Diel periodicity of nitrogen uptake by marine phytoplankton in nitrate-rich environments

It has been suggested that daily measurements of NO3‒ uptake by phytoplankton are indicative of the magnitude of flux of organic matter from the surface layers to the deep ocean (i.e. the magnitude of the biological pump). We used the 15N tracer technique to examine the magnitude of diel periodicity in the uptake of nitrogenous nutrients by microplankton in nutrient-rich environments of the oceanic subarctic Pacific and an upwelled plume off the British Columbia coast. In the subarctic Pacific, specific NO3‒ uptake rates, calculated at 3-h intervals for 24 h, demonstrated a clear diel pattern; average nighttime rate (0.0047 h‒1) was about half the average daylight rate (0.0085 h‒1). Off British Columbia, drogued drifter buoys were used to guide repeated sampling of the euphotic zone of an upwelled plume at 6-h intervals for >48 h. Specific rates of NO3‒ and NH4+ uptake by phytoplankton from the near-surface and the 30% Io light depth showed pronounced diel periodicity, whereas uptake rates of N were more or less constant over the day–night cycle at the bottom of the euphotic zone (1% Io). The average nighttime rates of the upper waters were 15–16 and 30–36 % of the NO3‒ and NH4+ daylight rates. Our results differ from several previous diel studies in that both NO3‒ and NH4+ uptake demonstrated definitive diel cycles with the magnitude of periodicity in NH4+ uptake being similar to that of NO3‒ uptake. We also found that NO3‒, not NH4+, seems to be the preferred source of N during daylight, while NH4+ is preferred at night. Estimates of daily NO3‒ uptake rates extrapolated from short-term (e.g. 4 h) incubations may considerably underestimate the magnitude of the biological pump, depending on the magnitude of diel periodicity in NO3‒ uptake.