SEASONAL-VARIATION IN CHEMICAL-DISTRIBUTIONS IN THE AMAZON (SOLIMOES) RIVER - A MULTIYEAR TIME-SERIES

The results of a 10-year time series study of the chemistry of the Amazon River mainstem near Manaus, Brazil, are presented. All variables measured showed distinct seasonal patterns linked to the discharge hydrograph except respiration rate and PO4-3 concentration. Stepwise multiple regression analysis showed that alkalinity, calcium, fine suspended sediment, and sulfate were correlated, primarily, with the percentage of the total water discharge that was derived from Andean drainages. Silicate, potassium, and the weight percentages of carbon and nitrogen in the coarse suspended sediment were correlated with the percentage of water attributable to local lowland drainages. These correlations suggest that seasonal cycles of these variables were controlled by their source strength. Coarse suspended sediment and the concentration all of the particulate carbon, nitrogen, and phosphorus species were correlated with river surface slope, suggesting that the seasonal cycles of these variables were controlled by river turbulence. The biogeochemically active elements O-2, CO2, NO3-, and SO4=, along with pH, Na, and Cl were all highly correlated with river discharge. The shapes of the seasonal cycles of O-2, and CO2 and much of their amplitude could be reproduced by a quasi steady state model in which respiration was balanced by air-water gas exchange. In the model, increases in river depth during the annual cycle result in increased depth-integrated respiration rates. This produces a drawdown of O-2 concentration, which increases air-water gas exchange, until the two processes are in balance. Thus the model produces seasonal cycles in which minimum dissolved O-2 and maximum dissolved CO2 coencide with high water and the converse at low water, in agreement with the observations. The remainder of the amplitude signal was probably either advected in from upstream or produced by lateral exchange with the fringing floodplain.