Variability in upwelling along the Pacific shelf of Panama and implications for the distribution of nutrients and chlorophyll

Seasonal dynamics of dissolved inorganic nutrients (NO3 , PO4 3 , and Si(OH)4) and chlorophyll a were investigated in the Pacific shelf of Panama. The shelf is divided into two large semi-open areas, namely the non-upwelling Gulf of Chiriquı´ and the upwelling Gulf of Panama. Four research cruises sampled the water column in cross-shelf transects during wet and dry seasons at each region. Hydrological measures varied spatially between regions and also varied temporally on a seasonal basis. Low concentrations of NO3 (<0.50 mM), PO4 3 (<0.27 mM), and chlorophyll a (<0.34 mg m 3) were typical near the surface in the Gulf of Chiriquı´ during both rainy and dry seasons, but in the Gulf of Panama nutrients and chlorophyll were low only during the rainy season. In contrast, during the dry season upwelling in the Gulf of Panama, high concentrations of NO3 (15 mM) and PO4 3 (1.2 mM) in the upper layer caused surface chlorophyll a to peak (1.5 mg m 3). Median Si(OH)4 concentrations in the upper layer ranged from about 4 mM in both regions during the rainy season to nearly 12 mM in the Gulf of Panama during the dry season upwelling. Both the N:P and N:Si molar ratios suggest that phytoplankton is N-limited except in the Gulf of Panama during upwelling. In both regions, a subsurface chlorophyll maximum (>0.5 mg m 3) typically developed close to the usually shallow thermocline during non-upwelling conditions. We found no evidence of significant nutrient input from freshwater runoff. The position of the thermocline is considered to be the main source of nutrients to the euphotic zone in both regions.