Profiles of conductivity and temperature were measured along a transect of 42 km along the Rio Dulce from Lake Izabal to Amatique Bay, in the coastal lowlands of Guatemala, Central America. Readings were taken three times during the dry season between March 22-June 13, 1972. It was
found that water conductivity along the transect increased as the dry season progressed. Tides, winds, gravitation forces, a low topographic gradient, and low discharge of fresh water at this time of the year are the factors responsible for the upstream movement of high conductivity water.
The vertical and horizontal stratification of brackish water disappears with the onset of the wet season. Marine invasions into the lake, and the presence of mature red mangroves (Rhizophora mangle) along the Rio Dulce, are explained in terms of the seasonal appearance of this high
conductivity water in the area. Pulses of primary production in the bays and coves surrounding the lake, and the consequent availability of concentrated food sources coincident with the occurrence of seasonal oligohaline waters inside the lake, are discussed as the causal factors for marine
invasions in tropical lakes.
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