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

Size-fractionated (<5 μm and >5 μm) uptake rates of 14C and of 15N-NO3, NH4 and urea by phytoplankton were estimated in a marginal sea (Gulf of St. Lawrence, Canada). For the three N sources, hourly uptake rates (ρ) were lower at night than during day. Diel periodicity of ρN was generally higher in the upper than lower euphotic zone, and often higher for NO3 than other N sources. Across seasons, the relationships between irradiance, ambient N concentration and diel periodicity were statistically significant for ρNO3 only. Biases introduced in the estimates of daily euphotic zone f-ratios by omitting nighttime or urea uptake from the calculations ranged from −30 to 47% and 1 to 95%, respectively. Biases were small (<10%) when daytime ρNO3 was high, but often large at low uptake rates. They were also different for the <5 and >5 μm 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 total N uptake and new production. When accounting for nighttime uptake, the daily uptake of NO3 was dominated by the large phytoplankton during pre-winter (77%) and spring (90%), and by the <5 μm fraction during summer (74%) and autumn (64%). On an annual basis, the <5 μm size fraction accounted 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 μm than the <5 μm fraction. Differences between size fractions were higher for the latter ratio, indicating that large phytoplankton can fix and potentially export more C per unit NO3 than the small size fraction.