Pan-Arctic Ocean Primary Production Constrained by Turbulent Nitrate Fluxes

Abstract

Arctic Ocean primary productivity is limited by light and inorganic nutrients. With sea ice cover declining in recent decades, nitrate limitation has been speculated to become more prominent. Although much has been learned about nitrate supply from general patterns of ocean circulation and water column stability, a quantitative analysis requires dedicated turbulence measurements that have only started to accumulate in the last dozen years. Here we present new observations of the turbulent vertical nitrate flux in the Laptev Sea, Baffin Bay, and Young Sound (North-East Greenland), supplementing a compilation of 13 published estimates throughout the Arctic Ocean. Combining all flux estimates with a Pan-Arctic database of in situ measurements of nitrate concentration and density, we found the annual nitrate inventory to be largely determined by the strength of stratification and by bathymetry. Nitrate fluxes explained the observed regional patterns and magnitudes of both new primary production and particle export on annual scales. We argue that with few regional exceptions, vertical turbulent nitrate fluxes can be a reliable proxy of Arctic primary production accessible through autonomous and large-scale measurements. They may also provide a framework to assess nutrient limitation scenarios based on clear energetic and mass budget constraints resulting from turbulent mixing and freshwater flows.

On the Variability of Stratification in the Freshwater-Influenced Laptev Sea Region

“…In contrast, higher concentrations of these nutrients were observed in bottom waters of the ROFI below the pycnocline (Pivovarov et al, 2005). The strong stratification within the ROFI inhibits vertical mixing and consequently vertical turbulent nitrate fluxes (Randelhoff et al, 2020). Thus, the concentration of inorganic nitrogen, which decreases strongly during the spring bloom, remains at low levels in the SML (Pivovarov et al, 2005;Nitishinsky et al, 2007;Thibodeau et al, 2017).…”

“…Longer open water seasons inarguably enhance the photosynthetic active radiation (PAR), which enhances the limiting role of nutrients in the future Arctic ecosystem. The supply of nutrients, however, apart from continental discharge, depends on vertical mixing rates and therefore is regulated by stratification on a Pan-Arctic scale (Randelhoff et al, 2020). Model projections by Slagstad et al (2015) on the future state of the Arctic Ocean ecosystem predict only regional increases in new production such as along the continental slopes.…”

The influence of Arctic Fe and Atlantic fixed N on summertime primary production in Fram Strait, North Greenland Sea

“…Conversely, in winter, deep mixing to depths > 100 m below the nutrient-depleted surface layer (10–25 m), replenishes surface waters of Fram Strait region with N 15 , 78 . Rates of surface WSC fixed N replenishment may be an order of magnitude greater during autumn/winter than summer 49 , 79 , making deep winter mixing likely the dominant factor controlling the extent of annual new primary production in the Fram Strait region 80 . Deep-water re-supply of surface Fe is more restricted, due to deeper ferriclines that are observed to depths > 150 m in the WSC of Fram Strait (Fig.…”

“…As an Arctic outflow region, Fram Strait is clearly strongly affected by processes occurring within the Arctic. Broad-scale changes to Arctic inflow, sea-ice cover, lateral advection via the Transpolar Drift and wind driven mixing will all have downstream effects on nutrient stoichiometry in Fram Strait 80 , 90 , 91 . How lateral and vertical nutrient supply will change in the future Arctic as a result of on-going changes to sea-ice retreat and stratification remains uncertain, as neither lateral transport of nutrients in the Transpolar Drift, or vertical turbulent supply of nutrients during summer are well quantified 13 , 75 , 92 .…”

“…Surface primary production in the Arctic Ocean due to increased summertime light availability has increased during the past few decades 3 , 93 . Second to light limitation, Arctic Ocean net primary production seems foremost limited by the availability of fixed N 15 , 80 and may at some point hinder future increases in productivity, although changes in nutrient supply ratios and/or phytoplankton species composition may result in other nutrients such as Si, PO 4 or dFe to become limiting factors as well 11 , 94 . In the future, the Arctic Ocean is projected to have less ice coverage and become fresher and more stratified, which would reduce winter mixing depth and the associated supply of deep water nutrients 8 , 95 .…”