EFFECTS OF NUTRIENT ENRICHMENT ON MICROALGAL COMMUNITY COMPOSITION IN A COASTAL SHALLOW-WATER SEDIMENT SYSTEM - AN EXPERIMENTAL-STUDY

An outdoor flow-through experimental set-up, was used to study the effect of increased concentrations of inorganic nitrogen (IN) and phosphorus (IP) on the microalgae of a sandy sediment. The compositional changes following nutrient addition were investigated in terms of the dominance of major taxonomic groups as well as the species composition of diatoms. Fresh samples were used to assess the viable biomass of major taxonomic and size groups, and cleaned diatom frustules were used for identification to species level. Nutrient addition to the water column stimulated the growth of filamentous cyanobacteria, diatoms and flagellates. Although statistically significant compositional changes with respect to both time and treatment were observed, the microflora was not subjected to any substantial shift in the dominance of major taxonomic groups. Instead, the major change occurred at the macroscopic level as a rapid growth of a filamentous green algal species. Diatoms which accounted for ca 50% of the microalgal biomass, were not outcompeted by any other algal group, probably because the Si-pool of the sediment counteracted secondary Si-limitation and because temperature was too low to allow cyanobacteria to fully dominate the biomass. Counts of both living cells and cleaned frustules showed that the diatom flora was dominated by cells in the size group 7 - 12-mu-m, however, depending on the counting method and the level of resolution, different compositional changes were noted. Biomass values of size groups of living diatom cells showed that N + P addition favoured larger cells. Frustule counts, which enable identification at the species level, revealed also that some small-sized species were enhanced by the nutrient addition. Canonical correspondence analysis (CCA) based on frustule counts, showed that the succession in both non-enriched and enriched containers was in the same direction, though faster and proceeding further with N + P addition. The CCA revealed that the P + N addition also favoured some small-sized taxa, such as Nitzchia and Amphora species within the size range 7 - 12-mu-m. Oxygen bubbles produced by the stimulated photosynthetic activity at the sediment surface appeared to selectively resuspend large motile diatom species, e.g. Cylindrotheca.