Changes in the structure of the epipelagic copepod community on the western coast of the Baja California Peninsula before and during El Niño 2015

This study describes changes in the community structure of epipelagic copepods along the west coast of the Baja California Peninsula before and during El Niño 2015. We tested the hypothesis that the area was undergoing tropicalization based on changes in species composition, diversity, and latitudinal distribution as well as changes in the abundance of herbivore and carnivore species. The warming effect on zooplankton communities was more evident north of Punta Eugenia in 2015 with a few changes in the southern portion between 2014 and 2015, it was also coincident with a higher influence of tropical waters up to Ensenada and a decrease in zooplankton biovolume values, with values lower than 60%, also lower than those observed in 2014. No significant changes in the species richness of the whole region were found. However, the main signs of tropicalization associated with El Niño 2015 were an increase in local species richness, an increase in tropical species abundance with an expansion to the north of their geographical distribution, and a decrease in the abundance of omnivorous-herbivorous species. These changes in community structure were related to the increase in warm water flow towards the north during 2015, a deeper thermocline, a decrease in chlorophyll-a concentration, as well as a deeper maximum of chlorophyll-a concentrations. The presence of the tropical species Pleuromamma abdominalis and Subeucalanus subcrassus with high abundance in the northern region suggest they are biological indicators of tropicalization processes during El Niño events.