Diel patterns of combined nitrogen uptake and intracellular storage of nitrate by phytoplankton in the open Skagerrak

Abstract

The relationship between the uptake rates of new (nitrate) and regenerated (ammonium and urea) N nutrients together with the intracellular storage of nitrate was investigated in the open Skagerrak in May 1987. All four stations which were studied exhibited a shallow pycnocline causing high nitrate concentrations in the upper mixed layer. At the most productive station, the total N uptake rates in the surface-water were in the range of 41–56 nmol N · 1 −1 · h −1, about twice as much as on the other station where N uptake rates were measured. There was also a difference in species composition between the most productive station, where the diatoms and dinoflagellates constituted ≈ 50% of the calculated phytoplankton C, and the other stations, where the same groups constituted 20% of the calculated phytoplankton C. Ammonium was generally preferred as a N source before urea and nitrate in the unfractionated samples, whereas in the size fraction < 3 μm the uptake was one-third each of ammonium, urea and nitrate. The intracellular storage of nitrate showed a pronounced diel variation with high values at midnight and midday. The variation with depth generally showed higher concentrations of intracellular nitrate in or below the pycnocline as compared to above it. It was also concluded from the intracellular pools that the phytoplankton population never was N-deficient.