Nitrogen deposition to the eastern Atlantic Ocean. The importance of south-easterly flow

Abstract

Converting measured concentrations into fluxes and using estimates of biological productivity in the coastal waters of the eastern Atlantic Ocean enables us to determine the role of the atmosphere as a source of biologically essential species, including nitrate and ammonium, to the marine biota. To understand the effects of the atmosphere as a source of nitrogen capable of promoting new production, we need to know both the seasonality of the input as well as the effects of extreme high deposition events which, while small in overall annual budget terms, maybe able to extend, or even promote, phytoplankton growth under nutrient depleted summer conditions. Aerosols and rainwater were collected at both Mace Head and at sea aboard RRS Challenger. Temporal patterns have been interpreted using airmass back trajectories which give the predicted air path prior to arrival at the sampling site. Low levels of both nitrate and ammonium are seen associated with marine westerly flow across the Atlantic and northerly air originating in the Arctic region. As expected, marine derived sodium, chloride, magnesium and seasalt sulphate are high during these periods. High concentration nitrate and ammonium events are seen associated with south-easterly flow where the airmass passes over the UK and northern Europe prior to arrival on the west coast of Ireland. In the polluted atmosphere, nitrate exists as nitric acid and as fine mode (<1 mm diameter) ammonium nitrate aerosol. In the coastal zone, nitric acid reacts with coarse mode seasalt aerosols to form coarse mode (>1 mm diameter) sodium nitrate: HNO 3 (g)+NaCl(s) NaNO 3 (s)+HCl(g). This seasalt displacement reaction not only enhances dry nitrate deposition through more efficient gravitational settling of large particles, but also increases the efficiency of precipitational scavenging via inertial impaction. By looking at the size distribution of nitrate, we can see evidence for the seasalt displacement reaction. Under the polluted south-easterly flow,~40-60% of the nitrate occurs in the coarse mode fraction. Under clean marine conditions, the seasalt displacement reaction results in almost complete conversion of nitrate from the fine to the coarse aerosol mode. By converting measured wet and dry nitrate, ammonium and organic nitrogen concentrations into fluxes and comparing the data with estimates of biological productivity in the surface waters, our data suggest that~30% of new production in eastern Atlantic surface waters off Ireland can be supported by atmospheric inputs in May 1997 and that most of the input occurs during short lived, high-concentration, south-easterly transport events. and rivers . While the vast major-

Modeling the Dynamics of Continental Shelf Carbon

“…Atmospheric nitrogen reaches the ocean through precipitation (wet deposition) and through direct contact of air with the surface (dry deposition) (Spokes et al 2000). Wet deposition is mainly in the form of nitrate (NO 3 − ), ammonium (NH 4 + ), and organic nitrogen and usually exceeds dry deposition (mainly as NO, NO 2 , HNO 3 , and NH 3 ) over long time periods.…”

Impact of biomass burning on ocean water quality in Southeast Asia through atmospheric deposition: eutrophication modeling

“…Furthermore, the atmospheric inorganic input is directly consumable by the algae, which is only true for parts of the river runoff. Pollutants accumulate in the air and can then be advected over marine areas with associated high dry deposition; if rainfall occurs at the time, particularly high depositions can occur (Spokes et al, 1993(Spokes et al, , 2000. Fig.…”

The role of dust in supplying nitrogen and phosphorus to the Southeast Mediterranean

“…While water column profiles can help to measure this process for certain elements (e.g., Al), it is not suitable for nutrients, where the atmospheric input is masked by biogeochemical processes. Nevertheless, it has been suggested that atmospherically derived nutrients are an important component of bioavailable nutrients to the upper water column, particularly (1) during large sporadic events of deposition, either of anthropogenic or natural character (Paerl 1997;Prospero et al 1996;Spokes et al 2000); (2) in oligotrophic seas; and (3) during seasons of low vertical mixing.…”

“…The accuracy of the Al and Fe method were evaluated on the basis of analyses of international standard reference materials: estuarine sediment 1646 (NIST), MESS-2 sediment (NRCC), and coal fly ash, which gave results within 5% of the certified values. Several previous studies have used detailed sampling from coastal sites and extrapolated the results to the adjacent marine areas (Martin et al 1989;Spokes et al 2000). The dry …”