Iron and phosphorus co-limit nitrogen fixation in the eastern tropical North Atlantic

Abstract

The role of iron in enhancing phytoplankton productivity in high nutrient, low chlorophyll oceanic regions was demonstrated first through iron-addition bioassay experiments and subsequently confirmed by large-scale iron fertilization experiments. Iron supply has been hypothesized to limit nitrogen fixation and hence oceanic primary productivity on geological timescales, providing an alternative to phosphorus as the ultimate limiting nutrient. Oceanographic observations have been interpreted both to confirm and refute this hypothesis, but direct experimental evidence is lacking. We conducted experiments to test this hypothesis during the Meteor 55 cruise to the tropical North Atlantic. This region is rich in diazotrophs and strongly impacted by Saharan dust input. Here we show that community primary productivity was nitrogen-limited, and that nitrogen fixation was co-limited by iron and phosphorus. Saharan dust addition stimulated nitrogen fixation, presumably by supplying both iron and phosphorus. Our results support the hypothesis that aeolian mineral dust deposition promotes nitrogen fixation in the eastern tropical North Atlantic.

Atmospheric P deposition to the subtropical North Atlantic: sources, properties, and relationship to N deposition

“…In the subtropical western North Atlantic, the strong seasonal cycle in P flux results in a large variation in the N/P ratios in deposition, with up to an eightfold difference in the interannual seasonal ratios and lower ratios during the summer months. The seasonal variation in P deposition may have important biological implications; for example, via the stimulation of N 2 fixation [Mills et al, 2004], although further biological studies are needed to more fully assess the bioavailability of atmospheric-derived P.…”

“…Selective removal of P from dissolved organic matter [Clark et al, 1998] and high N/P ratios of nutrients in the main thermocline [Ammerman et al, 2003;Fanning, 1992] are also observed. In some organisms, particularly diazotrophs, P is even thought to be limiting or colimiting growth [Mills et al, 2004;Moore et al, 2008;Sañudo-Wilhelmy et al, 2004].…”

Spatial variation in N<sub>2</sub>-fixation rate and diazotroph activity in the Tropical Atlantic

“…The best current information on the environmental factors controlling N 2 -fixation in the Atlantic comes from an elaborate set of bioassay experiments carried out by Mills et al (2004) on Cruise ME55. These experiments were designed to explore the role of iron and phosphorus in limiting N 2 fixation in surface waters of the Eastern Tropical Atlantic.…”

Combined uncertainty estimation for the determination of the dissolved iron amount content in seawater using flow injection with chemiluminescence detection

“…The dynamics of the ocean and its interaction with the atmosphere are strongly linked to the properties of seawater. Elements such as Fe limit marine primary production in half of the world ocean (Moore et al 2001) and thus may have a profound effect on plankton communities and the global carbon cycle (Martin and Fitzwater 1988; Mills et al 2004). More reliable determinations of micronutrient elements in marine waters are thus essential to enhance our understanding of their impact on ocean productivity and processes (e.g., ocean acidification).…”