Calculating annual deposition fluxes

To directly compare the two sites, annual deposition fluxes were calculated. For each 9-day collection, the flux was calculated from the measured concentration and the amount of rain i.e. ... 

The annual dry depositional fluxes were re-estimated for SC and STP using the long-term dataset. The new estimates are summarized in Table IV and, again the dry depositional fluxes were of the same magnitude as the wet depositional fluxes at SC and STP, although the new SC estimate was lower than that calculated previously using the intensive data. 

Nonetheless, methylmercury production can be proportional to Hg concentration (83, 89) and may be limited in lakes by the flux of reactive Hg(II) species across the sediment-water interface (52). Thus an increase in total Hg deposition could produce an equivalent response in Hg bioaccumulation, all other factors being equal. It may be significant in this regard that the average rate of increase in Hg residues in fish in Minnesota is of the same magnitude as that calculated here for atmospheric loading, roughly 3% annually since 1930 (19). 

Transport by Calcite. The annual flux of calcite into the uppermost trap (29 m below the lake surface) was estimated to be 35 g/m2, and the mean measured P content of this phase was estimated at 1.05 mg/g. A comparison of the calculated 29-m P flux, 37 mg/m2, with estimates of deposition to bottom sediment, indicated that 5-16 mg/m2 (13-43% of upper-water-column flux) was returned to the water column. This relatively small regeneration flux was not detected in profiles of meta- and hypolimnetic... 

Net sedimentation is defined as the flux of material incorporated into the permanent sediment record. 210Pb and 137Cs geochronologies indicate a mass sedimentation rate of 103 g/m2 per year for profundal sediments in Little Rock Lake. By using the mean Hg concentration (118 ng/g) in the top 1-cm slice of our bulk sediment profile, we estimated an annual net sedimentation of 12 xg of HgT/m2 per year. This net accumulation rate is similar to the calculated atmospheric input rate of about 10 xg/m2 per year (18, 19). Additionally, gross deposition rates (from sediment traps) exceeded these estimates by about a factor of 3 this rate suggests substantial internal recycling of material deposited at the sediment-water interface in this lake. 

To support the positive trend between 1993 and 1999, a simple scenario with Cd input and flux data for the whole Baltic Sea was estimated (Pohl and Hennings, 2005), leading to a Cddiss increase of 0.0105 nmol/(kg year) in Baltic Sea surface waters. Compared to the annual Cdjiss increase of0.0074 nmol/(kg year) between 1993 and 1999 as calculated above, this is a realistic value considering that data for atmospheric deposition, river input, and particulate export are still tenuous. 

The estimate for sea salt goes back to a detailed study of Eriksson (1959) of the geochemical cycles of chloride and sulfur. He calculated the rate of dry fallout of sea-salt particles from a vertical eddy diffusion model and then existing measurements of sea-salt concentrations over the ocean. This led to a global rate for dry deposition of 540 Tg/yr. Eriksson then argued that wet precipitation would remove a similar amount annually. It is now known, however, that wet precipitation is more effective than dry deposition in removing aerosol particles from the atmosphere, so that Eriksson s value must be an underestimate. The discussion in Section 10.3.5 suggests a flux rate for sea salt of about 5,000 Tg/yr. 

Using annual emission surveys and trajectory calculations, the concentration, deposition, and transboundary flux of sulphur compounds will be continuously estimated in order to evaluate the atmospheric transport and fate of sulphur dioxide. The model estimates will be compared with the dally measurements at the monitoring stations. 

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