Regional differences in modelled net production and shallow remineralization in the North Atlantic subtropical gyre

Abstract

Abstract. We used 5-yr concomitant data of tracer distribution from the BATS (Bermuda Time-series Study) and ES-TOC (European Station for Time-Series in the Ocean, Canary Islands) sites to build a 1-D tracer model conservation including horizontal advection, and then compute net production and shallow remineralization rates for both sites. Our main goal was to verify if differences in these rates are consistent with the lower export rates of particulate organic carbon observed at ESTOC. Net production rates computed below the mixed layer to 110 m from April to December for oxygen, dissolved inorganic carbon and nitrate at BATS (1.34 ± 0.79 mol O 2 m −2 , −1.73 ± 0.52 mol C m −2 and −125±36 mmol N m −2 ) were slightly higher for oxygen and carbon compared to ESTOC (1.03 ± 0.62 mol O 2 m −2 , −1.42 ± 0.30 mol C m −2 and −213 ± 56 mmol N m −2 ), although the differences were not statistically significant. Shallow remineralization rates between 110 and 250 m computed at ESTOC (−3.9 ± 1.0 mol O 2 m −2 , 1.53 ± 0.43 mol C m −2 and 38 ± 155 mmol N m −2 ) were statistically higher for oxygen compared to BATS (−1.81 ± 0.37 mol O 2 m −2 , 1.52 ± 0.30 mol C m −2 and 147 ± 43 mmol N m −2 ). The lateral advective flux divergence of tracers, which was more significant at ESTOC, was responsible for the differences in estimated oxygen remineralization rates between both stations. According to these results, the differences in net production and shallow remineralization cannot fully explain the differences in the flux of sinking organic matter observed between both stations, suggesting an additional consumption of nonsinking organic matter at ESTOC.

High sensitivity of ultra‐oligotrophic marine ecosystems to atmospheric nitrogen deposition

“…Higher simulated C:N ratios at NASE are consistent with indications of carbon overconsumption in the mixed layer available just to the north of NASE [ Koeve , 2006], excess carbon export at ESTOC [ Neuer et al , 2007], and C:N ratios of particulate export fluxes close to Redfield ratios measured at BATS [ Schnetzer and Steinberg , 2002]. A recent study reporting the comparison of net production and shallow remineralisation rates at BATS and ESTOC estimated from a 1D tracer budget approach for oxygen, DIC and nitrate suggests a C:N ratio for shallow remineralisation of 40 ± 162 at ESTOC and 10.2 ± 3.6 at BATS [ Fernández‐Castro et al , 2011]. The only time‐series of vertical export fluxes available at NASE derives from the sediment traps deployed at ESTOC, which is close to the high end of productivity inside the NASE region [ Mouriño‐Carballido and Neuer , 2008].…”

The <sup>3</sup>He flux gauge in the Sargasso Sea: a determination of physical nutrient fluxes to the euphotic zone at the Bermuda Atlantic Time-series Site

“…In addition, other researchers have measured export using 234 Th (Maiti et al, 2009), neutrally buoyant sediment traps (Owens et al, 2013), or apparent oxygen utilization rates (Stanley et al, 2012;Jenkins, 1980). Net community production has been estimated from the seasonal accumulation of O 2 / Ar (Spitzer and Jenkins, 1989) and the drawdown of dissolved inorganic carbon (Gruber et al, 1998;Brix et al, 2006;Fernandez-Castro et al, 2012). New production has been estimated from bottle incubations (Lipschultz, 2001;Lipschultz et al, 2002) and has also been studied using nitrogen isotopes (Fawcett et al, 2014;Knapp et al, 2008).…”

“…The seasonal accumulation of oxygen and argon has been used at other time periods to estimate the rate of net community production at BATS to be 2.2 to 3 mol C m −2 yr −1 (Spitzer and Jenkins, 1989;Luz and Barkan, 2009). Seasonal drawdown of DIC directly as well as the change in isotopic composition of 13 C of DIC have been used to estimate annual net community production fluxes of 1.7 to 4.9 mol C m −2 yr −1 (Gruber et al, 1998;Brix et al, 2006;Fernandez-Castro et al, 2012). The upper end of this range approximates the rate of new production we find here using the flux gauge technique.…”

The <sup>3</sup>He flux gauge in the Sargasso Sea: a determination of physical nutrient fluxes to the euphotic zone at the Bermuda Atlantic time series site

“…In addition, other researchers have measured export using 234 Th (Maiti et al, 2009), neutrally buoyant sediment traps (Owens et al, 2013), or apparent oxygen utilization rates (Stanley et al, 2012;Jenkins, 1980). Net community production has been estimated from the seasonal accumulation of O 2 / Ar (Spitzer and Jenkins, 1989) and the drawdown of dissolved inorganic carbon (Gruber et al, 1998;Brix et al, 2006;Fernandez-Castro et al, 2012). New production has been estimated from bottle incubations (Lipschultz, 2001;Lipschultz et al, 2002) and has also been studied using nitrogen isotopes (Fawcett et al, 2014;Knapp et al, 2008).…”

“…Laboratory experiments have determined the isotope effect in solution for helium in water as a function of temperature (Benson and Krause, 1980). Given that the helium isotope ratio may be further affected by isotopic fractionation in molecular diffusion (Bourg and Sposito, 2008) associated with the balance between wave-induced bubble trapping and air-sea exchange (Fuchs et al, 1987;Jenkins, 1988b), we have used our observations of the full suite of noble gases on these samples to develop a much more complete model of this dynamic equilibrium isotope effect. Thus, the dynamic solubility equilibrium value for 3 He, C eq , was determined by adding 3 He isotopes to a one-dimensional Price-Weller-Pinkel (PWP) model (Price et al, 1986) subject to 6-hourly NCEP (National Center for Environmental Prediction) reanalysis forcing (Kalnay et al, 1996) and QuikSCAT winds from the BATS site (Stanley et al, 2006(Stanley et al, , 2009b.…”