Seasonal variation in dissolved aluminium concentrations in coastal waters and biological limitation of the export of the riverine input of aluminium to the deep sea

Abstract

Three surveys in the English Channel between 0° and 5°30′W in November 1985 and May and August 1986 show that Al concentrations vary with the seasonal cycle in dissolved nutrient concentrations. Lowest levels were measured in August (10.5 nM mean) and highest in November (28 nM). In winter, the distribution of Al is determined by the mixing of low Al surface northeast Atlantic water with high Al river water. In summer, where a thermocline is present, Al concentrations are lower above the thermocline. The ratio Al to Si removed during the spring bloom is similar to that observed in diatom culture experiments. A continued decrease in Al after diatom production has ceased suggests that Al removal may be generally linked to primary production. The mechanism of removal is unclear. Profiles of Al against depth from deep-water stations in the northeast Atlantic, north of the area consistently effected by Saharan dust inputs, are very similar to those for Si above the main thermocline. A biologically mediated removal process maintains low concentrations of Al in these and coastal waters, limiting the export of riverine Al into the deep sea.