Nitrate supply and uptake in the Atlantic Arctic sea ice zone: seasonal cycle, mechanisms and drivers

Abstract

Nutrient supply to the surface ocean is a key factor regulating primary production in the Arctic Ocean under current conditions and with ongoing warming and sea ice losses. Here we present seasonal nitrate concentration and hydrographic data from two oceanographic moorings on the northern Barents shelf between autumn 2017 and summer 2018. The eastern mooring was sea ice-covered to varying degrees during autumn, winter and spring, and was characterized by more Arctic-like oceanographic conditions, while the western mooring was ice-free year-round and showed a greater influence of Atlantic water masses. The seasonal cycle in nitrate dynamics was similar under ice-influenced and ice-free conditions, with biological nitrate uptake beginning near-synchronously in early May, but important differences between the moorings were observed. Nitrate supply to the surface ocean preceding and during the period of rapid drawdown was greater at the ice-free more Atlantic-like western mooring, and nitrate drawdown occurred more slowly over a longer period of time. This suggests that with ongoing sea ice losses and Atlantification, the expected shift from more Arctic-like ice-influenced conditions to more Atlantic-like ice-free conditions is likely to increase nutrient availability and the duration of seasonal drawdown in this Arctic shelf region. The extent to which this increased nutrient availability and longer drawdown periods will lead to increases in total nitrate uptake, and support the projected increases in primary production, will depend on changes in upper ocean stratification and their effect on light availability to phytoplankton as changes in climate and the physical environment proceed. This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.

Nitrate assimilation and regeneration in the Barents Sea: insights from nitrate isotopes

“…For samples when nitrite was < 2.5 % of nitrate+nitrite, we correct our δ 18 O-NO 3 data for nitrite interference following Kemeny et al (2016). The δ 15 N-NO 3 +NO 2 samples were also corrected assuming a δ 15 N-NO 2 of −24 ‰ (Kemeny et al, 2016;Henley et al, 2017).…”

“…The northern Barents Sea is therefore transitioning from a cold, salinity-stratified shelf into a warmer, less-stratified Atlanticdominated climate regime (Lind et al, 2018), a process described as "Atlantification". These changes may increase nutrient availability to phytoplankton over the growing season (Henley et al, 2020;Randelhoff et al, 2018), which is increasingly a control on Arctic net primary production (NPP) (Lewis et al, 2020).…”

The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning

“…These changes can have substantial implications for ecosystem functioning by altering carbon drawdown and storage, trophic interactions, nutrient cycling and the integrity of Arctic assemblages. Here, Henley et al [31] document seasonal availability of nitrate in the surface ocean on the northern Barents Sea shelf. They show that, while availability varies little between ice-covered and ice-free locations, the productivity season in ice-free waters is extended by advection of nutrients in Atlantic waters.…”

Solar radiation and solar radiation driven cycles in warming and freshwater discharge control seasonal and inter‐annual phytoplankton chlorophyll a and taxonomic composition in a high Arctic fjord (Kongsfjorden, Spitsbergen)

“…Summer stratification (estimated as the density difference relative to the maximal spring density prior to stratification during week 14) was on average sixfold stronger than during spring, restricting access to deep nutrients. This is in contrast to the weaker summertime stratification in the Fram Strait, and sea ice free parts of the Barents Sea, where wind mixing can periodically entrain nutrients in the surface ocean (Stramska 2005; Henley et al 2020). As a consequence of the freshwater discharge, Kongsfjorden spring and summertime nutrient draw down patterns have a stronger resemblance to the sea ice covered northern Barents Sea shelf (Henley et al 2020).…”

“…This is in contrast to the weaker summertime stratification in the Fram Strait, and sea ice free parts of the Barents Sea, where wind mixing can periodically entrain nutrients in the surface ocean (Stramska 2005; Henley et al 2020). As a consequence of the freshwater discharge, Kongsfjorden spring and summertime nutrient draw down patterns have a stronger resemblance to the sea ice covered northern Barents Sea shelf (Henley et al 2020). Taxonomic composition differed markedly during spring and summer.…”