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 The extent to which water mass mixing contributes to the biological activity of the dark ocean is essentially unknown. Using a multiparameter water mass analysis, we examined the impact of water mass mixing on the nutrient distribution and microbial activity of the Northeast Atlantic Deep Water (NEADW) along an 8000 km long transect extending from 62°N to 5°S. Mixing of four water types (WT) and basin scale mineralization from the site where the WT where defined to the study area explained up to 95% of the variability in the distribution of inorganic nutrients and apparent oxygen utilization. Mixing-corrected average O2:N:P mineralization ratios of 127(±11):13.0(±0.7):1 in the core of the NEADW suggested preferential utilization of phosphorus compounds while dissolved organic carbon mineralization contributed a maximum of 20% to the oxygen demand of the NEADW. In conjunction with the calculated average mineralization ratios, our results indicate a major contribution of particulate organic matter to the biological activity in the NEADW. The variability in prokaryotic abundance, high nucleic acid containing cells, and prokaryotic heterotrophic production in the NEADW was explained by large scale (64–79%) and local mineralization processes (21–36%), consistent with the idea that deep-water prokaryotic communities are controlled by substrate supply. Overall, our results suggest a major impact of mixing on the distribution of inorganic nutrients and a weaker influence on the dissolved organic matter pool supporting prokaryotic activity in the NEADW.
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 The formation of cold dense waters in the Labrador and Greenland-Iceland-Norwegian Sea and the large-scale southward transport of North Atlantic Deep Water (NADW) drives the thermohaline circulation of the world's oceans, which plays a decisive role in the regulation of the Earth's climate [Bryden et al., 2005]. Accompanied with the formation of the NADW at a rate of about 17 Sverdrup [Smethie et al., 2000] is the export of relatively fresh dissolved organic matter (DOM) from the surface layer into the mesopelagic and bathypelagic realms [Carlson et al., 2010]. Other than particulate organic material (POM) that is capable of sinking, the vertical transport of DOM mainly depends on convective overturning and mixing of water masses. Thus, considering the constrained direct input of DOM from the surface to the deep waters, any increase or decrease of metabolically utilizable substrates for prokaryotes in the deeper strata of the water column is largely due to the solubilization of POM and mineralization of the available and steadily aging DOM during the evolution of the NADW in the global conveyor belt [Nagata et al., 2000].
 In general, the mineralization of DOM derived from POM is mediated by the abundant prokaryotes. The modern view of the microbial community comprising an active component [Herndl et al., 2005; Reinthaler et al., 2006] contrasts with the previous idea of slow-growing prokaryotes in the dark ocean [Jannasch and Taylor, 1984]. Although some authors indicated a highly reduced potential for heterotrophic productivity due to pressure effects and the highly refractory nature of the DOM pool in the dark ocean when compared to the surface layer [Bauer et al., 1992; Turley, 1993; Tamburini et al., 2013], on longer time scales, prokaryotes determine to a large extent the distribution and stoichiometry of the inorganic material in the dark ocean [Nagata et al., 2010].
 The decrease in dissolved organic carbon (DOC) and oxygen concentrations in the individual deep water masses as they age in the thermohaline circulation is an important indicator for heterotrophic metabolic activity of prokaryotes [Bendtsen et al., 2002]. Comparing the DOC decrease with the decrease in oxygen concentrations indicated that the contribution of DOC to dark ocean respiration is only about 10–20% [Arístegui et al., 2002; Carlson et al., 2010]. Most of the DOC in the dark ocean is refractory (> 90%) which is reflected by its radiocarbon age of > 4000 years [Williams and Druffel, 1987; Bauer et al., 1992]. However, the DOC may be differentiated into four pools in the dark ocean: a pool of semilabile DOC with a lifetime of 1.5 years, a pool of semirefractory DOC with a lifetime of 20 years, a pool of refractory DOC with a lifetime of 14,000 years, and an ultrarefractory pool with a lifetime of 40,000 years [Hansell, 2013]. Consequently, it is the semilabile and semirefractory DOC components that may eventually be assimilated by prokaryotes.
 Acknowledging the deficiencies in methodology and parameters influenced by different time and spatial scales, several recent reports point to major discrepancies between the available organic matter and the apparent metabolic requirements of the deep ocean prokaryotic community [Reinthaler et al., 2006; Steinberg et al., 2008; Burd et al., 2010; Reinthaler et al., 2010]. These studies show that the carbon demand of prokaryotes is orders of magnitude higher than the export primary production. The conversion of POM to DOM via extracellular ectoenzymes is the main mechanism for prokaryotes to obtain assimilable substrates. Thus, part of the DOM in the dark ocean must result from the cleavage of sinking organic matter particles and aggregates apart from the DOC injected into the dark ocean during water mass formation and convective overturn at lower latitudes [Baltar et al., 2009]. Hansell et al.  estimated that ~80% of organic matter transported from the surface to the deep ocean is in the form of POM with the remainder being DOM.
