Coccolithophore export production in Guaymas Basin, Gulf of California: response to climate forcing

Abstract

The flux and taxonomic composition of coccolithophores has been determined for sediment trap samples collected over a two-year period from Guaymas Basin, central Gulf of California. The Gulf of California is a subtropical marginal sea with exceptionally high rates of primary productivity. The annual biogenic sediment fluxes are largely dominated by silica, with diatoms as the major contributor to the opal flux. For the July 1990 through November 1992 study period, the total coccolithophore export production in Guaymas varied in response to the strength of El Niño/Southern Oscillation (ENSO). Coccolithophore production follows the general surface primary productivity pattern with relatively high fluxes from late fall to late winter, and low production in the summer when the water column is highly stratified and nutrient depleted. High fluxes of up to 10 9 coccoliths m −2 day −1 and 3×10 6 coccospheres m −2 day −1 indicate that coccolithophores are an important component of the phytoplankton population in Guaymas Basin. The intensification of ENSO conditions during 1992 altered the typical hydrography of the Gulf of California by increasing SST and reducing the strength of spring upwelling. This is reflected in reduced opal and diatom fluxes during this period. Conversely, these more oligotrophic conditions resulted in higher total coccolithophore fluxes during spring and summer 1992. Gephyrocapsa oceanica is present in the Gulf throughout the study period, but is dominant during weak or non-ENSO times. Emiliania huxleyi and, to a lesser degree, Florisphaera profunda are abundant during strong ENSO periods. Two major coccolithophore signals are indicative of strong ENSO conditions and could be used as paleo-ENSO indicators in the Gulf of California: (a) the switch in dominance between G. oceanica and E. huxleyi and (b) high abundances of the deep dwelling species F. profunda. The abundance of this latter species shows a strong inverse relationship with opal fluxes. A comparison of the annual coccolith flux with coccolith accumulation rates in the surface sediments indicates that only about 10–20% of the coccolith export production is preserved in the anoxic sediments of Guaymas Basin. This very low level of preservation is probably due to bacterial processes on the sea floor, which cause acidification of the bottom waters at the sediment/water interface. Coccolith accumulation rates are higher in surface sediments from the western coast of the central Gulf, suggesting both higher coccolithophore production and better preservation in this region, probably associated with lower diatom production. Tintinnids which agglutinate coccoliths are strongly selective for both coccolith size and species.