Phytoplankton size distribution and community structure: influence of nutrient input and sedimentary loss

This study examines the effects of nutrient availability and sedimentary loss on the dynamics of freshwater phytoplankton, from the physiological state at the cellular level to the competition issue at the community level. We studied the separated and combined responses to nutrient inputs and mixing, of a phytoplankton community, in terms of size fractionated photosynthetic activity and biomass, exported biomass and species composition. The community was composed of two dominant species, which differed in their sinking velocity. Experiments were conducted during 8 days in 12 water columns of 50 L, in light and temperature controlled conditions. The nutrient input was found to be the predominant factor affecting sedimentary fluxes and community composition, but mixing had also a significant effect on the community structure and sedimentary fluxes when combined with nutrient inputs. In this latter case, the large sinking species, Tribonema sp. (Xanthophyceae), developed, due to both a decrease of sedimentary loss by upward advection and the enhanced growth from nutrient inputs, as indicated by the higher photosynthetic activity measured using chlorophyll a in vivo fluorescence. The combined effect of both factors reduced the difference between growth and loss rates and increased the viability of the sinking populations.