Influence of silicate on particulate carbon production in phytoplankton

A mesocosm experiment was conducted over a period of 4 wk to examine how supply of silicate influences dominant species and biomass within the phytoplankton community. Four enclosures, filled with nutrient-depleted, post-bloom seawater, were fertilised with nitrate and phosphate, with the addition of silicate to 2 of the enclosures. The main difference in the phytoplankton community development between the 2 treatments was the occurrence of much stronger blooms of Emiliania huxleyi (Lohmann) Hay et Mohler in the 2 enclosures supplied only with nitrate and phosphate (NP1 and NP2), whereas diatoms were dominant in the 2 enclosures treated with silicate as well (NPS1 and NPS2). Phaeocystis pouchetii, which was observed primarily during the first week, disappeared in the NP enclosures, but persisted throughout the experiment, although in low numbers, in the NPS enclosures. Integrated over the whole experimental period, equal amounts of nitrate and phosphate were consumed in all 4 enclosures. Additional supply of silicate resulted in a 2-fold increase in carbon production in the NPS enclosures (6.1 and 7.0 g m(-3)) compared to the NP enclosures (2.6 and 3.2 g m(-3)). For each g silicate supplied to the enclosures, carbon production increased by 7.4 g. The ratio between calcification and photosynthesis (C:P), measured in the NP enclosures, reached maxima of 0.48 (NP1) and 0.42 (NP2) when the E. huxleyi bloom peaked, while the cell specific calcification ranged between 0.1 and 0.4 pg C h(-1) cell(-1) over the course of the experiment. Integrated over the experimental period, 0.5 and 0.3 g C m(-3), in NP1 and NP2 respectively, were fixed by calcification.