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

Abstract

Abstract.We have determined the latitudinal distribution of Trichodesmium spp. abundance and community N 2 fixation in the Atlantic Ocean along a meridional transect from ca. 30 • N to 30 • S in November-December 2007 and April-May 2008. The observations from both cruises were highly consistent in terms of absolute magnitude and latitudinal distribution, showing a strong association between Trichodesmium abundance and community N 2 fixation. The highest Trichodesmium abundances (mean = 220 trichomes L −1 ) and community N 2 fixation rates (mean = 60 µmol m −2 d −1 ) occurred in the Equatorial region between 5 • S-15 • N. In the South Atlantic gyre, Trichodesmium abundance was very low (ca. 1 trichome L −1 ) but N 2 fixation was always measurable, averaging 3 and 10 µmol m 2 d −1 in 2007 and 2008, respectively. We suggest that N 2 fixation in the South Atlantic was sustained by other, presumably unicellular, diazotrophs. Comparing these distributions with the geographical pattern in atmospheric dust deposition points to iron supply as the main factor determining the large scale latitudinal variability of Trichodesmium spp. abundance and N 2 fixation in the Atlantic Ocean. We observed a marked South to North decrease in surface phosphate concentration, which argues against a role for phosphorus availability in controlling the large scale distribution of N 2 fixation. Scaling up from all our measurements (42 stations) results in conservative estimates for total 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).

Isometric size-scaling of metabolic rate and the size abundance distribution of phytoplankton

“…The influence of the equatorial upwelling could be observed between 178 N and 58 S, where a shallower nitracline depth was found and nitrate concentrations in the UML increased, although without exceeding 0.2 mmol l 21 [33]. Nutrient-limited conditions thus prevailed throughout the cruise, and limiting nutrients were supplied to the euphotic layer through small and relatively continuous diffusive fluxes [19].…”

Five decades of N2 fixation research in the North Atlantic Ocean

“…However, upwelling-affected waters are colder, which likely explains the lower abundance of diazotrophic organisms in the NEA as compared to the NWA (Figure 3), and the lower N 2 fixation rates (generally <1 nmol N L −1 d −1 ; Moore et al, 2009;Fernández et al, 2010;Benavides et al, 2011). The low N 2 fixation rates of the NEA do not likely represent an important source of new production in this area (Benavides et al, 2013b).…”

“…Local accumulations of Trichodesmium have been observed in anticyclonic eddies across the NA (Davis and McGillicuddy, 2006), with an increasing abundance toward the West. In the NEA however, Trichodesmium is too scarce and usually present as free trichomes instead of colonies (Moore et al, 2009;Fernández et al, 2010;Benavides et al, 2011). This low abundance probably prevents any structuring effects from mesoscale activity (Agawin et al, 2013).…”

Filtration via Conventional Glass Fiber Filters in 15N2 Tracer Assays Fails to Capture All Nitrogen-Fixing Prokaryotes

“…Some investigators opt for not precombusting glass fiber filters prior to using them in BNF experiments (e.g., Fernández et al, 2010). This is due to the concern that the effective combustion temperature could vary for different filters inside the furnace (depending e.g., on how they are positioned and how close they are to the walls), and due to the belief that contamination of purchased filters is negligible and that the additional handling associated with precombustion could add further contamination.…”