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

This study examines the effects of nutrient availability andsedimentary loss on the dynamics of freshwater phytoplankton,from the physiological state at the cellular level to the competitionissue at the community level. We studied the separated and combinedresponses to nutrient inputs and mixing, of a phytoplanktoncommunity, in terms of size fractionated photosynthetic activityand biomass, exported biomass and species composition. The communitywas composed of two dominant species, which differed in theirsinking velocity. Experiments were conducted during 8 days in12 water columns of 50 L, in light and temperature controlledconditions. The nutrient input was found to be the predominantfactor affecting sedimentary fluxes and community composition,but mixing had also a significant effect on the community structureand sedimentary fluxes when combined with nutrient inputs. Inthis latter case, the large sinking species, Tribonema sp. (Xanthophyceae),developed, due to both a decrease of sedimentary loss by upwardadvection and the enhanced growth from nutrient inputs, as indicatedby the higher photosynthetic activity measured using chlorophylla in vivo fluorescence. The combined effect of both factorsreduced the difference between growth and loss rates and increasedthe viability of the sinking populations.