Size-differential uptake of nitrogen and carbon in a marginal sea (Gulf ofSt. Lawrence, Canada): Significance of diel periodicity and urea uptake

Size-fractionated( < 5 mu m and > 5 mu m) uptake rates of C-14 and of N-15- NO3, NH4 and urea by phytoplankton were estimated in a marginal sea (Gulf o f St. Lawrence, Canada). For the three N sources, hourly uptake rates (rho) were lower at night than during day. Diel periodicity of rho N was general ly higher in the upper than lower euphotic zone, and often higher for NO3 t han other N sources. Across seasons, the relationships between irradiance, ambient N concentration and diel periodicity were statistically significant for rho NO3 only. Biases introduced in the estimates of daily euphotic zon e f-ratios by omitting nighttime or urea uptake from the calculations range d from - 30 to 47% and 1 to 95%, respectively. Biases were small( < 10%) wh en daytime rho NO3 was high, but often large at low uptake rates. They were also different for the < 5 and > 5 mu m fractions, the most severe biases being for the large size fraction. Omission of nighttime and/or urea uptake thus yielded biased estimates for the contribution of size fractions to to tal N uptake and new production. When accounting for nighttime uptake, the daily uptake of NO3 was dominated by the large phytoplankton during pre-win ter (77%) and spring (90%), and by the < 5 mu m fraction during summer (74% ) and autumn (64%). On an annual basis, the < 5 mu m size fraction accounte d for 37, 64 and 51% of the total uptake of NO3, NH4 and urea, respectively . On average, the N-based (i.e. NO3 uptake/total N uptake) and C-based (i.e . new/total C production)f-ratios were higher for the > 5 mu m than the < 5 mu m fraction. Differences between size fractions were higher for the latt er ratio, indicating that large phytoplankton can fix and potentially expor t more C per unit NO3 than the small size fraction. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.