Ammonium regeneration and nitrification rates in the oligotrophic Atlantic Ocean: Implications for new production estimates

N regeneration was measured on a transect of the North and South Atlantic, from the United Kingdom to the Falkland Islands, that included extreme oligotrophic conditions. NH z4 and NO { 2 oxidation rates were measured from the surface and base of the photic zone at 16 stations, using an isotope dilution technique in conjunction with gas chromatography/mass spectrometry analysis. NH z4 regeneration rates were 10–160 nmol L 21 d21, NH z4 oxidation was ,1–10 nmol L21 d21, and NO { 2 oxidation was 1–30 nmol L 21 d21. Outside the oligotrophic central gyres, high rates of NH z4 regeneration were found in the northwest African upwelling region. In the central gyres, nitrification and NH z4 regeneration rates were sufficient to completely replace the dissolved inorganic nitrogen (DIN) pool within 1 d. These high turnover rates of the DIN pool indicate that both NH z4 regeneration and nitrification must be considered when estimating N assimilation rates by the 15N technique. Even in incubations of short duration, N assimilation rates are likely to be underestimated and may introduce error in the estimation of the F-ratio. In the majority of experiments with near-surface samples, nitrification could replace the NO 3 pool in less than 8 h, suggesting that a substantial majority of the NO { 3 pool in the surface oligotrophic Atlantic is derived from the regeneration of N. These findings increase the uncertainty of current new production estimates. The combined effect of the errors in estimating N-assimilation rates without determining N regeneration and the reclassification of NO 3 as a regenerated N source in oligotrophic regions may question the accuracy of C export estimations based on F-ratios. The paradigm of new production introduced by Dugdale and Goering (1967) has been a guiding principle in linking the nitrogen and carbon cycles in the oceans. It states that the assimilation of NO 3 rather than all other N sources is the important factor in determining the potential for the export of carbon from the surface to the deep ocean. It is essential to obtain accurate estimates of new production if the contribution of the oceans to the global carbon cycle is to be quantified, particularly as the Earth system is increasingly perturbed by anthropogenic activities. A general assumption made in the methodologies used to estimate new production is that the NO 3 utilized by phytoplankton is new and not regenerated, that is, that there is insignificant nitrification in the photic zone. In this paper, we test whether that assumption is valid for the oligotrophic Atlantic Ocean. According to the new production paradigm, ‘‘new N’’ is defined as a form of N that was previously unavailable to the phytoplankton community. NO 3 supplied from below the mixed layer is one form of new N. In contrast, NH z4 cannot contribute to new production since it is derived from recycling previously assimilated N, mainly through bacterial degradation of organic matter or from the excretion products of heterotrophic grazers. The assimilation of NO 3 or NH z 4 by the phytoplankton community therefore represents ‘‘new’’ or ‘‘regenerated’’ productivity that is related in the F-ratio (Eppley and Peterson 1979). Dugdale and Goering (1967) noted that the definition of new production would require revision if nitrification and N2 fixation proved to be of greater significance than was originally assumed. In recent years, evidence has accumulated that rates of nitrification are indeed greater than were originally assumed by Dugdale and Goering. Nitrification, which is the two-stage oxidation of NH z4 through NO { 2 to NO { 3 , was previously thought to occur in environments with relatively high (mmol L21) NH z4 concentrations and subsaturating dissolved oxygen concentrations. Such conditions are typically found in estuarine regions (de Bie et al. 2002) or in oxygen minimum zones of marine waters (Ward et al. 1989; Molina et al. 2005). Since ammonia-oxidizing bacteria are sensitive to light, inhibition was also thought to restrict this process to deep water (Olson 1981). However, nitrification has been reported in the photic zone of coastal and open ocean environments (Dore and Karl 1996; Diaz and Raimbault 2000). In a recent development, archaea have been found to be responsible for a significant component of NH z4 oxidation (Wuchter et al. 2006). All this evidence supports the view that nitrification is a substantially more important component of the N cycle in the surface ocean than has been assumed. This is important, particularly in studies of new production, because NO 3 derived from nitrification in the photic zone should be considered as a regenerated N source. It cannot be 1 Corresponding author (drcl@pml.ac.uk).