Abstract
Abstract. State-of-the-art global nutrient deposition fields are coupled here to the Pelagic Interactions Scheme for Carbon and Ecosystem Studies (PISCES) biogeochemistry model to investigate their effect on ocean biogeochemistry in the context of atmospheric forcings for pre-industrial, present, and future periods. PISCES, as part of the European Community Earth system model (EC-Earth) model suite, runs in offline mode using prescribed dynamical fields as simulated by the Nucleus for European Modelling of the Ocean (NEMO) ocean model. Present-day atmospheric deposition fluxes of inorganic N, Fe, and P into the global ocean account for ∼ 40 Tg N yr−1, ∼ 0.28 Tg Fe yr−1, and ∼ 0.10 Tg P yr−1. Pre-industrial atmospheric nutrient deposition fluxes are lower compared to the present day (∼ 51 %, ∼ 36 %, and ∼ 40 % for N, Fe, and P, respectively). However, the overall impact on global productivity is low (∼ 3 %) since a large part of marine productivity is driven by nutrients recycled in the upper ocean layer or other local factors. Prominent changes are, nevertheless, found for regional productivity. Reductions of up to 20 % occur in oligotrophic regions such as the subtropical gyres in the Northern Hemisphere under pre-industrial conditions. In the subpolar Pacific, reduced pre-industrial Fe fluxes lead to a substantial decline of siliceous diatom production and subsequent accumulation of Si, P, and N, in the subpolar gyre. Transport of these nutrient-enriched waters leads to strongly elevated production of calcareous nanophytoplankton further south and southeast, where iron no longer limits productivity. The North Pacific is found to be the most sensitive to variations in depositional fluxes, mainly because the water exchange with nutrient-rich polar waters is hampered by land bridges. By contrast, large amounts of unutilized nutrients are advected equatorward in the Southern Ocean and North Atlantic, making these regions less sensitive to external nutrient inputs. Despite the lower aerosol N : P ratios with respect to the Redfield ratio during the pre-industrial period, the nitrogen fixation decreased in the subtropical gyres mainly due to diminished iron supply. Future changes in air pollutants under the Representative Concentration Pathway 8.5 (RCP8.5) emission scenario result in a modest decrease of the atmospheric nutrients inputs into the global ocean compared to the present day (∼ 13 %, ∼ 14 %, and ∼ 20 % for N, Fe, and P, respectively), without significantly affecting the projected primary production in the model. Sensitivity simulations further show that the impact of atmospheric organic nutrients on the global oceanic productivity has turned out roughly as high as the present-day productivity increase since the pre-industrial era when only the inorganic nutrients' supply is considered in the model. On the other hand, variations in atmospheric phosphorus supply have almost no effect on the calculated oceanic productivity.
Highlights
Marine primary production is a critical component of the global carbon cycle and important for sustaining the habitability on Earth, it is vulnerable to environmental changes (e.g., Steinacher et al, 2010)
This study presents the implementation of state-of-the-art monthly mean atmospheric deposition fields in the global biogeochemistry model PISCES
For the present day, ∼ 40 Tg N yr−1, ∼ 0.28 Tg Fe yr−1, and ∼ 0.10 Tg P yr−1 of inorganic nitrogen, iron, and phosphorus atmospheric inputs to the global ocean are considered in PISCES
Summary
Marine primary production is a critical component of the global carbon cycle and important for sustaining the habitability on Earth, it is vulnerable to environmental changes (e.g., Steinacher et al, 2010). Global warming induced by greenhouse gas emissions has increased ocean stratification, reducing the supply of nutrients from subsurface waters and inhibiting the growth of phytoplankton in the surface ocean (Behrenfeld et al, 2006). Several studies have documented the importance of primary production on the surface ocean CO2 concentrations (e.g., Falkowski et al, 2000; Gruber, 2004; Gruber et al, 2009; Le Quéré et al, 2013; Smith, 2019) via the carbon uptake and sinking of the particulate organic matter to the deeper ocean. Significant uncertainties remain in the projected production among state-of-the-art model simulations, which can range between 2 % and 20 % for the Coupled Model Intercomparison Project phase 3 (CMIP3) and CMIP5 models (Fu et al, 2016; Steinacher et al, 2010), mainly due to the different responses of phytoplankton production to changes in water temperature and stratification (Gröger et al, 2013; Laufkötter et al, 2016; Steinacher et al, 2010)
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