Si cycle in the Cariaco Basin, Venezuela: Seasonal variability in silicate availability and the Si:C:N composition of sinking particles

A 9‐year time series of water column and sediment trap measurements was used to examine silicon cycling within the anoxic Cariaco Basin. The dynamic hydrographic regime 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 (NO 3 − ) rich waters into the photic zone which stimulates primary production and 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 μ M) in both Si(OH) 4 = and NO 3 − as a result of rapid utilization. In most years, the upper water column during winter and spring is marked by Si(OH) 4 = :NO 3 − and Si* values of less than 1. This indicates that silicate limitation in Cariaco Basin is most severe during upwelling and may restrict diatom production. Conversely, during the summer and fall when upwelling is reduced, Si(OH) 4 = :NO 3 − ratios in the upper 50 m of the water column exceed 10, implying that nitrate rather than silicate is acting to limit production during this time of year. On average, sinking particles 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 then remain relatively constant, suggesting minimal selective removal of elements with remineralization in the anoxic portion of the water column. Similar depth‐dependent changes 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 is most 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 nutrient utilization by diatoms and the contribution of diatoms to the total phytoplankton. The high ratios during upwelling suggest enhanced export of Si relative to C and N during this time of year.