bloom characterizes the plankton and nutrient dynamics in the Oyashio region, the westernmost part of the subarctic Pacific. Previous studies have shown that NO 3 was not depleted during the spring bloom, and an increase in the consumption ratio of Si(OH) 4 to NO 3 (ΔSi(OH) 4:ΔNO 3) was observed as the spring bloom progressed. The increase in ΔSi(OH) 4:ΔNO 3 has been suggested to be caused by growth stresses of diatoms, e.g. light limitation by self-shading. In the present study, incubation experiments of sea-surface water from the Oyashio region under saturated irradiance showed that NO 3 was depleted first and ΔSi(OH) 4:ΔNO 3 was more or less constant until the NO 3 depletion occurred. The increase in ΔSi(OH) 4:ΔNO 3 was observed after the NO 3 depletion had occurred in contrast with the field observation. This result of the increase in ΔSi(OH) 4:ΔNO 3 under saturated irradiance after NO 3 depletion suggests that the in situ increase in ΔSi(OH) 4:ΔNO 3 before the NO 3 depletion might be caused by light limitation for diatoms. Responses to a reduction in irradiance were examined using diatom species isolated from the Oyashio region. Variable responses to a reduced irradiance were observed for cell specific C, N, Si and chlorophyll a (Chl) contents. However, the examined diatom species showed similar tendencies for increases in Si:C and Si:N and decreases in C:Chl ratios with the reduction in irradiance. We conclude that light limitation changes the uptake ratio of nutrients and the elemental composition of diatoms and that light limitation is one of the factors influencing the physiology of diatoms and nutrient dynamics in the Oyashio region during the spring bloom.