A simple modelling approach towards hydrochemical seasonality of major cations in a Central Amazonian floodplain lake

To identify causes of hydrochemical seasonality, dynamic mass balance models are developed. They describe a hypothetical floodplain lake designed according to literature data supplemented by unpublished data on Lago Camaleao, an island channel lake in the lower Solimoes River. The conservative model accounts for advective and atmospheric cation fluxes, the non-conservative model additionally for biogenic cation fluxes 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 concentrations during falling water were not explained by the conservative model. Differences between predictions of the conservative, and the non-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. Evolution 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 non-conservative model. Hence, during rising water, macrophyte-derived influxes of K are the key factor for deviations from conservative expectation. During falling water, however, macrophyte decomposition is still poorly understood, and unknown processes causing Na, Ca, and Mg loadings might also procure additional K loading, and thus biogenic K fluxes might not be the sole cause for increasing concentrations.