THE BEHAVIOR OF IRON AND OTHER TRACE-ELEMENTS DURING THE IRONEX-I ANDPLUMEX EXPERIMENTS IN THE EQUATORIAL PACIFIC

Dissolved (< 0.4 mu m) and particulate (0.4-5 mu m and >5 mu m, leacha ble and refractory) trace elements were measured during the IronEx I a nd PlumEx experiments in October and November 1993 near the Galapagos Islands. Iron was measured in the enriched patch and at control statio ns over a 9 day period following fertilization. The dissolved iron was initially depleted at a rate that gave an iron half-life of 28-40 h. The loss rate gradually decreased, and dissolved iron concentrations d id not decrease below about 0.25 nmol kg(-1) throughout the experiment . These results were most consistent with a kinetic model that was sec ond order in iron concentration for the scavenging removal of each iro n fraction. Other trace elements measured did not change significantly either in concentration or partitioning during the IronEx I experimen t. Biological production tracked iron concentrations over time, which suggests that productivity within the fertilized patch was regulated b y the availability of iron. The PlumEx study consisted primarily of tw o transects, a meridional course to the east of the Galapagos Islands and a zonal section to the west. Surface dissolved iron was very low a t all stations except those near the Galapagos Islands. Sections of tr ace metal and other properties distinctly confirm the upwelling of the Equatorial Undercurrent as it intersects the islands. This upwelling has a great effect on the surface distributions of the trace metals on the west side of the islands. Productivity within the mixed layer of this region is highly correlated with iron and nitrate. West of the is lands highest productivity is found at stations with the highest iron. North of the Equatorial front, in nitrate-depleted waters, elevated i ron concentrations do not enhance productivity. (C) 1998 Elsevier Scie nce Ltd. All rights reserved.