Effects of nitrate : phosphate supply ratio and irradiance on the C : N : P stoichiometry of<i>Chaetoceros muelleri</i>

Abstract

The ratio of nitrate consumption to phosphate consumption (the draw-down ratio) during phytoplankton growth is often assumed to equal the Redfield ratio of 16 mol N : 1 mol P. As such, both nutrients are expected to be exhausted simultaneously during phytoplankton blooms in ocean waters where upwelling brings nitrate and phosphate to the surface in the Redfield ratio. We examined the extent to which the draw-down ratio can vary from 16 : 1 in batch cultures of Chaetoceros muelleri grown under two irradiances, and initial NO3 −: PO4 3− ratios of 5 and 35 mol N mol P − 1 in the culture media. Cell growth and nutrient concentration were measured together with particulate C, N and P. Exponential phase growth rates of 0.3 day − 1 and 1.8 day − 1 were observed under low and high light, respectively, while initial NO3 −: PO4 3− ratios did not affect growth rates. The NO3 − to PO4 3− draw-down ratio differed significantly from the Redfield ratio, with the ratio increasing with decreasing irradiance and increasing initial N : P ratio. During nutrient-replete exponential phase, the particulate C-to-N ratio (C : N) was independent of irradiance and initial nitrate-to-phosphate ratios. However, particulate C : N increased from about 6 mol C (mol N)−1 at the nutrient-replete early exponential phase to around 20 at the beginning of the stationary phase. At the initial NO3 −: PO4 3− of 35, particulate N : P increased from an initial value of around 5 for both irradiances to 35 – 45 at the end of the exponential phase.