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The time-space behavior of thermohaline properties of the water masses in the Gulf of Nicoya, a tropical estuary in the Costa Rican Pacific coast, was studied by sampling monthly from April 1992 to April 1993. The saline field has a seasonal maximum during April, a month before the maximum temperature is observed. Minimun values were observed during October and November, in the rainy season. A defined surface saline front is located towards the east of Negritos Islands; it is produced by the interaction of freshwater from the Tarcoles River and the oceanic waters that enter through the occidental coast of the gulf. The vertical distribution of temperature and salinity indicates a gulf whose internal area is highly stratified in the rainy season, and much less stratified, or even well mixed in the dry season. The outer area of the Gulf is stratified throughout the year. Se estudia el comportamiento espacio-temporal de las propiedades termohalinas de las masas de agua en el Golfo de Nicoya, un estuario tropical del Pacífico costarricense. Se utilizaron datos recogidos mensualmente durante el período comprendido entre abril de 1992 y abril de 1993. El campo salino presenta su máximo estacional durante el mes de abril, un mes antes del máximo de temperatura superficial del mar. El mínimo de salinidad se observó en octubre mientras que las temperaturas superficiales mostraron sus valores más bajos entre noviembre y enero. Un frente salino superficial muy bien definido es encontrado al este de las Islas Negritos, producto del encuentro de las aguas dulces provenientes del Río Grande de Tárcoles y las aguas de carácter oceánico que se introducen por la costa occidental del Golfo. Finalmente, el análisis de las distribuciones verticales de temperatura y salinidad muestran a un Golfo cuya parte interna es altamente estratificada en época lluviosa, y ligeramente estratificada o bien mezclada durante la época seca. La parte externa del Golfo presenta una estratificación permanente durante todo el año.

In March and October of 2000, under the DIPAL II project (Proyecto para el Desarrollo Integral de la Pesca Artesanal en la Región Autónoma del Atlántico Sur), two hydrographic surveys were carried out in the Bay of Bluefields to study their hydrography during the dry and rainy seasons. Water temperature, salinity and turbidity were determined both at the surface and the bottom of the bay. The results obtained are consistent with previous studies carried out in this area. In March, water temperature and salinity were lower and higher, respectively, than in October. Water turbidity increased with increased fresh water input as a result of a greater movement of suspended sediments and organic matter into the water body. Saline wedges were observed in deep strata during the two months of sampling in the adjacent areas to the bars of The Bluff and Hone Sound. In the first case the wedge extended to the northwest up to the area of Bluefields, while in the second case it extended to the west reaching the western coast of the bay. The vertical gradient of salinity was stronger in October, when the superficial flow of fresh water in the whole bay was more intense. A circulation pattern related to the salinity field was recognised: fresh water introduced by the Caño Negro and Escondido rivers moves along the western coast of the bay, while sea water enters the bay towards the northwest, throughout the whole water column, at the bars of The Bluff and Hone Sound. Fresh water introduced by the River Torsuani moves along the eastern coast towards the open sea at the southern end of the bay.