Abstract
Global warming resulting from the release of anthropogenic carbon dioxide is rapidly changing the Arctic Ocean. Over the last decade sea ice declined in extent and thickness. As a result, improved light availability has increased Arctic net primary production, including in under-ice phytoplankton blooms. During the GEOTRACES cruise PS94 in the summer of 2015 we measured dissolved iron (DFe), nitrate and phosphate throughout the central part of the Eurasian Arctic. In the deeper waters concentrations of DFe were higher, which we relate to resuspension on the continental slope in the Nansen Basin and hydrothermal activity at the Gakkel Ridge. The main source of DFe in the surface was the Trans Polar Drift (TPD), resulting in concentrations up to 4.42 nM. Nevertheless, using nutrient ratios we show that a large under-ice bloom in the Nansen basin was limited by Fe. Fe limitation potentially prevented up to 54% of the available nitrate and nitrite from being used for primary production. In the Barents Sea, Fe is expected to be the first nutrient to be depleted as well. Changes in the Arctic biogeochemical cycle of Fe due to retreating ice may therefore have large consequences for primary production, the Arctic ecosystem and the subsequent drawdown of carbon dioxide.
Highlights
The Arctic Ocean is the most rapidly changing region of our planet due to recent global warming (IPCC, 2013); yet the central Arctic belongs to the least studied parts of the Earth
In the Central Arctic the Polar Surface Water (PSW) flows into the direction of Fram Strait in the Trans Polar Drift (TPD) (Gordienko and Laktionov, 1969; Gregor et al, 1998; Macdonald et al, 2005)
The most important source of dissolved Fe (DFe) to the surface of the Arctic Ocean is the TPD, transporting river water with high concentrations of Fe complexed by organic ligands (Slagter et al, 2017) from the Arctic shelf seas toward Fram Strait
Summary
The Arctic Ocean is the most rapidly changing region of our planet due to recent global warming (IPCC, 2013); yet the central Arctic belongs to the least studied parts of the Earth. Arctic sea ice has been observed to decline in extent (Stroeve et al, 2012; IPCC, 2014; Serreze et al, 2016) and thickness (Haas et al, 2008; Serreze and Stroeve, 2015) and changed from multi-year sea ice into more first-year sea ice (Maslanik et al, 2011). Increased light penetration and nutrient availability during spring from earlier ice breakup enhances primary production in the Arctic Ocean and its adjacent shelf seas (Bhatt et al, 2014). The assumption has been for a long time that primary productivity is negligible in waters beneath ice because of insufficient light (Arrigo and Van Dijken, 2011).
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