Biogeochemical processes in Amazon shelf waters: chemical distributions and uptake rates of silicon, carbon and nitrogen

Biogeochemical processes in the Amazon River/ocean mixing zone were examined during four AmasSeds cruises between August 1989 and November 1991. On the Amazon shelf, the distributions of chlorophyll-a, oxygen supersaturation, pH and the biogenic-silica content of suspended matter all showed coherent patterns highlighting areas of high primary productivity. Phytoplankton blooms occurred seaward of the high-turbidity waters (suspended-solid concentration >10 mg 11¯) and the extent of the bloom depended on the supply of nitrate from the Amazon River, as well as the time available to phytoplankton in the “optimal-growth zone” (a region of relatively low turbidity and high nutrients). Of the four AmasSeds field cruises, the largest phytoplankton bloom occurred during March 1990, when the nitrate flux to the shelf was highest and the plume residence time was the longest. The biogenic-silica standing crop in Amazon shelf waters (area of 8.9 × 1010 m2) varied from 29 to 66 × 108 moles during the two-year study period, whereas the depth-integrated silicate anomaly on the shelf (i.e. the difference between the observed nutrient concentration and the salinity-predicted value from the ideal mixing of riverine and oceanic end members) varied from +5 to −170 × 108 moles. Chlorophyll-a inventories on the shelf ranged from 10 to 36 × 108 g. The maximum extent of nutrient removal occurred during the March 1990 cruise, consistent with the maximum standing crops of biogenic silica and chlorophyll-a. Excess silicate and excess inorganic-nitrogen concentrations (relative to the ideal mixing line) commonly occurred in inner-shelf waters (<20 m water depth) and were attributed to resuspension of nutrient-rich porewaters by tides and waves. If the standing crops of these excess nutrient distributions are divided by the residence time for inner-shelf waters (estimated to be ∼14 days), the resulting fluxes suggest that seabed resuspension provided on the order of 5–20% of the silicate and inorganic nitrogen reaching the shelf. A comparison of the magnitude and the locations of the silicate depletions with the biogenic-silica standing crops suggests that for most cruises a mechanism separating dissolved and particulate siliceous phases must be operating on the shelf. Aggregation, zooplankton grazing and/or nutrient limitation (which increases the sinking rates of diatoms) are likely causes for the settling of the siliceous particles into the landward-moving bottom waters. Distributions of biogenic silica and organic carbon in surface suspended matter correlated poorly (linear regressionR2 <0.06) during all sampling periods. The likely explanation for this observation is the occurrence of non-siliceous phytoplankton and the grazing by zooplankton (which strip organic matter from the siliceous frustule). The Amazon shelf exhibited some of the highest carbon and silicon production rates in the marine realm. Rates of primary productivity during the AmasSeds cruises were as high as 250 mmol C m−3 day−1, whereas silicate uptake rates were as high as 15 mmol m−3 day−1. Field measurements of carbon and silicon production rates were consistent with the changes in particulate organic-carbon and biogenic-silica concentrations that occurred along the trajectory of the plume during the phytoplankton's 2–7 transit through the “optimal-growth zone” of the shelf.