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

Abstract

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...

Estimating Carbon Flux From Optically Recording Total Particle Volume at Depths Below the Primary Pycnocline

“…Due to the strong katabatic winds in TNB in the Ross Sea, especially during the second visit to the site, the main pycnocline eroded and particles were mixed deeper into the water column ( Figures 2C,D, 3D). This process is similar to that previously described as the "mixed-layer pump" in the North Atlantic and in the Arctic where particles are convectively mixed deep into the water column (Gardner et al, 1995;Koeve et al, 2002;Giering et al, 2016;Wiedmann et al, 2017). Based on synchronous sediment trap and current meter observations, late summer-autumn wind-induced upper water column mixing has also been identified provoking enhanced organic C fluxes in the western Ross Sea (Langone et al, 2003).…”

Still Arctic? - The changing Barents Sea

“…The potential of mixing-induced nutrient supply was indicated around 74 N in 2012, where increased surface nutrient concentrations suggested recent mixing facilitated by the hydrographical conditions (Figure 8). Such episodic mixing in more weakly stratified AW has been suggested by Sakshaug and Slagstad (1991) to explain the high productivity in the southern Barents Sea and demonstrated in more recent studies (Fer and Drinkwater, 2014;Wiedmann et al, 2017). The depletion in nutrients was stronger farthest north (Figure 8).…”

Pan-Arctic Ocean Primary Production Constrained by Turbulent Nitrate Fluxes

“…In areas where the water column overturned in winter, summer fluxes were an order of magnitude below winter values. A notable exception was observed at one station in the Barents Sea south of the polar front (Wiedmann et al, 2017), where the water was weakly stratified even in summer and hence nitrate fluxes were probably at least as high as in winter with 5 mmol N m −2 d −1 (Table 1), although sample size (N = 1) was not sufficient to draw further conclusions.…”