The exchange of gases and particles across the air-sea interface can have important impacts on the biogeochemistry of both the oceans and atmosphere. For example, air-sea transfer of CO₂ is a large flux in both the natural and man-perturbed cycles of this radiatively active gas. Further, the flux of dimethyl sulphide (DMS, produced by marine algae) from sea to air plays an important role in the production of atmospheric acidity and cloud condensation nuclei (CCN). In the case of transfer of particles, the oceans are a sink for terrestrial mineral dust; a major source of iron which appears to be a limiting nutrient for marine primary production in some important oceanic areas.

The air-sea exchange of trace gases (CO₂, DMS, and many others) is closely coupled with their production or consumption by the biota in the surface oceans. Hence the processes regulating marine biological production are crucial to understanding, and quantifying, gas exchange. Key factors are the fluxes of the major (N, P, Si) and minor (Fe, Mn, Zn) nutrient elements, some of which in turn are being supplied from the atmosphere, notably N and Fe. Thus, there is an intimate coupling between the inward and outward fluxes of material across the sea surface, which make their combined study, as proposed here, both appropriate and rewarding.

It is well known that concentration distributions of biogeochemical variables in the oceans exhibit marked 'patchiness' on a wide range of temporal and spatial scales. Although much of this variability is intrinsic to the driving forces and biogeochemical processes, 'patchiness' also arises from the way that parameters are observed and measured. Traditionally, biogeochemical observations have been made from ships or fixed platforms. These give insight into variations over the large scale, but only limited information on the nature of processes, since sequential measurements will almost certainly not be in the same body of water biologically (or biogeochemically) speaking. Thus, the nature of many biogeochemical signals is blurred by inadequate deconvolution of the chemical and biological processes from the spatial and temporal variability.

To solve this fundamental problem it is necessary to carry out as near as possible a truly lagrangian experiment, in which the observing system moves with the body of water. Tracking water movement with drogued buoys alone is often unsuccessful, particularly under rough conditions, and gives no information on the effects of small-scale lateral and vertical mixing on the biogeochemical signals. Studies of iron enrichment in the equatorial Pacific (IronEx I and II) and recent studies of bloom development during the PRIME study in the N. E. Atlantic have shown the success and value of deployment of sulphur hexafluoride (SF₆) as a deliberate tracer for small-scale lagrangian experiments in the open ocean. It is, however, not enough to make detailed process studies at the micro-scale level in such lagrangian experiments if we also require a full understanding of biogeochemical systems. In addition it is necessary to have a proper measure of the meso-scale variability within which to interpret the small-scale observations.

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