Effects of suspended sediments on geochemical processes near the mouth of the Amazon River: examination of biological silica uptake and the fate of particle-reactive elements

Abstract

The effects of suspended-sediment concentration on the biological uptake of silica and the chemical behavior of particle-reactive elements are examined for the continental shelf waters near the mouth of the Amazon River. Dissolved silica and suspended-sediment concentrations measured during periods of high and low discharge indicate that uptake of dissolved silica begins where the suspended-sediment concentration falls below 10–20 mg l 1. Near the mouth of the Amazon River the 10 mg l 1 turbidity contour and the region of initial silica uptake both occur 20–40 km farther offshore during periods of high discharge (May and June) than during periods of low discharge (October and November). The location of initial silica uptake along an across-shelf transect near the river mouth varies by as much as 20 km on a time scale of days to weeks, which is comparable to the seasonal variation. Silica uptake, chlorophyll a concentration, and suspended-sediment concentration are poorly correlated along this transect (correlation coefficients range from +0.03 to −0.3). The poor correlations may be a result of grazing by zooplankton. Phaeopigment:chlorophyll a ratios increase offshore from 0 to 0.7, which supports the hypothesis that zooplankton grazing may be responsible for the low chlorophyll concentrations in the low-turbidity offshore waters exhibiting extensive silica removal. Excess 210Pb inventories for box cores collected from an across-shelf transect just south of the river mouth range from 21 to 124 dpm cm 2, which are 2–10 times greater than the inventory supported by atmospheric supply and in-situ production in the overlying water column. Riverine supply and lateral transport from offshore waters are the main sources of excess 210Pb to the Amazon continental shelf. 210Pb and 234Th data indicate that the high particulate flux from the Amazon River scavenges particle-reactive species not only from river water and continental shelf water but also scavenges these elements from offshore open-ocean water which flows onto the shelf.