From September 2005 to November 2007 we studied the meteorological typologies and the physical, chemical and zooplankton oceanography of Golfo Dulce, a unique aquatic ecosystem in the South Pacific of Costa Rica. The water circulation pattern in Golfo Dulce is typical of fjord-type estuaries, with flux stratification within the water column. Surface water currents reached speeds near 18cm/s in the vicinity of the sill and diminished inward to the northwest at around 5cm/s. The east-west flux was predominant, whereas near Puerto Jiménez the speed was a moderated 5.4cm/s with a south direction. Higher speeds were found near the sill (17.8cm/s) and at the bottom (2.0cm/s to 4.8cm/s). One station was visited once, five were visited five times each. Dissolved oxygen was not absent in deep waters (70m to 140m), but concentrations were low (0.25mg/L to 1.10mg/L). Nutrient concentrations and distribution had the usual horizontal estuarine patterns and for the vertical level phosphate and silicate concentrations increased with depth. Decomposition of organic matter was evident because nitrate concentrations were higher in deep waters. Surface nutrient concentrations indicated the gulf has good environmental quality that sustains a variety of life. Copepods dominated zooplankton (44%-83.6%). The strongest equatorial westerlies were detected between September and October, coincident with higher wave heights. The trade winds blowing from December to April reduced the energy of entering waves and therefore the mean wave height. The annual rainfall cycle was the typical bimodal pattern from the Central America Pacific, with maxima in May and October, except for the midsummer interruption, and minimum rain was on February. Golfo Dulce is a unique environment in the Eastern Tropical Pacific, shows healthy environmental conditions, and is threatened; therefore we urge the establishment of zoning policy for human activities, and to improve protection under the principles of sustainable use.

Keywords: tropical fjord, Tropical East Pacific, water currents, wind system, nutrients, zooplankton.