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

Abstract

Nutrient enrichment experiments were conducted m May and June of 1993 at 8 stations along a North Atlantlc transect, from Morocco to Nova Scotia, Canada. Variable fluorescence (F,/F,,) 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 p,,. 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 quantlfy the physiological response to nutrient additions. Results ind~cated nitrogen linxtation 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 relat~ve to unamended controls lnd~cated physiological limitation by nitrogen.

Basin-scale variability of phytoplankton biomass, production and growth in the Atlantic Ocean

“…It has been suggested that open ocean algae may have adapted to low nutrient conditions by losing their ability to grow fast (Jackson, 1980). However, Brand and Guillard (1981) demonstrated that several species of microalgae isolated from oligotrophic waters have the same growth potential as those cultured in the laboratory, and numerous studies in the central gyres have found growth responses of phytoplankton to experimental nutrient additions (Eppley et al, 1973;DiTullio et al, 1993;Graziano et al, 1996;Dufour et al, 1999). During the AMT cruises, we found dramatic increases in productivity and growth in some areas (e.g.…”

Processes and patterns of oceanic nutrient limitation

“…In the low-biomass (oligotrophic) waters that dominate the lowlatitude oceans, phytoplankton biomass and productivity typically only increase following experimental addition of nitrogen 47,52,56 . In contrast, in high-nitrate low-chlorophyll waters (Fig.…”

Analogous nutrient limitations in unicellular diazotrophs and Prochlorococcus in the South Pacific Ocean

“…Nitrogen is thought to be the most significant limiting nutrient in oligotrophic oceans with low nutrient and low chlorophyll, suggested by results from nutrient bioassays conducted in the North Atlantic (Graziano et al, 1996;Mills et al, 2004;Davey et al, 2008) and Pacific Oceans. The presence and activity of microorganisms fixing atmospheric N 2 in oligotrophic oceans indicates scarcity of bioavailable N and relative sufficiency of other essential nutrients that allows growth of these microorganisms.…”