Open ocean atmospheric deposition

Trace metals are introduced to the ocean by atmospheric feUout, river runoff, and hydrothermal activity. The latter two are sources of soluble metals, which are primarily reduced species. Upon introduction into seawater, these metals react with O2 and are converted to insoluble oxides. Some of these precipitates settle to the seafloor to become part of the sediments others adsorb onto surfaces of sinking and sedimentary particles to form crusts, nodules, and thin coatings. Since reaction rates are slow, the metals can be transported considerable distances before becoming part of the sediments. In the case of the metals carried into the ocean by river runoff, a significant fraction is deposited on the outer continental shelf and slope. Hydrothermal emissions constitute most of the somce of the metals in the hydrogenous precipitates that form in the open ocean. 

Inputs of new N into coastal systems are more diverse than inputs to the open ocean and include compounds considered regenerated N sources in the open ocean. New N can be dehvered by rivers, runoff events, and atmospheric deposition and can include a whole spectrum of N compounds including NH4 , urea, and DON (e.g., Anderson et al., 2002 Glibert et al., 2001, 2005c MuIhoUand et al., 2007). Similarly NH4 derived from natural processes and anthropogenic activities can support new production in estuarine and coastal systems (see Chapter 9 by Seitzinger and Harrison and Chapter 11 by Paerl and Piehler, this volume). 

Atmospheric N deposition can be an important source of N to coastal and open ocean ecosystems. The potential importance of N in atmospheric deposition has been recognized for over twenty years (e.g., CorreU and Ford, 1982 Duce, 1986 Paerl, 1985). Recognition of the importance of atmospheric deposition as a source of N to coastal waters increased rapidly following the analysis by Fisher and Oppenheimer (1991) for a number of coastal systems, including Chesapeake Bay. Atmospheric deposition to watersheds contributes to diffuse N loads in rivers as discussed previously in this chapter. In this section we are specifically referring to N deposited (wet and dry) direcdy to the surface of coastal and open ocean waters. 

For open ocean regions atmospheric deposition is calculated as a percent of biological N2-fixation plus atmospheric deposition, assuming river N inputs are removed within coastal and shelf sediments. For continental shelves the range includes uncertainties in river N inputs for open ocean estimates, the range is that calculated with and without including DON in rainwater. See Fig. 6.6 for the contribution of atmospheric deposition in watersheds to river N export. 

Open ocean regions receive significant amounts of land-based N from atmospheric deposition ( 25 Tg N yr Dentener pers. comm.). This estimate of atmospheric deposition does not include dissolved organic nitrogen (DON), which may account for from 20% to as much as 80% of the total dissolved N in rainwater in remote marine locations (Cornell et al., 1995, 2001, 2003 Mace et al. 2003). Keeping in mind that this estimate of DON is uncertain because of the... 

The continental shelves receive N from the open ocean (820 x 10 molyear ), from estuaries (250 x 10 mol year ), from major rivers (350 x 10 mol year ) and from atmospheric deposition (130 x 10 mol year ). Some is lost to the sediments (120 x 10 mol year ) and fish catch (32 x 10 mol year ), but the majority is removed from the system via sedimentary denitrification (1400 x 10 mol year ). Nitrogen introduced to the shelves from the open ocean appears to contribute the most to shelf denitrification (Seitzinger and Gibhn, 1996). 

Several comprehensive studies of N assimilation in the North Pacific trades biome have been conducted over the past several decades. Gundersen and his colleagues (1974, 1976) were the first to estabhsh N2 fixation as a source of new N to the open ocean ecosystem, and concluded that it was a more important source of fixed N than wet deposition from the atmosphere (see Case Studies section). They also made measurements of the rates of nitrification, denitrification and assimilatory nitrate-reduction. These latter experiments involved the addition of fairly high concentrations of exogenous N substrates (NH4 , N02, NOa ) and extended incubations (days to months), so the rates reported must be viewed as potential rates at best. 

Inputs, outputs and exchanges of N with systems adjacent to salt marshes are generally much smaller in magnitude than internal fluxes (Table 22.7). The source and relative importance of various external inputs of N to salt marshes varies from system to system. While the input of N from rivers is potentially large, most of this N is probably not taken up by salt marshes but is processed in aquatic portions of estuaries or routed to the open ocean. On average, the largest input is from N fixation (2-15 g N m year ), followed by atmospheric deposition (0.5-2.2 g N year ). Groundwater inputs are a major source of N in some smaller salt marshes with developed uplands such as found in the northeastern United States. 

There remains an intriguing inconsistency between experiments related to the mechanisms for mercury removal. Many lab, field, and model efforts indicate that the lifetime of mercury in the atmosphere must be 1 -2 yr, but there exist a number of plausible removal mechanisms (such as foliar mercury uptake followed by litterfall) that suggest the flux from the atmosphere is more consistent with lifetimes that are less than 1 yr. The likely resolution of this problem hes in the observation that majority of the Earth s surface is covered by areas that are not temperate or boreal forests, including the open ocean and tropical regions. The deposition to the ocean is consistent with an atmospheric residence time in excess of 1 yr, while the mercury cychng within tropical forests is understudied. 

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