EFFECTS OF INCREASED CONCENTRATION OF NITRATE AND PHOSPHATE DURING A SPRINGBLOOM EXPERIMENT IN MESOCOSM

A mesocosm experiment was carried out in the early spring of 1991 (19 February to 20 March) in Raunefjorden, Norway. The experiment consiste d of four enclosures of which two were initially supplied with nitrate and phosphate corresponding to an increase in concentration of 6 and 0.2 muM, respectively. Effects of an increased concentration of nitrat e and phosphate on the development of the annual phytoplankton springb loom was investigated. Measurement of light, temperature, salinity, nu trients, chlorophyll (chl) a, primary production, phytoplankton enumer ation and identification were performed daily or every other day. Aver age daily irradiance during the experiment was low (4.8 mol.m-2.d-1). Maximum concentrations of biomass (chl a) and primary production were 11.5 mug chl a.1(-1) and 109 mug C.1(-1).d-1. The initial phytoplankto n community in all enclosures were dominated by diatoms, mainly Skelet onema costatum (Grev.) Cleve, and maximum cell number of this species was 11.2.10(6) 1(-1). After 1 wk, the diatoms were replaced by flagell ates, due to silicate deficiency (< 2 muM). The fertilized enclosures were dominated by the haptophyte Phaeocystis cf. pouchetti (18.10(6) c ells 1(-1)), and the non-fertilized enclosures were dominated by unide ntified flagellates, together with cryptophytes and prasinophytes. Hig h abundance of choanoflagellates and microzooplankton were also regist ered in the fertilized enclosures (3.3.10(6) 1(-1) and 11.10(6) 1(-1), respectively). This indicates that the microzooplankton may have cont rolled the growth of flagellates by grazing in the fertilized enclosur es. The effect of an increased concentration of nitrate and phosphate was not an increase in biomass (chl a) or primary production, but a ch ange in the species composition. The species composition changed from a diatom community dominated by S. costatum to a flagellate community dominated by P. cf. pouchetii. These results also suggest that major l imiting factors for the biomass and primary production have been silic ate deficiency, low irradiance and temperature.