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NUTRIENT AND CHLOROPHYLL DYNAMICS IN RELATION TO THE HYDROLOGICAL PATTERN IN PACIFIC CENTRAL AMERICA (PANAMA)

Luis D’Croz and Aaron O’Dea
Smithsonian Tropical Research Institute
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Strong wind-jets from the Caribbean and the Gulf of Mexico cross Central America through topographic depressions in the cordillera during the boreal winter, pushing Pacific coastal waters offshore, lowering sea levels, and causing strong coastal upwelling. Where high mountains impede the winds, this phenomenon does not occur. The Panamanian Pacific shelf is an excellent example of this variability. The coast is divided in two large areas, the Gulf of Panama (upwelling) and the Gulf of Chiriquí (non-upwelling). To investigate nutrient and chlorophyll dynamics between the two gulfs we sampled the water column in cross-shelf transects during upwelling and non-upwelling seasons in each region, measuring temperature, salinity, dissolved nutrients, and chlorophyll a concentrations. In both Gulfs during non upwelling conditions, surface level nutrients are poor and the chlorophyll maximum occurs at around 30 m where the thermocline intersects the photic zone. When wind strength increased, strong upwelling was observed in the Gulf of Panama. The thermocline migrated upwards and surface waters became nutrient enriched and chlorophyll a level rise. In the Gulf of Chiriquí, wind strength was weaker and surface waters did not become enriched with nutrients and surface chlorophyll a levels remained low. However, we did observe a shallowing of the thermocline in the Gulf of Chiriquí, but unlike in the Gulf of Panama wind mixing was not strong enough to result in sea-surface cooling and nutrient enrichment. We do not rule out that the shallowing of the thermocline in the Gulf of Chiriquí is likely to increase the chances that pockets of deep water occasionally migrate into surface waters leading to restricted and ephemeral upwelling-like conditions. By including data on rainfall in both regions we conclude that the position of the thermocline is the most important factor in determining nutrient levels in the euphotic zone in both regions.