ECOLOGY AND DEVELOPMENT OF TWO COASTAL LAGOONS IN NORTHWEST MEXICO

Resumen

Long and narow inlet-channels into Morancarit and Huizachc-Caimanero Lagoon result in development of only inner lagoon water masses reflected by inner lagoon and marsh assemblage, of foraminifera. Occurrence of an inner lagoon assemblage of foraminifera in Yavaros Lagoon, a small lagoon with free access to the open-ocean, is due to drainage from an agricultural area. Huizache-Caimanero Lagoon is filled with water during the rainy season. At this time most of the estuarine water enters from thc Caimanero inlet-channel and most of the outflow of runoff water is through the Huizachc inlet-channel. This flow is reflected by the occurrence of one open ocean species of Foraminifera near the Caimanero inlet-channel, an inner lagoon assemblage in the Caimanero basin and no Foraminifera in the Huizache basin. Experirnental flooding of Etchoropo Lagoon with estuarine and irrigation water resulted in very large standing stocks of Foraminifera, where none had previously existed, within three months after flooding and rapid turnover of the population. The Yavaros area lagoons developed on the delta of the Mayo River and source of sediment for the lagoon barrier is from the west. The Etchoropo and Moroncarit basins are almost full of sediment; the Yavaros basin is rapidly filling with sediment brought in by the drains from the irrigated arca. The Huizache -Caimanero Lagoon is essentially filled with sediment. The inner lagon barrier was formed by sediments from the north when the Presidio River was farther south and the Baluarte River was farther north than now. Channels traversing this inner barrier record a flood and relocation of the river effluents. The outer barrier was deposited by sediment derived from the Baluarte River recording a change in direction of net sediment transport. A recent shift in positions of the Presidio and Baluarte Rivers is believed to have resulted from an intense flood. These events correlate with those described previously in adjacent Nayarit State by other authors.