Meltwater as a driver of changing nickel availability in the polar ocean?

Pelagic dissolved (< 0.2 µm) nickel (dNi) concentrations are rarely depleted below 2 nmol L−1 and normally remain tightly correlated with the macronutrients dissolved silicic acid and phosphate. Nickel is therefore widely disregarded as a possible limit on primary metabolism in modern-day aquatic environments. However, here we demonstrate that low dNi concentrations can arise in some polar regions due to low Ni concentrations in meltwater and thus there are environmental contexts in which Ni availability may plausibly constrain primary producers. Here we characterize the dNi concentrations of meltwater across a range of Arctic localities and present new pelagic measurements from Disko Bay (western Greenland) where previous measurements of dNi indicated the lowest concentrations ever measured in the Arctic or Atlantic. Across 10 (sub)Arctic glacier-fed streams/rivers we find a broad range of mean dNi concentrations from 0.40–132 nmol L−1. Most of the surveyed glacier-fed streams had dNi concentrations within the range 3–13 nmol L−1, which is comparable to major river systems worldwide. Yet three evidenced much lower concentrations with mean dNi < 2 nmol L−1 which would act to dilute dNi concentrations almost anywhere in the ocean. Similarly, meltwater from iceberg fragments in southwest and west Greenland had very low dNi concentrations (mean 0.6 nmol L−1 and 0.2 nmol L−1, respectively). Meltwater therefore appears to be a possible driver of low dNi concentrations, especially when icebergs, rather than runoff enriched with dNi from bedrock weathering, are a dominant freshwater source. However, dNi flux budgets for Arctic fjords reveal that vertical entrainment of deep, Ni-rich waters driven by subglacial discharge plumes often also constitutes a measurable fraction of dNi supply to surface waters, limiting the impact of low-Ni meltwater except under very specific ice-melt stratified scenarios. Whilst in the modern-day ocean there appear to be only limited localized scenarios in which dilution by meltwater might deplete dNi concentrations to levels which could plausibly constrain primary producers, low dNi conditions might have been more prevalent during deglaciation events in Earth’s past when regions of the ocean were stratified by large meltwater fluxes.