Deep-Sea Research, Vol. 28A. No. 11, pp. 1339 to 1374, 1981. 0198 0149/81/111339 36 $02.00/0 Printed in Great Britain. © 1981 Pergamon Press Ltd. The chemical mass balance in the Amazon plume I: The nutrients J. M. EDMOND*, E. A. BOYLE*, B. GRANT* and R. F. STALLARD* (Received 18 August 1980; in revised form 5 March 1981; accepted 9 April 1981) Abstract In May and June 1976, the Amazon plume was dominated by a diatom bloom centered on the 10x 10 -3 isohaline. The bloom was apparently initiated by the increased transparency produced by the rapid settling of the fluvial detrital load. The bloom removes the nitrate and phosphate from the surficial layer. The underlying salt wedge was enriched by remineralisation of planktonic debris. A balance between the dissolved and particulate material shows that the regeneration was essentially complete for carbon and phosphorus; but about 50",, of the nitrogen was unaccounted for, having been solubilised to species other than nitrate and nitrite. Only 20°Joof the silica removed as diatoms (~ 25% of the river dissolved flux) could be accounted for as salt wedge enrichment. The rest must have been incorporated in the sediment. The composition of the river waters was strongly affected, before entry to the mixing zone, by remineralisation of a large fraction of the fluvial particulate organic material in the broad area of multi-channel flow above the mouth. INTRODUCTION A FUNDAMENTALproblem in any natural science is the establishment of the relevant conservation equations. In marine geochemistry these take the form of mass, or better, flux balances. Significant effort has gone into establishing the river input in these balances, however, only recently has the importance of estuarine processes to the net value of the fluvial flux been recognized. Precipitation and dissolution reactions, biologic or inorganic, during the mixing of river and ocean waters can have a major effect on the amount of continental dissolved material introduced into the deep ocean. Most of the geochemical work in estuaries has been directed at silica. Earlier studies were marked by a variety of interpretations postulating removal of dissolved silica during mixing, either by precipitation of clay minerals, (BIEN, CONTOIS and THOMAS, 1958; LIss and SPENCER, 1970), uptake and removal by diatoms (STEFANSSON and RICHARDS, 1963; WOLLASTand DE BROEU, 1971 ; VAN BENNEKOM, KRIJGSMAN VAN HARTINGSVELD, VAN DER VEER and VAN VOORST, 1974), or by unspecified mechanisms (Ltss and POINTON, 1973). None of the previous data (BOYLE, COLLIER, DENGLER, EDMOND, NG and STALLARD, 1974) proved the estuarine removal of dissolved silica unambiguously. Rather, all features of the data could be explained in terms of simple mixing between water types of differing compositions, the latter being determined by non-estuarine processes. This conclusion was supported by the work of FANNINGand PILSON (1973). Subsequently, CONOMOSand PETERSON (1974)demonstrated seasonal removal of silica by diatoms in the Sacramento River and San Francisco Bay. SIMPSON, HAMMOND, DECK and WILLIAMS (1975) reported a similar phenomenon in the Hudson. MILLIMAN and BOYLE * Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. 1339