Response of siliceous microplankton from the Santa Barbara Basin to the 1997-98 El Nino event

We report on fluxes of siliceous microorganisms (diatoms, radiolarians, and silicoflagellates), organic carbon, calcium carbonate, biogenic silica, an d lithogenic particles in the Santa Barbara Basin (SBB; 34 degrees4'N, 120 degrees 02'W), offshore of California, in a sediment trap set 540 m deep. W e describe changes in particle fluxes, emphasizing the period from 1996 to early 1998, and compare flux values and species composition for non-El Nino (1996) and El Nino (1997-98) conditions. The California coastal waters wer e strongly influenced by El Nino conditions beginning late in the summer of 1997. Terrigenous input to our sampling site, as measured by the lithogeni c flux, was significantly higher during the El Nino period, presumably refl ecting higher rainfall and run off into the basin. Samples front December 1 997 to February 1998 contained large amounts of detritus, Chrysophyte cysts , benthic diatoms, and estuarine benthic foraminifers, indicating that cons iderable material was flushed front the river mouths during large storms. B oth opal and organic carbon fluxes mirrored the productivity cycle, with hi gh fluxes during the spring-summer upwelling period and low fluxes during f all and winter. However, for the winter of 1998 organic carbon fluxes were unusually high, and coincided with a February peak in the carbonate flux an d high abundance of arenaceous tintinnids. Opal fluxes decreased, and major changes in the contribution of siliceous microplankton assemblages to the biogenic opal flux were observed during El Nino conditions: (1) Diatom flux es were an order of magnitude lower, and species richness was higher than i n the 1996 non-El Nino period. (2) The flux of radiolarians was 20% lower i n late 1997-early 1998 when compared to the 1993-96 period. (3) The fall-wi nter peak in silicoflagellate fluxes, seen annually from 1993 to 1996, was missing ill 1997. In addition, major changes in species composition were ob served, including a significant increase in the proportion of warm-water fl ora and fauna, and a decrease in the relative contribution of the siliceous microorganisms indicative of spring upwelling conditions in the SBB.