Do Trichodesmium spp. populations in the North Atlantic export most of the nitrogen they fix?

Abstract

A new observational synthesis of diazotrophic biomass and nitrogen fixation provides the opportunity for systematic quantitative evaluation of these aspects in biogeochemical models. One such model of the Atlantic Ocean is scrutinized, and the simulated biomass is found to be an order of magnitude too low. Initial attempts to increase biomass levels through decreasing grazing and other loss terms caused an unrealistic buildup of nitrate in the upper ocean. Two key changes to the model structure facilitated a closer match to the observed biomass and nitrogen fixation rates: addition of a pathway for export of diazotrophically fixed organic material and uptake of inorganic nitrogen by the diazotroph population. These changes, along with a few other revisions to existing model parameterizations, facilitate more accurate simulation of basin-scale distributions of diazotrophic biomass, as well as mesoscale variations contained therein. The resulting solutions suggest that the Trichodesmium spp. populations of the North Atlantic export the vast majority of the nitrogen they fix, a finding that awaits assessment through direct observation.

Trichodesmium latitudinal distribution on the shelf break in the southwestern Atlantic Ocean during spring and autumn

“…High Trichodesmium densities and slicks like the one described in the SBB, associated to Brazil Current dynamics, have also been reported in other western boundary currents systems [ Dupouy et al ., ; McGillicuddy , ; Shiozaki et al ., ]. The shelf break upwelling modes, generated by eddies and meanders or encroachment, are common phenomena observed in the Gulf Stream, Kuroshio Current, off the southeastern African coast, and in the East Australian Current [ Palma and Matano , ].…”

The Trichodesmium microbiome can modulate host N2 fixation

“…In the oligotrophic western North Atlantic however, high‐iron concentrations relative to phosphorus lead to phosphorus depletion, and Trichodesmium distribution and N 2 fixation is thought to be more strongly influenced by phosphorus availability, relative to other systems (Sañudo‐Wilhelmy et al ; Moore et al ; Rouco et al , ). Despite the fact that resource controls on Trichodesmium eco‐physiology are well established, using geochemistry to predict and model the distribution and activities of this organism remains challenging (Capone et al ; McGillicuddy ; Snow et al ). Recent studies have suggested that some of the challenges associated with modeling Trichodesmium dynamics are in part due to the fact that its physiology is tightly linked to that of its microbiome (Frischkorn et al ; Lee et al ).…”

Mesoscale eddies and Trichodesmium spp. distributions in the southwestern North Atlantic

“…[] observations were of a decaying anticyclone in the North Pacific subtropical gyre, perhaps associated with upwelling of phosphorus and iron. The Davis and McGillicuddy [] observations of elevated Trichodesmium colony abundance in warm and salty anticyclones in the northern limb of the North Atlantic subtropical gyre have recently been attributed to a combination of down‐gradient transport of biomass and phosphorus by anticyclones originating in the Gulf Stream [ McGillicuddy , ]. The present study attributes enhanced Trichodesmium biomass in anticyclonic eddies to phosphorus and iron supply through transport of Amazon plume water to the southwestern North Atlantic.…”