Contrasting winter and summer wet zooplankton biomass in a semi-enclosed bay in the southwestern Gulf of California

Zooplankton biomass is an indirect proxy of secondary production and a measure that allows estimates to be made about the availability of matter in the food web. This study compares the wet biomass of zooplankton within the Bay of La Paz, the largest and deepest coastal water body in the Gulf of California, Mexico. It analyzes the influence of the physical forcing in zooplankton biomass and distribution. Two research cruises were conducted in February 2006 and August 2009, recording high-resolution hydrographic data and collecting zooplankton. The hydrographic properties of the seawater column changed in both seasons. The circulation pattern was dominated by a well-defined cyclonic eddy with different azimuthal velocities, being more intense in summer (75 cm s^-1) than in winter (20 cm s^-1). Zooplankton biomass showed slightly lower values in winter (43.4 g 100 m^-3) than in summer (45.5 g 100 m^-3). Its horizontal distribution showed that the highest values were observed in the shallow stations, close to the coast, and in the connection with the Gulf of California. However, high secondary values were observed at the eddy’s peripheries, describing a circular distribution pattern following their circumference, which could be attributed to 1) eddies retain zooplankton organisms and advect them to their periphery, and 2) mixing processes at the eddies peripheries ensure food (phytoplankton) for zooplankton organisms.