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During April and May 1993, the typical south and southwestward directed flow over the Texas shelf interacted with current rings located just off the Texas shelf in the Gulf of Mexico. The rings, a cyclone/anticyclone eddy pair, were oriented such that water was transported onto the shelf between the rings and water was drawn from the shelf on the rings' offshore-directed limbs. A shelf convergence zone transported water to the shelf edge where some of the water became entrained between the cyclone/anticyclone pair. Active shelf-slope exchange such as that seen in the April-May 1993 episode may be a more typical example of synoptic shelf circulation than the general circulation pattern suggested by averaging multiannual observations. Nutrient and particle distributions were influenced by the interactions of the shelf current with the rings. Where the cyclonic and anticyclonic eddies directed flow off the shelf, particles were transported off the shelf. Additionally, particles in the bottom nepheloid layer were transported off the shelf in the shoreward limb of the cyclone, even though currents in this limb were directed along the isobaths. Nutrient concentrations, hence nutrient fluxes, were generally low in near-surface waters every-where in the study area. At mid-depth and in near-bottom waters, nutrient fluxes were highest near the cyclone because of higher current speeds in this feature and higher nutrient concentrations at depth.

Primary production by marine microalgae is believed to be a critical factor regulating atmospheric carbon dioxide levels and associated climatic changes1. Assessments of photosynthesis in the open ocean2–4, and the related export of organic carbon to the deep ocean (new or non-regenerative production5–7, vary by as much as an order of magnitude (refs 2 and 6, compare with refs 3–5 and 7). Discrepancies are attributed4,6–8to different temporal and spatial scales reflected by instantaneous rate measurements, as opposed to seasonally averaged measurements based on subsurface changes in chemical tracers. Satellite extrapolations of primary production can be used to characterize and quantify temporal and spatial variability9–11. But time differentials between satellite and ship measurements, as well as regional and seasonal variations in empirical relationships, have so far limited the precision of such extrapolations9,12. We conducted extensive ship sampling of chlorophyll a and primary production in the western Mediterranean Sea contemporaneous with Nimbus-7 coastal zone colour scanner imagery. Our approach resulted in an empirical model for estimating integrated water-column primary production from satellite imagery. Precision was adequate to resolve short-term fluctuations in primary production associated with a meso-scale circulation feature.

J. Berthelin (ed.). 1991. Diversity of Environmental Biogeochemistry, Developments in Geochemistry 6. Elsevier, New York, 537 pp. Hardcover, US$143.00, DM250.00.John Postgate, FRS. 1992. Microbes and Man, 3rd ed. Cambridge University Press, Cambridge, UK, 297 pp. Hardcover, $49.95; paper, $15.95.