NITROGEN LIMITATION OF NORTH-ATLANTIC PHYTOPLANKTON - ANALYSIS OF PHYSIOLOGICAL CONDITION IN NUTRIENT ENRICHMENT EXPERIMENTS

Nutrient enrichment experiments were conducted in May and June of 1993 at 8 stations along a North Atlantic transect, from Morocco to Nova S cotia, Canada. Variable fluorescence (F-v/F-m) was measured in order t o estimate the health or physiological state of the population as a wh ole. Low values across the transect indicated nutrient limited photosy nthetic efficiency and probable growth rates ranging from 10 to about 50 % of mu(max). Where the lowest value was measured, over the Grand B anks of Newfoundland, Canada, nitrogen addition to incubated samples r esulted in large, significant increases in photochemical efficiency. N umbers and cell-specific fluorescence of 3 major groups of picophytopl ankton were studied using flow cytometry, in order to further quantify the physiological response to nutrient additions. Results indicated n itrogen limitation of physiology and/or abundance of small eukaryotes, cyanobacteria, and prochlorophytes. Abundance (cell numbers) and cell ular fluorescence of the 3 groups responded differently to nutrient ad ditions. Prochlorophytes showed the greatest response to incubation in terms of cell numbers, responding especially to nitrogen addition. By contrast, cyanobacterial numbers did not change from initial values o r with treatment, although cell pigment content did. Cellular fluoresc ence as measured by the flow cytometer reflected cell pigment content in most experiments. Increased cellular fluorescence of all groups in nitrogen-amended treatments relative to unamended controls indicated p hysiological limitation by nitrogen.