Si Cycle in the Cariaco Basin, Venezuela: Seasonal Variability in Silicate Availability and the Si:C:N Composition of Sinking Particles

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A 9-year time series of water column and sediment trap measurements was used toexamine silicon cycling within the anoxic Cariaco Basin. The dynamic hydrographicregime within Cariaco Basin results in strong seasonal changes in nutrient availability and the Si:C:N of sinking particles. Upwelling in early winter injects silicic acid (Si(OH)4)and nitrate (NO3) rich waters into the photic zone which stimulates primary productionand results in opal fluxes in excess of 4 mmol Si m-2 d-1. However, even during upwelling, surface waters tend to be depleted (<1>mM) in both Si(OH)4 and NO3 as aresult of rapid utilization. In most years, the upper water column during winter and springis marked by Si(OH)4 :NO3 and Si* values of less than 1. This indicates that silicatelimitation in Cariaco Basin is most severe during upwelling and may restrict diatomproduction. Conversely, during the summer and fall when upwelling is reduced, Si(OH)4:NO3 ratios in the upper 50 m of the water column exceed 10, implying that nitrate ratherthan silicate is acting to limit production during this time of year. On average, sinkingparticles collected at 150-m depth in the Cariaco Basin have Si:C and Si:N values of 0.17± 0.01 and 1.14 ± 0.10, respectively. These ratios increase with depth to 400 m and thenremain relatively constant, suggesting minimal selective removal of elements withremineralization in the anoxic portion of the water column. Similar depth-dependentchanges in these ratios are seen in surface sediments from the basin. Seasonally,particulate Si:C and Si:N are highest during the early part of the year when upwelling ismost intense, while both ratios decrease to their lowest values during summer and fall.The observed seasonal variability in these ratios is due to changes in both nutrientutilization by diatoms and the contribution of diatoms to the total phytoplankton. Thehigh ratios during upwelling suggest enhanced export of Si relative to C and N during thistime of year.

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