Between September 2003 and April 2005, the bathymetry and the behavior of the temperature, salinity, dissolved oxygen and oxygen percent saturation were studied at Marino Ballena National Park (MBNP). The park has an average depth of 14.9±9.1m, and maximal depth of 37.3 m. The internal section of the bay is shallow, not reaching more than 20m, while the external section posses a more pronounced slope, reaching easily 37 m. Mean values of superficial salinity were: 29.1±4.3‰, temperature 29.7±1.3ºC, dissolved oxygen 6.7±1.3 mg/l, and oxygen percent saturation 115.6±22.1%. Significant differences were only detected between the internal and external stations for salinity and temperature according to depth, during the rainy season. The lower values of salinity and temperature were observed on the rainy season, and the highest on the dry season, with a continuous of dissolved oxygen and saturation. The similarity between the stations and the fact that significant differences were only for salinity and temperature according to depth, suggests that the waters of the park behave in similar manner over time and that the area is very dynamic and homogenous, with currents strong enough to mix the water column on the park. The park is located in a very productive area, resulting in a high value of oxygen percent saturation (120%). This suggests that there are good conditions for the development of phytoplankton, zooplankton and other groups
Keywords: bathymetry, temperature, salinity, dissolved oxygen, marine park, Pacific, Costa Rica