Influence of a cyclonic gyre and a thermohaline front on the population structure of Centropages furcatus in Bay of La Paz, Gulf of California

The population structure of Centropages furcatus associated with hydrographic structures in late spring 2004 was analyzed. The distribution of the different copepodite stages of C. furcatus was analyzed from zooplankton samples obtained with a bongo net. The hydrography and geostrophic velocity in Bay of La Paz relative to the bottom depth revealed the presence of a cyclonic gyre in the Alfonso basin area and a thermohaline front near Boca Grande. The distribution of vertically integrated chlorophyll a concentration showed maximum values at the center of the gyre and in the region of the front, making evident that both hydrographic structures stimulated biological productivity because of the flux of nutrients into the euphotic layer in the gyre and the nutrient accumulation in the thermohaline front. The results showed that the spatial variation of C. furcatus was influenced by both hydrographic structures. The differential distribution of the copepodite stages was noticeable in the cyclonic gyre, with the highest density for the first copepodite stages at the center of the gyre because of the high chlorophyll a concentration due to the nutrient enrichment in the euphotic layer, as a result of Ekman pumping. Copepod density was higher in the thermohaline front than inside the cyclonic gyre because the front serves as a reproductive spot for the species, maintaining a stable adult population density inside the bay and in the Gulf of California.