Impacts of atmospheric nutrient deposition on marine productivity: Roles of nitrogen, phosphorus, and iron

Abstract

[1] Nutrients are supplied to the mixed layer of the open ocean by either atmospheric deposition or mixing from deeper waters, and these nutrients drive nitrogen and carbon fixation. To evaluate the importance of atmospheric deposition, we estimate marine nitrogen and carbon fixation from present-day simulations of atmospheric deposition of nitrogen, phosphorus, and iron. These are compared with observed rates of marine nitrogen and carbon fixation. We find that Fe deposition is more important than P deposition in supporting N fixation. Estimated rates of atmospherically supported carbon fixation are considerably lower than rates of marine carbon fixation derived from remote sensing, indicating the subsidiary role atmospheric deposition plays in total C uptake by the oceans. Nonetheless, in high-nutrient, low-chlorophyll areas, the contribution of atmospheric deposition of Fe to the surface ocean could account for about 50% of C fixation. In marine areas typically thought to be N limited, potential C fixation supported by atmospheric deposition of N is only ∼1%-2% of observed rates. Although these systems are N-limited, the amount of N supplied from below appears to be much larger than that deposited from above. Atmospheric deposition of Fe has the potential to augment atmospherically supported rates of C fixation in N-limited areas. In these areas, atmospheric Fe relieves the Fe limitation of diazotrophic organisms, thus contributing to the rate of N fixation. The most important uncertainties in understanding the relative importance of different atmospheric nutrients are poorly understood speciation and solubility of Fe as well as the N:Fe ratio of diazotrophic organisms.Citation: Okin, G. S., et al. (2011), Impacts of atmospheric nutrient deposition on marine productivity: Roles of nitrogen, phosphorus, and iron, Global Biogeochem. Cycles, 25, GB2022,

Atmospheric P deposition to the subtropical North Atlantic: sources, properties, and relationship to N deposition

“…However, high "excess P" in dry deposition was only observed in the upper-end estimates of inorganic P inputs (i.e., where P = TP) and may not be environmentally relevant, since not all of this P is likely to be bioavailable. In most cases in Barbados and in all cases in Miami, deposition was a net source of excess N to surface waters, a result that fits well with model expectations [Okin et al, 2011].…”

“…Therefore, even though it is a relatively small source of P compared to other sources [Okin et al, 2011], atmospheric P can have a large impact on long-term nutrient limitation patterns if its relationship to new, nonrecycled, nitrogen (N) changes as it is currently doing. Over the past 150 years, the deposition of N compounds is estimated to have doubled [Duce et al, 2008], while the deposition of P species has remained relatively steady [Baker et al, 2003;Mahowald et al, 2008].…”

How well can we quantify dust deposition to the ocean?

“…Efforts to evaluate the supply of dust to the ocean are motivated in large part by the desire to assess the role of aerosols as a source of iron, an essential nutrient whose abundance is thought to influence rates and geographical patterns of photosynthesis and nitrogen fixation [1][2][3][4][5][6][7]. Ocean productivity, in turn, is a major component of the global carbon cycle [8], so there is a recognized need to include dust supply in global biogeochemical models [9].…”

“…Owing to the paucity and difficulty of observations, it is generally necessary to rely on model estimates of dust deposition to assess its impact on marine ecosystems and biogeochemical cycles [2,19]. A number of different approaches have been employed to assess the accuracy of dust models, revealing substantial inconsistencies in dust emission, horizontal and vertical distribution of dust, atmospheric lifetime and dust deposition, both among models and between models and observations [17,[19][20][21][22][23][24].…”

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“…and macro-nutrients to the open ocean (Okin et al 2011) and is a key component of the international GEOTRACES program (GEOTRACES Planning Group 2006). A priority of the GEOT-RACES program is to quantify both major and trace elements (e.g., Al, Fe, Ti, V, Zn, Pb, and Hg) and species such as nitrate and sulfate in marine aerosols.…”