A SIMPLE MODELING APPROACH TOWARDS HYDROCHEMICAL SEASONALITY OF MAJORCATIONS IN A CENTRAL AMAZONIAN FLOODPLAIN LAKE

To identify causes of hydrochemical seasonality, dynamic mass balance models are developed. They describe a hypothetical floodplain lake des igned according to literature data supplemented by unpublished data on Lago Camaleao, an island channel lake in the lower Solimoes River. Th e conservative model accounts for advective and atmospheric cation flu xes, the non-conservative model additionally for biogenic cation fluxe s due to growth and decomposition of the macrophyte vegetation. During much of the hydroperiod, seasonal evolutions observed for Na, Ca, and Mg, were in accordance with conservative expectation. Increasing conc entrations during falling water were not explained by the conservative model. Differences between predictions of the conservative, and the n on-conservative model were negligible. Thus, for the evolution of Na, Ca, and Mg, biogenic fluxes are insignificant. We suggest that cation loading during falling water is caused by an abiotic process. Evolutio n of K was non-conservative during much of the hydroperiod. Deviations from conservative expectation due to K loading during rising, as well as during falling water, were in accordance with predictions of the n on-conservative model. Hence, during rising water, macrophyte-derived influxes of K are the key factor for deviations from conservative expe ctation. During falling water, however, macrophyte decomposition is st ill poorly understood, and unknown processes causing Na, Ca, and Mg lo adings might also procure additional K loading, and thus biogenic K fl uxes might not be the sole cause for increasing concentrations.