Abstract

The relationship between water column processes and sedimentation was investigated using a five year time series of bi-weekly water column measurements and continuous sediment trap collections in the Santa Barbara Basin, California. Conditions during the strong El Niño period of 1997–98 were compared to those during the previous years and the post El Niño period. Suspended particulate concentrations of chlorophyll a (chl a), particulate organic carbon (POC), particulate organic nitrogen (PON) and biogenic silica (bSi) normally underwent a seasonal cycle characterized by high phytoplankton abundance in the spring, dominated by diatoms, followed by lower concentrations of biogenic particles throughout the rest of the year. Maxima in sinking fluxes of POC, PON, bSi and lithogenic silica (lSi) generally occurred during the summer. Prior to the El Niño period, molar ratios of C/N, Si/C and Si/N were all higher in sinking particulate material relative to particulate material suspended in the upper 75 m. Si/N and Si/C ratios were highest in the spring and summer in both surface and sinking pools. During the 1997–98 El Niño, the seasonal evolution of the density structure of surface waters was altered by the presence of a water mass high in temperature and low in salinity. The depression of the thermocline resulted in concentrations of nitrate, phosphate and dissolved silicon in the upper 75 m becoming lower that those measured in other years. Mean chl a and bSi concentrations integrated from the surface to 75 m were low on an annual basis, but there were no clear changes in the seasonality of suspended particle concentrations. Perhaps unexpectedly, fluxes of POC, PON and lSi at 470 m increased during the El Niño period. Lower C/N ratios and shorter turnover times suggest increases in the export ratios of POC and PON at that time. We hypothesize that the increase in lSi flux, despite the absence of elevated concentrations of lSi in the upper 75 m, resulted from the lateral advection of particles into the region from the increased riverine discharges at the margins of the basin and subsequent scavenging of the small particles by organic material. Decreases in ratios of C/N, Si/C and Si/N in sinking particles that occurred during the El Niño were sustained until the end of the time series in June 1999, and may have resulted from a shift toward a less diatom dominated pool of sinking particles.

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