How much can riverine biogeochemical fluxes affect the arctic ocean acidification?

Abstract

Arctic rivers carry not only large amounts of freshwater but also biogeochemical materials into the ocean and play important roles in Arctic ocean acidification (OA). This study quantitatively evaluated the effects of riverine biogeochemical fluxes (R-BGC; carbon and nutrients) on the Arctic marine carbonate system and OA using multi-decadal experiments (1979–2018) with a pan-Arctic sea ice–ocean model. Improved initial and lateral boundary conditions of carbonate properties, observation-based riverine biogeochemical data, and land model-based interannually varying riverine freshwater discharge were adopted to enable more realistic experiments. The model simulated negative trends in aragonite saturation state (Ω) and pH in most regions of Arctic Ocean regardless of R-BGC. The increased riverine freshwater promoted more OA through the higher dilution effect. Compared to the experiment with riverine discharge of only freshwater, the inclusion of R-BGC caused positive anomalies in Ω and pH (by ∼0.14 and ∼0.03 in central basins, and by ∼0.15 and ∼0.06 in shelf seas, respectively). In the central basins, these anomalies were caused mostly by carbon (total alkalinity and dissolved inorganic carbon) of the R-BGC. In the shelf seas, nutrient (nitrate and silicate) fluxes also contributed ∼14% and ∼32% of the anomalies owing to the enhanced primary production and a corresponding reduction in seawater pCO₂. R-BGC mitigated OA (ΔΩ = −1.53 × 10⁻³ year⁻¹ and ΔpH = −0.56 × 10⁻³ year⁻¹) in regions where riverine freshwater was accumulated (i.e., the Canada Basin, Chukchi Cap, Eurasian Basin, and East Siberian Sea). This study stressed the importance of including R-BGC for OA model projection.

Key Points

• Simulations with only riverine freshwater overestimate Arctic ocean acidification and negative trends in aragonite saturation state and pH
• Addition of riverine biogeochemical fluxes, especially total alkalinity and dissolved inorganic carbon, can reduce this overestimation
• Riverine nutrient fluxes can also reduce ∼14% and ∼32% of the overestimation in aragonite saturation state and pH in shelf seas

Plain Language Summary

Ocean acidification (OA) is caused primarily by atmospheric CO₂ absorption at the sea surface. In the Arctic Ocean, riverine biogeochemical inputs (R-BGC; carbon and nutrients carried by riverine water), in addition to freshwater discharge, affect OA. This study quantified the effects of R-BGC on OA using a couple of model simulations for 1979–2018. Advancing from previous studies, this study conducted more realistic experiments by using improved initial and lateral boundary conditions of carbonate properties, observation-based riverine biogeochemical data, and the land model-based interannually varying riverine freshwater discharge data. R-BGC impacts on OA were found mainly in the surface waters of the Arctic, especially in coastal regions close to the river mouths. During 1989–2018, negative trends in carbonate saturation state and pH (both indicating OA) were simulated regardless of R-BGC in most regions of Arctic Ocean. Compared to a simulation with riverine input of only freshwater, R-BGC mitigated the simulated OA in the regions with increased riverine freshwater content. This study stressed the importance of R-BGC for model-based OA estimation and future projections. It will improve the understanding of OA under climate change and aid in informing marine ecosystem-based management.

Zhang Y., Yamamoto-Kawai M., Watanabe E. & Park H., 2024. How much can riverine biogeochemical fluxes affect the arctic ocean acidification? JGR Oceans 129(6): e2023JC020404. doi: 10.1029/2023JC020404. Article.

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