Turbulent nitrate fluxes in the Amundsen Gulf during ice‐covered conditions

This study compiles colocated oceanic observations of high‐resolution vertical profiles of nitrate concentration and turbulent microstructure around the Svalbard shelf slope, covering both the permanently ice‐free Fram Strait and the pack ice north of Svalbard. The authors present an overview over the seasonal evolution of the distribution of nitrate and its relation to upper ocean stratification. The average upward turbulent diffusive nitrate flux across the seasonal nitracline during the Arctic summer season is derived, with average values of 0.3 and 0.7 mmol m−2 d−1 for stations with and without ice cover, respectively. The increase under ice‐free conditions is attributed to different patterns of stratification under sea ice versus open water. The nitrate flux obtained from microstructure measurements lacked a seasonal signal. However, bottle incubations indicate that August nitrate uptake was reduced by more than an order of magnitude relative to the May values. It remains inconclusive whether the new production was limited by an unidentified factor other than NO3− supply in late summer, or the uptake was underestimated by the incubation method.

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“…Bourgault et al . [] estimate autumn

F_{N}

in the ice‐covered southeast Beaufort Sea, and find a flux of 0.5 mmol m −2 d −1 , which is about twice as large as our estimate for

F_{N}

under ice‐covered conditions. The season and regional hydrography are somewhat different and so the flux magnitudes compare reasonably well.…”

Section: Discussionsupporting

confidence: 60%

Vertical fluxes of nitrate in the seasonal nitracline of the Atlantic sector of the Arctic Ocean

et al. 2016

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“…However, these high upward nitrate fluxes diverge from the common understanding of nitrate replenishment in the Arctic Ocean and sub-Arctic seas. Convective winter mixing is usually assumed to be the major driver of the upward nutrient flux and replenishment of the nitrate concentrations in the surface layers (Louanchi and Najjar, 2001), because thermal and meltwater stratification tends to hamper deep vertical mixing during summer (Martin et al, 2010a;Bourgault et al 2011;Painter et al, 2014;Randelhoff et al 2016).…”

Section: Impact Of Water Column Stratification and Vertical Turbulentmentioning

confidence: 99%

Upward nitrate flux and downward particulate organic carbon flux under contrasting situations of stratification and turbulent mixing in an Arctic shelf sea

Wiedmann

Tremblay

Sundfjord

et al. 2017

Elementa: Science of the Anthropocene

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Wiedmann, I, et al 2017 Upward nitrate flux and downward particulate organic carbon flux under contrasting situations of stratification and turbulent mixing in an Arctic shelf sea. Elem Sci Anth, 5: 43, DOI: https://doi.org/10.1525/elementa.235 IntroductionThe declining sea ice cover in Arctic seas (Arrigo and van Dijken, 2015; IPCC, 2013) affects the pelagic marine ecosystem in contrasting ways. Sea ice melt strengthens the water column stratification and hampers the upward nitrate flux into the surface layer (Tremblay et al., 2015), while an absent sea ice cover allows various winddriven processes (e.g. wind-driven shear, breaking waves) to induce vertical mixing and shelf-break upwelling (Carmack and McLaughlin, 2011;Rainville et al., 2011;Falk-Petersen et al., 2015). Such mixing and upwelling can generate strong upward nitrate fluxes into the surface layers (Hales et al., 2005;Randelhoff et al., 2016). Dependent on the intensity of the nutrient renewal in the surface layer, the plankton abundance and composition may change, which in turn can modify the downward flux of particulate organic carbon (POC). However, definitive regulating mechanisms of the POC flux are still under discussion (Carmack and Wassmann, 2006;Wassmann and Reigstad, 2011; Forest et al., 2013).We conducted a field study in the Barents Sea, an Arctic shelf sea, to investigate the upward flux of nitrate and the downward flux of POC in contrasting field situations of ice cover, hydrography, mixing, and plankton abundance and composition. Arctic-derived water masses (temperature T < 0°C, salinity S = 34. 4-34.8;Loeng, 1991) influence the northern Barents Sea, which is seasonally ice-covered (annual maximum extension found during March-April; Kvingedal, 2005). In late spring and summer, the sea ice recedes northwards and a phytoplankton bloom commonly occurs in the marginal ice zone, where the waters are well-lit and contain high, winter-accumulated, nutrient concentrations after the ice break-up. This bloom is often associated with a major downward POC flux, because senescent stages, resting stages and aggregates of diatoms, the often prevailing microalgae, have high sinking velocities (Eppley et al., 1967;Bienfang, 1981; Iversen and Ploug, 2013 Increased sea ice melt alters vertical surface-mixing processes in Arctic seas. More melt water strengthens the stratification, but an absent ice cover also exposes the uppermost part of the water column to windinduced mixing processes. We conducted a field study in the Barents Sea, an Arctic shelf sea, to examine the effects of stratification and vertical mixing processes on 1) the upward nitrate flux (into surface layers <65 m) and 2) the downward flux of particulate organic carbon (POC) to ≤200 m. In the Arcticinfluenced, drift ice-covered northern Barents Sea, we found a low upward nitrate flux into the surface layers (<0.1 mmol nitrate m -2 d ). We suggest that strong wind events during our field study induced vertical mixing processes and triggered upwards nitrate flux, while a combination of d...

