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'.
Highlights
An overall increase in primary production has been documented over recent decades using satellite-based estimates, and ascribed to the widening seasonal ice zone and longer phytoplankton productive season over a larger area of the Arctic Ocean [9,10,11,12,13]
Primary production is regulated by seasonal variability in light, nutrient availability and sea surface temperature, each of which is intricately linked to the seasonal sea ice cycle and its coupling with upper ocean physics [14,17,18,19,20,21,22,23]
The phytoplankton spring bloom is initiated when PAR increases after the polar night, nutrient concentrations are high after autumn/winter replenishment, temperature increases and mixed layer depth decreases as surface waters are stratified by solar heating and/or sea ice meltwater input [27,28,29,30,31,32]
Summary
The Arctic is one of the fastest-warming regions on Earth, with atmospheric warming occurring at approximately double the global average rate [1], due to amplification of anthropogenic climate change by sea ice losses and strong albedo feedbacks on upper ocean heat uptake [2]. The phytoplankton spring bloom is initiated when PAR increases after the polar night, nutrient concentrations are high after autumn/winter replenishment, temperature increases and mixed layer depth decreases as surface waters are stratified by solar heating and/or sea ice meltwater input [27,28,29,30,31,32]. Nutrient supply is expected to increase and support increases in primary production in less-stratified regions, along the shelf breaks, in response to projected increases in frequency and intensity of episodic vertical mixing and upwelling as ice retreats northwards, storminess increases and the ice-free season extends [13,20,22,26,39,43,44,45]. Recent evidence from the shelf-slope north and west of
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