Impact of a dipole on the phytoplankton community in a semi-enclosed basin of the southern Gulf of California, Mexico

The present study assesses the impact of a dipole on the abundance and distribution of phytoplankton groups as well as the chlorophyll-a concentration in the Bay of La Paz, Gulf of California, Mexico. Based on in situ observations obtained in a multidisciplinary research cruise during the summer of 2008, a mesoscale dipole (cyclone-anticyclone) was observed; the cyclone had ∼25km diameter and tangential speed of ∼45cms−1, while the anticyclone had ∼15km diameter and tangential speed of ∼40cms−1. Strong gradients in conservative temperature and density were observed between both structures, suggesting the presence of an oceanic thermohaline front. Differences in phytoplankton distribution showed minimum abundance of diatoms in the southern bay and close to Roca Partida Island, and maximum in the periphery in the northern cold core. The maximum abundance of dinoflagellates and silicoflagellates occurred at the frontal zone. The chlorophyll-a concentration was high in the region associated with the frontal zone. Although mesoscale eddies are ubiquitous processes in the Bay of La Paz, this study represents the first observational report of the impacts of a dipole on the phytoplankton structure and chlorophyll-a in the region. The observations presented here indicate the existence of a strong association between the mesoscale processes and the phytoplankton community in the study area. This study highlights the value of efforts to improve projections of physical forcing and its influence on the planktonic ecosystem.