Contrasts in sedimentation flux below the southern California Current in late 1996 and during the El Niño event of 1997–1998

The vertical flux of particulate matter at 330 m depth in San La´ zaro Basin off southern Baja California ranged from 63 to 587 mgm 2 d 1 between August and November 1996. Organic carbon contents were between 5.6 and 14.8%, yielding flux rates of 9e40 mgCm 2 d 1. In December 1997 and January 1998, at the height of the strong El Nin˜ o event, the respective fluxes (47e202 mgm 2 d 1 and 3e8 mgCm 2 d 1) were comparable. The FebruaryeJune 1998 records, however, revealed sharply reduced mass (1e6 mgm 2 d 1) and organic carbon (0.2e0.8 mgCm 2 d 1) fluxes. The organics collected in 1996 were predominantly autochthonous (d13C ¼ 22&; C=N ¼ 8). The variations in d15N (8.3e11.0&) suggest an alternation of new and regenerated production, possibly associated with fluctuations in the intensity of deep mixing during that autumn. The relatively high organic matter fluxes in December 1997 appear to be associated with regenerated production. The average composition from February to June 1998 (d13C ¼ 23:6&; 15N ¼ 11:7&; C=N ¼ 10:5) indicates degraded material of marine origin. The maximum d15N value found (w14&) suggests that deeper, denitrified waters were brought to the surface and possibly advected laterally. Regime changes in the waters of the basin occur at 6e10 week intervals, evidenced by concurrent shifts in most of the measured parameters, including fecal pellet types and metal chemistry. The marine snow-dominated detritus collected showed a shift from a mixed diatom-rich-radiolarian-coccolith assemblage in late 1996 to a coccolith-dominated assemblage, including the contents of fecal pellets, during the 1997e1998 El-Nin˜ o period. TeS profiles, plankton analysis and chlorophyll contents of the upper water column indicated that the strong phytoplankton bloom, normally associated with seasonal upwelling along the Pacific coast of Baja, did not occur during the spring of 1998. The persistence of oligotrophic conditions during the 1997e1998 El Nin˜ o event favored the dominance of nanoplankton and reduced the vertical flux of particles.