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The study of aerosols and rainwater presented here demonstrates that episodic atmospheric deposition events associated with southeasterly flow are quantitatively significant for large areas of the North Atlantic Ocean. This paper considers aluminium and manganese, with predominantly crustal sources, and lead and zinc, which are mobilised into the atmosphere primarily through anthropogenic activity. High levels of all trace metals are associated with southeasterly flow from Europe as the air passes over heavily populated and industrialised regions before reaching the northeast Atlantic Ocean. Fluxes calculated using the 1% HNO3 acid soluble metal concentration show that, although the climatological norm for this area is westerly flow, short-lived southeasterly transport events dominate the input of trace metals to this ocean region. This material may be toxic to phytoplankton or may be represent a new source of nutrients to the biological community. A significant decrease in atmospheric lead levels in polluted air is seen between June 1996 and May 1997, reflecting the decrease in use of leaded fuels in Europe. Comparing atmospheric flux values to sediment trap metal fluxes shows that the atmosphere represents the dominant source of zinc to the deep ocean, whereas an additional, non-atmospheric, manganese source this required, perhaps from mobilisation of sedimentary material from the continental shelf or long range advection of manganese rich Saharan material.

Experiments have been conducted to determine whether the speciation of the nitrate-nitrite and iodate-iodide redox couples is influenced by light and organic matter in both deionised and sea waters. The results obtained indicate that photochemical production of nitrite occurs and that the rate of this reaction is dependent on the concentration of organic matter present in the system. The findings suggest that nitrate undergoes not only direct photochemical reaction, acting itself as a primary chromophore, but also a photochemically induced reduction through organic matter. This reaction is, however, unlikely to influence productivity to any great extent as most phytoplankton can use both species as nutrients. It may, however, be a source of the highly reactive hydroxyl radical and of NO which has the potential to degas to the atmosphere and there help to regulate ozone concentration. Experiments conducted on the iodine redox system similarly indicate that photochemically induced production of iodide occurs. The results show that the reaction is not biologically mediated and that the presence of organic matter is essential for iodide photoproduction. At the concentrations of added iodate used in the experiments no direct 1:1 relationship between iodate and iodide is seen. It is suggested that, under these conditions, organic matter is not only essential for the photochemical production of iodide but that it is also the limiting species for the reduction reaction.

Experiments have been conducted to determine whether the speciation of the manganese and iron redox couples can be influenced by light and organic matter in seawater. The results indicate that photochemically induced reduction of Mn(IV) to Mn(II) occurs through the mediation of humic acids. The rate of the reaction is dependent on many variables, some of which are identified here. Stirring plays an important role in the reductive dissolution process, the effect of which is dependent on the organic loading. It is suggested that this results from an adsorptive competition between organic matter and photoproduced reductants such as H2O2. Photochemically induced reduction reaction, at least in part, controls the speciation and thus biological availability of manganese in surface seawater. The results obtained from studies of the iron redox system show generally poor reproducibility and no conclusive evidence for photochemically induced reduction of Fe(III) under seawater conditions. These results are perhaps understandable given the fast reoxidation reactions by hydrogen peroxide and oxygen at seawater pH and the use of membrane filtration to separate soluble and insoluble iron. Reproducible results are obtained in stirred seawater and show a gradual decrease in levels of 0.2 μm filterable iron over time. The light replicates show a slower rate of iron loss from solution than in the dark. The possible implications of these results for iron bioavailability in seawater are discussed.