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

Abstract

Abstract.A variety of N 2 -fixers occur in oligotrophic waters and these diazotrophs make a substantial contribution to the nitrogen budget of the upper water column. A synthesis of previously published and new rate measurements for the North Atlantic provides insight into the role of two different groups of N 2 -fixers (Trichodesmium and small diazotrophs) in supporting N 2 fixation in the tropical Atlantic. The highest rates of N 2 fixation occurred in the western part of the basin, but the full data set showed no significant difference between the eastern and western parts of the basin in overall rates of N 2 -fixation by the two groups of diazotrophs. N 2 -fixation by Trichodesmium was strongly dominant in the western part of the basin while small diazotrophs played a much larger role to the east of 40 • W. The reasons for this shift in dominance are unclear, as is the identity of the small organisms fixing N 2 in the water column.

Latitudinal distribution of <i>Trichodesmium</i> spp. and N<sub>2</sub> fixation in the Atlantic Ocean

“…The mean rates of N 2 fixation we measured in the Equatorial and North gyre regions (55-66 and 11-25 µmol m −2 d −1 , respectively) are similar to those recently reported by Moore et al (2009), who studied also the eastern region of the Atlantic, but lower than the mean rates observed in other studies more focused on the western tropical Atlantic Montoya et al, 2007). As a rough calculation, we scaled up from all our measurements (42 stations) to the whole Atlantic Ocean (40 • N-40 • S) by multiplying the measured daily rates by 365 and using the location of the continental shelf to define the region's longitudinal limits.…”

“…We estimate an annual N 2 fixation of ∼6 TgN yr −1 in the North Atlantic (0-40 • N) and ∼1.2 TgN yr −1 in the South Atlantic (0-40 • S). These are rather conservative estimates because they do not take into account the occurrence of Trichodesmium blooms and also because our surveys did not cover the Western tropical Atlantic, where higher Trichodesmium abundances have been reported Montoya et al, 2007). In addition, a substantial fraction of the N 2 fixed by diazotrophs can be released as dissolved organic nitrogen (DON), whose subsequent remineralization represents an additional source of new nitrogen for the ecosystem (Glibert and Bronk, 1994).…”

Five decades of N2 fixation research in the North Atlantic Ocean

“…The intensity of the turbulence and the depth of mixed layer are driven by the wind speed (Mellor and Yamada, 1982). Thus, nitrate uptake by phytoplankton is not directly coupled to N 2 fixation, but the Frontiers in Marine Science | www.frontiersin.orgenhanced stability of the water column in the NWA as compared to the NEA translates into a lessened nitrate diffusion, which eventually makes the contribution of N 2 fixation to total fixed nitrogen inputs greater than that derived from nitrate diffusion (Capone et al, 2005;Montoya et al, 2007). In addition, the inherent seasonal variability of the NA, and the intermittent nature of mesoscale and submesoscale features may result in a highly variable contribution of N 2 fixation to new production (Mouriño-Carballido et al, 2011;Painter et al, 2013).…”

High-resolution measurements of atmospheric molecular hydrogen and its isotopic composition at the West African coast of Mauritania

“…(Wilson et al, 2010a;Punshon and Moore, 2008b;Scranton et al, 1982), one of the major N 2 fixers in the oceans (Capone et al, 2005). Estimates suggest an annual new N input of at least 1.6 × 10 12 mol N to the tropical North Atlantic, which means a substantial fraction of approximately 10-20 % of the global N 2 fixation of 100-200 Tg yr −1 (Capone et al, 2005;Karl et al, 2002;Deutsch et al, 2007), and measurements on H 2 production by Trichodesmium indicate that a major part of the dissolved H 2 in the ocean is coming from this species, although it cannot account for the total rate of N 2 fixation (Wilson et al, 2010a;Punshon and Moore, 2008b;Montoya et al, 2007). Trichodesmium spp.…”