Nitrogen limitation of North Atlantic phytoplankton: Analysis of physiological condition in nutrient enrichment experiments

Abstract

Nutrient enrichment experiments were conducted in May and June of 1993 at 8 stations along a North Atlantic transect, from Morocco to Nova Scotia, Canada. Variable fluorescence (Fv/Fm) was measured in order to estimate the health or physiological state of the population as a whole. Low values across the transect indicated nutrient limited photosynthetic efficiency and probable growth rates ranging from 10 to about 50% of μmax. Where the lowest value was measured, over the Grand Banks of Newfoundland, Canada, nitrogen addition to incubated samples resulted in large, significant increases in photochemical efficiency. Numbers and cell-specific fluorescence of 3 major groups of picophytoplankton were studied using flow cytometry, in order to further quantify the physiological response to nutrient additions. Results indicated nitrogen limitation of physiology and/or abundance of small eukaryotes, cyanobacteria, and prochlorophytes. Abundance (cell numbers) and cellular fluorescence of the 3 groups responded differently to nutrient additions 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 or with treatment, although cell pigment content did. Cellular fluorescence 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 physiological limitation by nitrogen.