Differential zooplankton aggregation due to relative vorticity in a semi-enclosed bay

The relationship between a mesoscale cyclonic eddy and the abundance and distribution of zooplankton in the Bay of La Paz, Gulf of California, in the summer of 2009 is presented. The water density distribution, the geostrophic current velocity and the vertical component of relative vorticity indicate a mesoscale cyclonic eddy located in the Alfonso Basin, with a diameter of ∼30 km. The zooplankton trophic groups along the bay presented a radial distribution. Using a Bray-Curtis similarity cluster analysis, three different zones were delimited: 1) the Alfonso Basin where the cyclonic eddy is centered, 2) the strip from southern bay to the Boca Grande, which is the periphery of the eddy, and 3) outside the bay. The results indicated that the mesoscale cyclonic eddy might be the dominant physical feature influencing the horizontal distribution of the different trophic groups of zooplankton in the Bay of La Paz. Our results confirm that the mesoscale cyclonic eddy retains a biological composition that is markedly different from that of its surroundings. Zooplankton biomass was substantially higher inside the eddy than at its periphery. However, outside the bay; the biomass was comparable to that found in the center of the eddy. The trophic group composition exhibits a predominance of herbivorous zooplankton in the center of the eddy, a predominance of omnivorous zooplankton in its periphery, and a high abundance of carnivorous zooplankton in the region adjacent to the Gulf of California.