Effects of increased concentration of nitrate and phosphate during a springbloom experiment in mesocosm

Abstract

A mesocosm experiment was carried out in the early spring of 1991 (19 February to 20 March) in Raunefjorden, Norway. The experiment consisted of four enclosures of which two were initially supplied with nitrate and phosphate corresponding to an increase in concentration of 6 and 0.2 μM, respectively. Effects of an increased concentration of nitrate and phosphate on the development of the annual phytoplankton springbloom was investigated. Measurement of light, temperature, salinity, nutrients, chlorophyll (chl) a, primary production, phytoplankton enumeration and identification were performed daily or every other day. Average 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 μg chl a · 1 −1 and 109 μg C ·1 −1 · d −1. The initial phytoplankton community in all enclosures were dominated by diatoms, mainly Skeletonema 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 flagellates, due to silicate deficiency (< 2 μM). The fertilized enclosures were dominated by the haptophyte Phaeocystis cf. pouchetii (18 · 10 6 cells 1 −1), and the non-fertilized enclosures were dominated by unidentified flagellates, together with cryptophytes and prasinophytes. High abundance of choanoflagellates and microzooplankton were also registered in the fertilized enclosures (3.3 · 10 6 1 −1 and 11 · 10 6 1 −1, respectively). This indicates that the microzooplankton may have controlled the growth of flagellates by grazing in the fertilized enclosures. The effect of an increased concentration of nitrate and phosphate was not an increase in biomass (chl a) or primary production, but a change 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 limiting factors for the biomass and primary production have been silicate deficiency, low irradiance and temperature.