Nutrient cycling in the Arctic and Subarctic oceans: a stable isotope study

Abstract

Anthropogenic global warming is actively changing nutrient supply and the food web of the Arctic Ocean and the subpolar regions. This study uses the stable isotopes of dissolved silicon and nitrate, two vital nutrients for marine life, to investigate the marine biogeochemical cycling of nutrients in these regions. This work analyses datasets acquired from 7 oceanographic expeditions in three key regions: the Laptev Sea shelf, polar outflow waters of the Fram Strait (79˚N) and a full transect across the subpolar North Atlantic (50-60˚N). Hydrographic data, alongside concentrations of nitrate (NO3), dissolved silicon (DSi) and their isotopic composition (d15N-NO3, d18O-NO3, d30Si(OH)4) is presented to provide spatially and temporally integrated information on biogeochemical cycling in these regions. The overall objective of this work is to determine the processes which control nutrient budgets and cycling in the Arctic Ocean, export to the subpolar regions and the sensitivity of these processes to ongoing climate change.

On the shallow Eurasian shelves of the Arctic Ocean, nitrogen is strongly depleted. This results from intense biological utilisation and significant benthic denitrification in the coastal regions, coupled with nitrogen-poor freshwater sources. Primary production in these regions is limited by N availability as a result of this. This puts a biological control on the extent of DSi utilisation in surface waters and modulates its export to the central Arctic Ocean. Over 40% of riverine DSi supplied by the Lena river is consumed and buried into the sediments of the Laptev shelf, enabled by vigorous recycling of nitrogen. Extrapolating these burial rates to the Eurasian shelf leads to an excess riverine DSi export of 3.10 ± 0.71 kmol/s through the Transpolar Drift to the central Arctic Ocean and outflowing currents.

Consequently, Eurasian rivers significantly contribute to the DSi inventory of outflow polar surface waters, providing 40 ± 4% of the total DSi. By contrast, Pacific sources, which were previously estimated to be an important source of export of DSi, only contribute to 8 ± 1% of the total inventory. Glacial DSi influence from melting of the Greenland Ice sheet was found to be negligible. The Si budget is thus primarily controlled by biological processes on Arctic shelves, which currently act to enrich the d30Si(OH)4 outflowing water masses by 0.1‰ compared to Atlantic inflow (1.7‰). Climate change is increasing riverine inputs of DSi faster than N. As the export of DSi from the Arctic Ocean is dependent on N-availability, outflow waters could transport a larger flux of DSi in the future, with lowered isotopic signature.

In the subpolar North Atlantic, nutrient properties of surface waters are integrated into the deep through convective water mass formation. Thus, biological assimilation and regeneration of nutrient stocks at high and low latitudes impact the nutrient inventory of North Atlantic deep waters. Surface waters of the North Atlantic have lighter d30Si(OH)4 (1.7‰) than predicted considering its nutrient deplete nature. Important processes at low latitudes act to dilute DSi concentrations of Atlantic surface waters and dampen their isotopic signature. This signal is integrated with the one of heavily utilised surface waters from the subpolar regions and the Nordic Seas into the deep North Atlantic. In recent years, deviation of the Labrador Current to the subpolar North Atlantic has reduced N assimilation. The freshwater content of the subpolar regions is predicted to increase from increased glacial melt and freshwater supply. This can act to increase stratification and decrease primary production of the region in the future. Due to the interconnectivity of the subpolar regions on the global scale, this can be reflected into the deep convective waters of the Atlantic and affect nutrient availability in the Eurasian Arctic.