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“…This suggest that from a physical point of view, the buoyant vertical displacement of eggs is dominant over the turbulent diffusion mechanism, and only turbulent events about a 100 times above the background value could effectively influence their distribution. The role of turbulent mixing in egg distribution (aggregation or spreading) is likely minimal and the use of a parameterization such as the one presented in Bourgault et al (2011) could have been sufficient here. The effect of turbulence may only become important for denser eggs rising very slowly toward the surface or directly after the spawning if it occurs in a thin layer pattern, hence producing a high concentration gradient.…”

Section: Dynamical Interactions In the Water Columnmentioning

confidence: 99%

“…For two long-term stations the number of casts used in the average profile were respectively 24 and 25, whereas at least 5 profiles were used to build the 18 other mean profiles. Missing values in averaged profiles (e.g., near the surface or below 250 m) were dealt with according to Equation (1) from Bourgault et al (2011). Further details about this dataset can be found in their study.…”

Section: Simulationsmentioning

confidence: 99%

“…We first performed a scale analysis of the two first terms on the right-hand side of Equation (7), i.e., egg buoyancy and vertical turbulence. For the simulation scenario illustrated in Figure 4, we can estimate the vertical gradient in egg concentration with w ε [4, 9.3] m d −1 (see Results) and K = 3.4×10 m −6 s −1 (background turbulent diffusivity) from (Bourgault et al, 2011). This suggest that from a physical point of view, the buoyant vertical displacement of eggs is dominant over the turbulent diffusion mechanism, and only turbulent events about a 100 times above the background value could effectively influence their distribution.…”

Section: Dynamical Interactions In the Water Columnmentioning

confidence: 99%

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Impacts of Intraguild Predation on Arctic Copepod Communities

et al. 2016

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Communities of large copepods form an essential hub of matter and energy fluxes in Arctic marine food webs. Intraguild predation on eggs and early larval stages occurs among the different species of those communities and it has been hypothesized to impact its structure and function. In order to better understand the interactions between dominant copepod species in the Arctic, we conducted laboratory experiments that quantified intraguild predation between the conspicuous and omnivorous Metridia longa and the dominant Calanus hyperboreus. We recorded individual egg ingestion rates for several conditions of temperature, egg concentration, and alternative food presence. In each of these experiments, at least some females ingested eggs but individual ingestion rates were highly variable. The global mean ingestion rate of M. longa on C. hyperboreus eggs was 5.8 eggs ind −1 d −1 , or an estimated 37% of M. longa daily metabolic need. Among the different factors tested and the various individual traits considered (prosome length, condition index), only the egg concentration had a significant and positive effect on ingestion rates. We further explored the potential ecological impacts of intraguild predation in a simple 1D numerical model of C. hyperboreus eggs vertical distribution in the Amundsen Gulf. Our modeling results showed an asymmetric relationship in that M. longa has little potential impact on the recruitment of C. hyperboreus (<3% egg standing stock removed by IGP at most) whereas the eggs intercepted by the former can account for a significant portion of its metabolic requirement during winter (up to a third).

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Turbulent nitrate fluxes in the Amundsen Gulf during ice‐covered conditions

Cited by 27 publications

References 26 publications

Vertical fluxes of nitrate in the seasonal nitracline of the Atlantic sector of the Arctic Ocean

Vertical fluxes of nitrate in the seasonal nitracline of the Atlantic sector of the Arctic Ocean

Upward nitrate flux and downward particulate organic carbon flux under contrasting situations of stratification and turbulent mixing in an Arctic shelf sea

Impacts of Intraguild Predation on Arctic Copepod Communities

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