 In this respect, analyzing the stoichiometry of the major biogenic elements in the oceans is a useful tool to assess the mineralization of organic matter in the framework of the biological pump [Anderson and Sarmiento, 1994]. While Anderson and Sarmiento  considered that mineralization ratios are essentially constant with depth and basin, suggesting that large, fast-sinking phytoplankton-derived material of Redfieldian elemental composition is exported from the surface ocean and consumed in all the depth horizons, other authors concluded that there are remarkable changes in the nutrient mineralization ratios in the deep waters of the different ocean basins [Li and Peng, 2002] or different depths within the same basin [Shaffer et al., 1999]. However, the estimation of nutrient regeneration ratios from dissolved nutrient concentrations can be distorted by the method used to eliminate the effect of the conservative mixing of water masses with different initial nutrient concentrations [Schneider et al., 2005]. Direct measurements of microbial activity by prokaryotes in the dark ocean, however, are not implemented in models on DOM and/or POM mineralization, one reason being the lack of data on the conversion efficiency from particulate to dissolved organic matter by microbes.
 Here we study the deep pelagic realm of the NE Atlantic basin from 60°N to the equator following the core of Northeast Atlantic Deep Water (NEADW) (Figure 1). This core is identified as a deep vertical salinity maximum at about σ3 = 41.42 or 2700 dbar [van Aken, 2000] in between the overlying lower salinity Labrador Sea Water (LSW) and underlying Lower Deep Water (LDW). The linear trend in the potential temperature-salinity relationship for the NEADW [Mantyla, 1994] derives from the sluggish circulation below 2500 dbar in the NE Atlantic basin, although some coherent patterns are discernable as an eastward current over the Mid-Atlantic Ridge (MAR) at the Charlie-Gibbs Fracture Zone (CGFZ), a southward slope current near the MAR and the continental slope of Africa [Paillet and Mercier, 1997] over a general cyclonic deep and abyssal circulation [Reid, 1994].
Figure 1. Occupied stations tracking the Northeast Atlantic Deep Water (NEADW). The flow of the main water masses encountered in the deep North Atlantic is indicated by white lines. Cruises were conducted in fall of the years 2002 (brown dots), 2005 (red dots), 2006 (blue dots), and 2007 (orange dots). Abbreviations of water masses see text. CGFZ: Charlie-Gibbs Fracture Zone; AFZ: Azores Frontal Zone; MAR: Mid-Atlantic Ridge.
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 The NEADW is a body of water defined by mixing of four water types (WT), each one characterized by a unique combination of thermohaline and chemical property values. When these properties are taken in the source region of the WT, it is called a source water type (SWT). The four WT that contribute to NEADW are Labrador Sea Water (LSW), Iceland-Scotland overflow water (ISOW), Mediterranean Water (MW), and Lower Deep Water (LDW) [van Aken, 2000]. LSW is a proper SWT but ISOW, MW, and LDW are WT. The property values of the ISOW were taken at the sills between Iceland and Scotland, where it forms by entrainment of the Norwegian overflow. MW was defined at about 1000 m in the Gulf of Cadiz after the intense mixing of the eastern North Atlantic central water with the Mediterranean overflow water that spills at the Strait of Gibraltar. The properties of the LDW were taken at the entry of the Vema Channel at 11°N of the Mid-Atlantic Ridge (see Table S1 in the supporting information).
 If NEADW would be a distinct water mass transported through the deep NE Atlantic with negligible mixing, then the O2:C:N:P stoichiometry of the mineralization of biogenic materials could be assessed by simply comparing the dissolved oxygen and nutrient distributions. However, the previously described WT contribute to the salinity maximum of the NEADW [van Aken, 2000]. We hypothesized that the mixing WT does not only mask the latitudinal and longitudinal patterns of variability of nutrient mineralization but also prokaryotic activity that is intrinsic to the pure NEADW. We used a multiparameter water mass modeling approach that is based on predefined WT and thus allows calculating the proportions of the different WT influencing the NEADW. The ultimate goal was to explore the link between prokaryotic biomass and activity and nutrient mineralization and stoichiometry along the NEADW core in the NE Atlantic basin that is corrected for water mass mixing and hence yielding a better approximation of microbe-mediated biogeochemical cycling in the NEADW.