Role of environmental seasonality in the turnover of a cetacean community in the southwestern Gulf of California

La Paz Bay is a distinct region within the Gulf of California whose rich cetacean community exhibits an intense annual overturn. We studied the environmental conditions that could drive this change over the course of a year. Cetacean biomass was estimated from monthly surveys, with concurrent collection of water-column measurements of temperature, salinity, nutrients, chlorophyll a (chl a), and biogenic matter fluxes. The water-column structure showed 3 major conditions: deep mixing during winter, stratified isopycnal shoaling in spring and early summer, and deep stratification during late summer and autumn. Chl a and relative fluxes of biogenic silica and calcium carbonate indicated a seasonal succession of primary producers in response to the observed evolution of hydrography. During the periods of mixing and isopycnal shoaling, the bay provided suitable habitat for blue whales, bottlenose dolphins, and common dolphins, while fin whales, Bryde's whales, and short-finned pilot whales were numerically dominant during the period of stratification. To provide a regional context to the observed seasonality, we fitted temporal least-squares to an 11 yr monthly time series of satellite-derived wind, sea surface temperature (SST), and chlorophyll concentration (CHL). Within the bay, the SST followed the annual mon-soonal shift in the wind, whereas CHL showed a bi-modal pattern, with a main peak occurring under mixing conditions in winter and a second peak under isopycnal shoaling in spring/early summer. The regional fitting suggested that the latter period was driven by a localized intra-seasonal phenomenon that could be responsible for the higher biological richness of the bay compared to the surrounding gulf.