Approximately half of the freshwater discharged from the Greenland and Antarctic Ice Sheets enters the ocean subsurface as a result of basal ice melt, or runoff draining via the grounding line of a deep ice shelf or marine-terminating glacier. Around Antarctica and parts of northern Greenland, this freshwater then experiences prolonged residence times in large cavities beneath floating ice tongues. Due to the inaccessibility of these cavities, it is unclear how they moderate the freshwater associated supply of nutrients such as iron (Fe) to the ocean. Here, we show that subglacial dissolved Fe export from Nioghalvfjerdsbrae (the ‘79°N Glacier’) is decoupled from particulate inputs including freshwater Fe supply, likely due to the prolonged ~162-day residence time of Atlantic water beneath Greenland’s largest floating ice-tongue. Our findings indicate that the overturning rate and particle-dissolved phase exchanges in ice cavities exert a dominant control on subglacial nutrient supply to shelf regions.

The duration and magnitude of the North Atlantic spring bloom impacts both higher trophic levels and oceanic carbon sequestration. Nutrient exhaustion offers a general explanation for bloom termination, but detail on which nutrients and their relative influence on phytoplankton productivity, community structure, and physiology is lacking. Here, we address this using nutrient addition bioassay experiments conducted across the midlatitude North Atlantic in June 2017 (late spring). In four out of six experiments, phytoplankton accumulated over 48–72 h following individual additions of either iron (Fe) or nitrogen (N). In the remaining two experiments, Fe and N were serially limiting, that is, their combined addition sequentially enhanced phytoplankton accumulation. Silicic acid (Si) added in combination with N + Fe led to further chlorophyll a (Chl a) enhancement at two sites. Conversely, addition of zinc, manganese, cobalt, vitamin B12, or phosphate in combination with N + Fe did not. At two sites, the simultaneous supply of all six nutrients, in combination with N + Fe, also led to no further Chl a enhancement, but did result in an additional 30–60% particulate carbon accumulation. This particulate carbon accumulation was not matched by a Redfield equivalent of particulate N, characteristic of high C:N organic exudates that enhance cell aggregation and sinking. Our results suggest that growth rates of larger phytoplankton were primarily limited by Fe and/or N, making the availability of these nutrients the main bottom‐up factors contributing to spring bloom termination. In addition, the simultaneous availability of other nutrients could modify bloom characteristics and carbon export efficiency.

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Lead (Pb) is a toxic element to humans (Carrington et al., 2019;Wani et al., 2015) and accumulation in marine biota a pathway of exposure (Burger et al., 2012;Zimmer et al., 2011). Lead emissions from coal combustion and the use of tetraethyllead as an additive to gasoline through the twentieth century (McConnell & Edwards, 2008;Pacyna & Pacyna, 2001) resulted in large scale atmospheric deposition of anthropogenic Pb across the surface ocean (Boyle et al., 2014). Whilst Pb concentrations across the North Atlantic are now declining, they remain above pre-industrial concentrations (Kelly et al., 2009;Noble et al., 2015). The ultimate fate of Pb in the marine environment is governed by its high affinity to particles (Dewey et al., 2021;Yang et al., 2015), which results in a dissolved Pb (dPb) distribution strongly affected by lateral transfer within sinking matter and burial in sediments (Bruland et al., 2013). However, a growing number of studies have shown intermittent release of dPb from shelf sediments (Cobelo-García & Prego, 2004;Kalnejais et al., 2007;Martino et al., 2002). This suggests that shelf sediments affected by a legacy of anthropogenic Pb deposition may continue to act as a dPb source to the water column (Rusiecka et al., 2018;Vieira et al., 2019).

Trace metals (TMs) are important to the functioning of marine ecosystems, with a range of TMs required as micronutrients by phytoplankton and serving as essential cofactors in metalloenzymes (Sunda, 1989(Sunda, , 2012. Iron (Fe), cobalt (Co), and manganese (Mn) are potentially (co-)limiting oceanic primary production (Browning