MEPS 173:95-106 (1998)  -  doi:10.3354/meps173095

ABSTRACT: The growth of 2 different algal size classes was studied in July/August 1994 along a transect from the Dogger Bank to the Shetland Islands in the thermally stratified part of the North Sea. Size-differential growth rates were estimated on the basis of independent measurements of carbon and nitrogen uptake. In general, the major nutrients were low in the upper mixed layer. At 10 m depth, average values for phosphate, ammonium, urea, and nitrate were 0.047 ± 0.019 (n = 32), 0.17 ± 0.05 (n = 27), 0.15 ± 0.09 (n = 26), and 0.16 ± 0.13 (n = 27) µM, respectively. Nitrate, urea, and ammonium uptake by the total plankton community were 9, 31 and 60% of the total nitrogen uptake, respectively. The <5 µm fraction showed a comparable speciation of nitrate, urea and ammonium uptake being 8, 32 and 60%, respectively. The average chlorophyll a (chl a) concentration was 0.68 ± 0.47 µg l^-1 (n = 34), and 71 % of the total chl a was in the <5 µm fraction. The average specific growth rate of <5 µm phytoplankton (0.39 ± 0.17 d^-1; n = 19) was not significantly different (0.25 > p > 0.1) from the specific growth rate of the >5 µm fraction (0.43 ± 0.12 d^-1; n = 15). At 40 m depth, i.e. just below the chl a maximum near the thermocline, major nutrients were present at non-limiting concentrations and photosynthetic active radiation (PAR) ranged between 2 and 5% of the surface PAR. Average values for phosphate, ammonium, urea, and nitrate were 0.40 ± 0.13 (n = 32), 1.40 ± 0.90 (n = 34), 0.15 ± 0.06 (n = 34), and 2.61 ± 1.79 (n = 34) µM, respectively. At 40 m depth, more nitrate was consumed than at 10 m depth: nitrate, urea, and ammonium uptake by the total community were 17, 16 and 67% of the total nitrogen uptake, respectively. The <5 µm fraction showed a lower preference for nitrate with nitrate, urea and ammonium uptake being 12, 14 and 74% of the total nitrogen uptake by this size fraction, respectively. The average chl a at 40 m was 0.63 ± 0.42 (n = 34), and also at this depth 71% of the total chl a was in the <5 µm fraction. The average specific growth rate of phytoplankton was lower at greater depth: 0.17 ± 0.09 d^-1 (n = 15) for the <5 µm fraction. The specific growth rate of the phytoplankton >5 µm was 0.10 ± 0.06 d^-1 (n = 15), which was significantly lower than the value for the smaller size fraction (p < 0.025). Net primary production showed large variation amongst the stations with average values for the <5 µm fraction and the total community of 0.37 ± 0.19 g C m^-2 d^-1 (n = 20) and 0.50 ± 0.26 g C m^-2 d^-1 (n = 20), respectively. In combination with the companion paper it is concluded that size-partitioning of algal growth rate appears to depend on the character of the growth rate limiting factor. Smaller algae showed faster growth than larger ones in light-controlled environments, regardless of the nitrogen source predominantly used. In the nutrient-controlled surface layers, no size-partitioning of algal growth rate was present when ammonium was the major nitrogen source. At some stations, where nitrogen limitation co-occurred with enhanced nitrate consumption, larger algae did grow faster than smaller ones. However, the algal biomass in different size classes was not related to the estimated growth rates of these different categories. Apparently, larger algae do not dominate during summer in the surface layers of the stratified central North Sea, since mesozooplankton densities are high compared to spring and vertical mixing in the photic zone is low. At the lower part of the photic zone, the larger algae do not dominate since they grow much slower than the smaller species due to light-limitation.

KEY WORDS: Phytoplankton growth · Size fractionation · Nitrogen uptake · Carbon uptake · North Sea