Effect of different nutrient combinations on phytoplankton development in microcosms

Abstract

Two sets of microcosm experiments were carried out, during winter and autumn 1993, to assay the effect of nutrient loadings, with different ratios among N, P and Si, on the succession patterns of enclosed natural communities of phytoplankton. The experimental setup consisted of 8 Perspex tubes which were filled with 30 L of coastal water. Four nutrient treatments, NPSi, NP, NSi and PSi, were randomly assigned to replicate tubes and added on the first day of the experiment: each pair of tubes received nitrate (N), phosphate (P) and silicate (Si) as indicated. A major chlorophyll 3 concentration peak appeared 2-3 days after enclosure in al1 treatments; the magnitude of the peak was highest in the NPSi tubes and lowest in the PSi and NSi ones. Centric diatoms formed the bulk of this initial peak, while dinotlagellates presented their maxima in later phases of the experiment. A principal component analysis showed that successional changes and the initial composition of the inoculum were the most important factors explaining phytoplankton variability in the pooled data set for both experiments. Discriminant analyses based on the abundance of the most frequent taxa could be used to identify phytoplankton composition differences among treatments, within each experiment. However, there were no obvious regularities concerning species or group-specific effects of nutrient enrichment ratios. This points out that the outcome of fertilization events is highly dependent on the initial conditions of the community