Carbon and nitrogen uptake and export in the equatorial Pacific at 150°W: Evidence of an efficient regenerated production cycle

Abstract

which can fuel 20%-100% of the nitrate uptake. Sinking particles represented <10% of total carbon fixation and -10%-50% of new production in terms of carbon and nitrogen. From these discrepancies it was suggested that (1) new production rates were overestimated because of the high level of nitrification that provided "regenerated nitrate" and (2) advection of dissolved organic carbon and nitrogen played an important role in export. The specific hydrodynamical circulation, a conveyor belt generated by upwelling at the equator and downwelling some degrees south, associated with biological in situ rcmineralization of ammonium and nitrate as well, appeared to be a very efficient system for recycling inorganic nitrogen in the euphotic layer and thus for supporting the high regenerated production levels. On the other hand, the high nitrate/silicate ratios (>1.5) observed in the upwelling waters seemed to indicate that silicate is not efficiently recycled in this specific circulation system because of its low regeneration rate as well as its sink via rapidly sedimenting diatoms cell walls; this may be also true for iron. This reinforces the idea of Si and/or Fe limitation which was put forward earlier to explain low new production levels in the equatorial Pacific. IntroductionNew production, defined as the fraction of primary production driven by the input of new nutrients (usually nitrate) into the euphotic zone [Dugdale and Goering, 1967], and export production, defined as the fraction of primary production exported as particles (carbon and nitrogen) [Eppley and Peterson, 1979], are important variables that characterize the efficiency of carbon and nitrogen cycling and particle export from the biological food web in the ocean. These fractions of photosynthetic production play a role in the transport of atmospheric carbon dioxide to the ocean interior, and their quantification is

Elemental and isotopic characterization of dissolved and particulate organic matter in a unique California upwelling system: Importance of size and composition in the export of labile material

“…While this may seem unexpected, previous studies have shown similar results for coastal and open ocean environments (Hansell et al 1993;Hansell and Carlson 2001;Hill and Wheeler 2002). For example, concomitant increases in DON as a function of decreasing nitrate values have been observed on transects across the equatorial upwelling zone in the Pacific Ocean (Hansell and Waterhouse 1997;Libby and Wheeler 1997;Raimbault et al 1999). This was attributed to new production from upwelled equatorial nitrate and then subsequent algal DON exudation during advection to nutrient-deplete higher latitudes.…”

A novel clade of Prochlorococcus found in high nutrient low chlorophyll waters in the South and Equatorial Pacific Ocean

“…Although the number of sequences analyzed is too low to be statistically significant, this distribution may hint at a preference for the HNLC2 clade for However, HNLC Prochlorococcus was correlated to ammonium uptake and regeneration rates, which were in fact measured in the same samples in the BIOSOPE cruise, whereas there were no significant relationships of eMIT9312 to these parameters. Although we did not have actual measurements for the Equatorial Pacific samples where the HNLC clade was present, other studies have shown high rates of ammonium uptake and regeneration in this region (Raimbault et al, 1999). Along with the positive correlation of HNLC Prochlorococcus to Chla and Synechococcus, these observations lead us to hypothesize that HNLC is adapted to less oligotrophic environments and may be dependent on higher ammonium availability.…”

Spatial and temporal variations in dissolved and particulate organic nitrogen in the equatorial Pacific: biological and physical influences

“…Overall, the surface DON concentrations show relatively weak meridional variations in the central and eastern equatorial Pacific. Interestingly, there is a considerable difference (approximately 2 mmol m −3 ) in the surface DON concentration between the two reports with relatively higher values along 125 • W and 140 • W during September-November 1992 (Libby and Wheeler, 1997) than along 150 • W during November 1994 (Raimbault et al, 1999). One would ask if such a difference is ascribed to the spatial variability (i.e., 125 • W-140 • W vs. 150 • W), the interannual variability (i.e., 1992 vs. 1994), or both?…”

“…As Libby and Wheeler (1997) stated, "we have only a cursory knowledge of the distributions of dissolved and particulate organic nitrogen in this region," and "a comprehensive evaluation of both inorganic and organic N-pools is important to achieve an understanding of the transport and regeneration of nitrogen in the equatorial Pacific." While there are several field studies which have measured particulate organic nitrogen (PON) in the equatorial Pacific (Libby and Wheeler, 1997;Eppley et al, 1992;Raimbault et al, 1999;Pena et al, 1991Pena et al, , 1992, only two studies have reported concentrations of dissolved organic nitrogen (DON) (Raimbault et al, 1999;Libby and Wheeler, 1997). The limited data show little variations in both PON and DON concentrations below 150 m in the central equatorial Pacific (Raimbault et al, 1999).…”

“…While there are several field studies which have measured particulate organic nitrogen (PON) in the equatorial Pacific (Libby and Wheeler, 1997;Eppley et al, 1992;Raimbault et al, 1999;Pena et al, 1991Pena et al, , 1992, only two studies have reported concentrations of dissolved organic nitrogen (DON) (Raimbault et al, 1999;Libby and Wheeler, 1997). The limited data show little variations in both PON and DON concentrations below 150 m in the central equatorial Pacific (Raimbault et al, 1999). Overall, observations show considerable variations in the surface PON (>0.7 µm) concentrations (0.4-1.6 mmol m −3 ) but a relatively small range of the surface DON concentration (4-9 mmol m −3 ).…”