Size-fractionated uptake of ammonium, nitrate and urea and phytoplankton growth in the North Sea during spring 1994

The growth of 2 different algal size classes was studied in March/April 1994 during the establishment of the spring bloom along a transect from the Dogger Bank to the Shetland Islands in the North Sea. Size-differential growth rates were estimated on the basis of independent measurements of carbon and nitrogen uptake. At the shallower stations near the Dogger Bank (DB) area, chlorophyll a (chl a) levels were up to 5.8 mu g l(-1). In the bloom 89 % of the chi a was in the >5 mu m fraction. In the central North Sea (cNS) and near the Shetland Islands (SI) total chi a was 0.52 and 0.38 mu g l(-1), respectively; 60 % was in the >5 mu m fraction. Depth-integrated primary production at the DB, cNS and SI was 46, 145 and 149 mg C m(-2) d(-1), respectively, for the <5 Fun fraction, and 392, 254, and 282 mg C m(-2) d(-1) for the total phytoplankton communities (n = 6, 24 and 8, respectively). The major nitrogen source was nitrate according to N-15 uptake. The average f-ratios calculated from nitrate uptake at, respectively, the DB, cNS and SI were 83 (n = 4), 71 (n = 12), and 61% (n = 5) of the total nitrogen uptake by the total phytoplankton community. The <5 mu m fraction had a lower preference for nitrate except for the populations near the Shetland Islands with values of 71 (n = 4), 63 (n = 12), and 62 % (n = 5), respectively, of the total nitrogen uptake. Ammonium was taken up about twice as fast as urea by both the <5 mu m and the total fraction. The average specific growth rate of phytoplankton, calculated on the basis of nitrogen uptake, along the transect was 0.27 +/- 0.11 d(-1) (n = 36) for the <5 mu m fraction with a maximum value of 0.73 d(-1). The larger size fraction had a significantly (p<0.0005) lower average specific growth rate (0.17 +/- 0.11 d(-1)) with a maximum value of 0.70 d(-1). The chi a specific carbon uptake rates yielded similar estimates of the specific growth rate: 0.29 +/-: 0.10 d(-1) (n = 28) for the smaller size fraction and 0.15 +/- 0.05 d(-1) (n = 28) for the >5 mu m size fraction. Since the calculated specific growth rates, based on either nitrogen uptake or inorganic carbon uptake, were in good agreement with each other for both size fractions, it was concluded that smaller algae apparently grow faster than larger ones in this typically light-controlled environment. The >5 mu m fraction, however, dominated the bloom at the Dogger Bank. Our findings consolidate the concept of size differential control of phytoplankton communities under typical spring bloom conditions which originally was demonstrated in a coastal area (Riegman et al. 1993; Neth J Sea Res 31:255-265).