Causes of seasonality in the chemistry of a lake on the Orinoco River floodplain, Venezuela1

Abstract

Lake Tinco, which lies on the Orinoco floodplain, passes through four seasonal phases (filling, through‐flow, drainage, and isolation), during which the lake volume changes 20‐fold. During filling and through‐flow, when the lake is inundated by river water, floating grasses (Paspalum repens) and phytoplankton grow rapidly, causing depletion of inorganic N and P between inflow and outflow, although phytoplankton biomass does not accumulate because of flushing. Inundation of the surrounding forest has no quantifiable effects on concentrations of major ions or macronutrients. Following through‐flow, phytoplankton biomass accumulates, and high abundances of blue‐green algae increase N in the lake by fixation (15 kg ha‐1). Quantities of some elements released by decomposition of floating grasses over a 4‐week period are very high in relation to lake‐water inventories (K, 126%; Cl, 66%; organic C, 170%; N, 74%; and P, 89%). Mass balance for Lake Tinco shows, on an annual basis, net retention of N (27%)— partly because most nitrogen fixation is limited to the isolation phase—and P (39%), and net yield of inorganic C (24%). There is neither detectable net retention nor yield of organic C, major cations, sulfate, or chloride. The major chemical transformation of river water during passage through the lake is conversion of inorganic N and P to organic forms.