CHAPTER 2 • The Chemical and Physical Properties of Marine Aerosols: An Introduction
63
2.4.6
Atmospheric Deposition and the Nitrogen-Nutrient Budget in the Oceans
The primary productivity in surface waters of the ocean depends on the availability
of nutrients (see for example, Levitus et a1.1993), the most important being phosphate
and nitrate. Historically, the major sources of nutrients in the photic zone were considered to be the fixation of nitrogen in surface waters; the upward transport of nutrients from deeper waters; and the recycling of nutrients in the surface waters. However, Duce (1986) showed that atmospheric inputs could be important to some ocean
regions. Using a simple model, he made some preliminary estimates of the possible
impact of the deposition of various aerosol species on the surface waters and compared these atmospheric inputs to ocean sources such as upwelling. Duce found that
in some oligotrophic regions, the atmospheric input of NO; and Fe could contribute a
substantial and sometimes major fraction of these species to surface waters and thereby
have an impact on biogeochemical processes. Of particular interest is the role of these
species in primary productivity. In this section, we consider the possible significance
of NO x and NHx species. In Section 2.5 we consider the role of Fe deposition on the
ocean nutrient cycle.
The huge increase in anthropogenic emissions of NO x and NHx has resulted in major
disruptions of the nitrogen-nutrient cycle over the continents, and there is concern
about the impact of these inputs on coastal and ocean processes. It is known that pollutant nitrogen species such as NO; and NH! (Paerl199S) as well as various DON species (Seitzinger and Sanders 1999) can serve as a nutrients for microorganisms. The
impact on coastal waters has been particularly notable (Paerl199S; Howarth et a1.1996)
but the importance to the larger ocean has been more difficult to characterize. Recently, an effort was made to assess the nitrogen budgets for the North Atlantic and
the surrounding watersheds (Galloway et a1.1996). Prospero et al. (1996) estimated that
the total present-day (natural and anthropogenic) deposition of NO y to the North Atlantic Ocean (NAO) watershed, shelf, and open ocean is 670 Gmol yr-l, 79 Gmol yr-l,
and 360 Gmol yr -1, respectively; in addition, the estimated deposition of NHx to these
same regions is 390 Gmol yr- 1 , 55 Gmol yr- 1 , and 260 Gmol yr- 1 • In contrast, Michaels
et al. (1996) estimate that the source of NO; within the main thermocline of the NAO
due to N-fIxation is in the range of) 700 to 6400 Gmol yr- 1 • Thus, the total atmospheric
deposition of nitrogen-nutrient species to the open ocean, about 600 Gmol yr- 1 , constitutes a substantial fraction (roughly 10-15%) of the estimate production above the
thermocline. If the deposition of DON is comparable to that of NO; and NH! as suggested above, then the total nutrient-nitrogen deposition from the atmosphere is
doubled and would constitute 20-30% of the open-ocean in-water source.
Thus, it appears that atmospheric transport and deposition has indeed had a major impact on the N-cycle in the NAO compared to pre-human times. Atmospheric
transport has also impacted the N -cycle in other remote ocean regions. Figure 2.8
shows the deposition rate (mmol m- 2 yr- 1 ) of NO y and NHx in latitude bands extending from the western North Pacific across North America and the Atlantic to Europe
and Africa. Huge amounts of NOx and NHx are deposited on the continents. Also, the
deposition rate to the NAO is substantially higher than to the Pacific. Note also the
ratio of NOxINH x ' The ratio is much higher over North America compared to Europe.
This reflects the very different character of the emissions from these two regions, a
63
2.4.6
Atmospheric Deposition and the Nitrogen-Nutrient Budget in the Oceans
The primary productivity in surface waters of the ocean depends on the availability
of nutrients (see for example, Levitus et a1.1993), the most important being phosphate
and nitrate. Historically, the major sources of nutrients in the photic zone were considered to be the fixation of nitrogen in surface waters; the upward transport of nutrients from deeper waters; and the recycling of nutrients in the surface waters. However, Duce (1986) showed that atmospheric inputs could be important to some ocean
regions. Using a simple model, he made some preliminary estimates of the possible
impact of the deposition of various aerosol species on the surface waters and compared these atmospheric inputs to ocean sources such as upwelling. Duce found that
in some oligotrophic regions, the atmospheric input of NO; and Fe could contribute a
substantial and sometimes major fraction of these species to surface waters and thereby
have an impact on biogeochemical processes. Of particular interest is the role of these
species in primary productivity. In this section, we consider the possible significance
of NO x and NHx species. In Section 2.5 we consider the role of Fe deposition on the
ocean nutrient cycle.
The huge increase in anthropogenic emissions of NO x and NHx has resulted in major
disruptions of the nitrogen-nutrient cycle over the continents, and there is concern
about the impact of these inputs on coastal and ocean processes. It is known that pollutant nitrogen species such as NO; and NH! (Paerl199S) as well as various DON species (Seitzinger and Sanders 1999) can serve as a nutrients for microorganisms. The
impact on coastal waters has been particularly notable (Paerl199S; Howarth et a1.1996)
but the importance to the larger ocean has been more difficult to characterize. Recently, an effort was made to assess the nitrogen budgets for the North Atlantic and
the surrounding watersheds (Galloway et a1.1996). Prospero et al. (1996) estimated that
the total present-day (natural and anthropogenic) deposition of NO y to the North Atlantic Ocean (NAO) watershed, shelf, and open ocean is 670 Gmol yr-l, 79 Gmol yr-l,
and 360 Gmol yr -1, respectively; in addition, the estimated deposition of NHx to these
same regions is 390 Gmol yr- 1 , 55 Gmol yr- 1 , and 260 Gmol yr- 1 • In contrast, Michaels
et al. (1996) estimate that the source of NO; within the main thermocline of the NAO
due to N-fIxation is in the range of) 700 to 6400 Gmol yr- 1 • Thus, the total atmospheric
deposition of nitrogen-nutrient species to the open ocean, about 600 Gmol yr- 1 , constitutes a substantial fraction (roughly 10-15%) of the estimate production above the
thermocline. If the deposition of DON is comparable to that of NO; and NH! as suggested above, then the total nutrient-nitrogen deposition from the atmosphere is
doubled and would constitute 20-30% of the open-ocean in-water source.
Thus, it appears that atmospheric transport and deposition has indeed had a major impact on the N-cycle in the NAO compared to pre-human times. Atmospheric
transport has also impacted the N -cycle in other remote ocean regions. Figure 2.8
shows the deposition rate (mmol m- 2 yr- 1 ) of NO y and NHx in latitude bands extending from the western North Pacific across North America and the Atlantic to Europe
and Africa. Huge amounts of NOx and NHx are deposited on the continents. Also, the
deposition rate to the NAO is substantially higher than to the Pacific. Note also the
ratio of NOxINH x ' The ratio is much higher over North America compared to Europe.
This reflects the very different character of the emissions from these two regions, a