Estimating nutrient deposition

The term 'atmospheric deposition' includes nutrients in several forms.  Gaseous forms of nutrients (NHx, NOy) and small particles may be scavenged by water droplets and fall out of the atmosphere with precipitation as wet deposition.  Dry deposition of nutrients on dust and organic matter particles or other aerosols also occurs.  Different forms of nutrients have different transport properties and points of origin.   Reduced forms of nitrogen (NHx) are more rapidly scavenged from the air and thus do not carry as far as oxidized forms of nitrogen.  Phosphorus is associated with mineral dust, most of which has its origin in arid, poorly vegetated regions of the earth.  Gaseous forms of nitrogen are produced in industrial regions by combustion and agricultural regions by livestock.  Good discussions of this issue can be found in: Prospero et al., 1996; Charlson, 1992;Duce, 1991.

To the extent that a coastal ecosystem is coupled to an adjoining terrestrial watershed by river flow, it may see the effect of atmospheric deposition of nutrients over the entire catchment as well as the direct effects over its own surface area. In such cases, the direct deposition of nutrients in both wet (i.e. in precipitation) and dry forms may be small in comparison to the terrestrial contribution. To some degree, nutrients deposited on terrestrial ecosystems are sequestered locally, either in living tissue, organic matter, or relatively slowly moving groundwater storage.  Some of these nutrients may be harvested (as forest or agricultural products) and leave the system through human action.  Some nutrients (e.g. nitrate), through the action of some biological processes such as denitrification, may be returned to the atmosphere locally in gaseous form.  Despite these factors, in many cases, a significant proportion of atmospherically deposited nutrients makes its way to the sea.  Estimating these proportions is a challenging problem for terrestrial ecologists (Howarth et al., 1996).  

Apart from the problem of determining the proportion of riverborne nutrient fluxes that can be attributed to atmospheric deposition is the relatively simple problem of estimating direct deposition on the coastal ecosystem surface.  Here, 'relatively simple' means 'far from trivial', because the spatial and temporal variability of these fluxes is very high. As in the case of precipitation and evaporation, the direct effects of wet and dry deposition of nutrients is especially important for coastal systems with large surface areas of open water in comparison to the associated watershed area, especially in arid environments, in which the local terrestrial regions may be significant sources of nutrient-laden dust.

Deposition estimates for coastal systems are difficult to find - the long-term atmospheric chemistry monitoring stations are on land.  Some global and regional maps of average deposition are available (fig 1), but extrapolating land-based estimates of deposition to coastal areas may be problematic. Progress is being made with deposition estimates using complex mathematical models of these processes, but their reliability is uncertain.  If an estimate of local nutrient deposition flux is not available from a reference, we suggest a few rules of thumb:

• ignore it (i.e. assume that it is zero). This is not likely to be true, but in the absence of any data, it may be a good enough approximation for the budget calculation, especially if point sources of nutrients or riverine fluxes are known to be large, or if the surface area of the coastal ecosystem is small compared to the associated catchment (watershed) area.
• For nitrogen species, if an average rainfall concentration is known, or can be estimated, calculate the wet-deposition flux as the product of concentration and average annual rainfall input (volume), and double this value to estimate the combined wet and dry deposition (i.e. dry deposition is assumed to be approximately equal to wet deposition)

Some sources of deposition data on the web (there are many more):

• National Atmospheric Deposition Program
• NOAA air resources laboratory (ARL)
• ARL coastal ecosystems program
• International Global Atmospheric Chemistry Program (IGAC)
• IGAC data directory

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Last Updated 12 May 2009 by DPS