Low rates of nitrogen fixation in eastern tropical South Pacific surface waters

Angela N. Knapp, Karen L. Casciotti, William M. Berelson, Maria G. Prokopenko, and Douglas G. Capone

ENVIRONMENTAL SCIENCE

Significance

We present direct, field-based measurements of low nitrogen fixation rates in the eastern tropical South Pacific (ETSP) Ocean demonstrating that N₂ fixation plays a minor role supporting export production regionally. These results are in contrast to indirect estimates that the highest global rates of N₂ fixation occur in the ETSP. The low N₂-fixation rates occur in a region with relatively high surface ocean phosphate concentrations (and low nitrate concentrations) but where atmospheric iron deposition rates are diminishingly low. Consequently, these results indicate that the ETSP hosts a minor fraction of global N₂-fixation fluxes and that low nitrate to phosphate concentration ratios alone are insufficient to support high N₂-fixation fluxes.

Abstract

An extensive region of the Eastern Tropical South Pacific (ETSP) Ocean has surface waters that are nitrate-poor yet phosphate-rich. It has been proposed that this distribution of surface nutrients provides a geochemical niche favorable for N₂ fixation, the primary source of nitrogen to the ocean. Here, we present results from two cruises to the ETSP where rates of N₂ fixation and its contribution to export production were determined with a suite of geochemical and biological measurements. N₂ fixation was only detectable using nitrogen isotopic mass balances at two of six stations, and rates ranged from 0 to 23 µmol N m⁻² d⁻¹ based on sediment trap fluxes. Whereas the fractional importance of N₂ fixation did not change, the N₂-fixation rates at these two stations were several-fold higher when scaled to other productivity metrics. Regardless of the choice of productivity metric these N₂-fixation rates are low compared with other oligotrophic locations, and the nitrogen isotope budgets indicate that N₂ fixation supports no more than 20% of export production regionally. Although euphotic zone-integrated short-term N₂-fixation rates were higher, up to 100 µmol N m⁻² d⁻¹, and detected N₂ fixation at all six stations, studies of nitrogenase gene abundance and expression from the same cruises align with the geochemical data and together indicate that N₂ fixation is a minor source of new nitrogen to surface waters of the ETSP. This finding is consistent with the hypothesis that, despite a relative abundance of phosphate, iron may limit N₂ fixation in the ETSP.

See: http://www.pnas.org/content/113/16/4398.abstract.html?etoc

PNAS April 19 2016; vol. 113; no.16: 4398–4403

Fig. 1.

Biogeochemical characterization of the ETSP upper thermocline. Water column profiles of potential density (dashed black line), NO−3+NO−2 concentration (open circles), δ¹⁵N_NO3+NO2 (filled circles), δ¹⁵N_PNsink with error bars (±1 SD) (filled triangles), and CTD fluorescence trace from the 2010 cruise (solid green line) or discrete chlorophyll a measurements from the 2011 cruise (filled diamonds) for 2010 Station 5 (100°W, 20°S) (A), 2010 Station 3 (90°W, 20°S) (B), 2010 Station 1 (80°W, 20°S) (C), 2011 Station 5 (100°W, 20°S) (D), 2011 Station 13 (82°W, 15°S) (E), and 2011 Station 1 (80°W, 20°S) (F).