Manganese in coastal rainwater: speciation,photochemistry and deposition to seawater

Concentrations of manganese in 56 rain events in Wilmington, NC, USA rainwater from April 1, 2005 to March 31, 2006 were 11 ± 3 nM for dissolved Mn and 1.2 ± 0.4 nM for particulate Mn. Concentrations of both forms of Mn were higher in terrestrial storms relative to marine events. This observation along with the positive correlation of Mn with pollutant indicators suggests anthropogenic inputs to rain at this location, as has been observed at other locations. The ratio of Mn_part/Mn_diss was threefold larger in summer relative to winter rain, which matched the increase of particulate to dissolved Fe in rainwater suggesting influence of Saharan dust during the summer. Like Fe in rain, Mn undergoes photoreduction in rainwater, which has also been shown to be important in Mn cycling in seawater. The flux of Mn removed from the atmosphere via wet deposition is 1.5 × 10⁻⁵ moles m⁻² yr⁻¹ at this location, which is approximately twice the flux reported from two rainwater studies conducted in the early 1980s on Bermuda. Atmospheric input of Mn to the oceans is important because Mn like Fe is an essential and potentially limiting nutrient. Experiments mixing authentic rainwater and seawater demonstrate that rainwater dissolved Mn does not rapidly precipitate in seawater suggesting wet deposition is an important source of soluble, stable Mn to surface seawater.