Temporal variability of abundance and non-predation mortality of some major groups of crustacean zooplankton (Diplostraca, Copepoda, and Decapoda) in Magdalena Bay, Baja California, Mexico

Abstract Estimates of the prevalence of dead zooplankton are increasingly necessary in marine ecological studies. We aimed to identify the relative proportions of live/dead individuals of three of the historically most abundant taxa of zooplankton in Magdalena Bay, Mexico: copepods, decapods, and diplostracans. Abundance data and environmental variability were recorded monthly at neap tides during 2015–2018. Samples were stained with neutral red to distinguish living from dead individuals. Copepods were the most abundant group and showed a significant annual variability in their abundance during the study period. Influenced by a preceding warm spell and the El Niño event of 2015–2016, the lowest abundances and the highest recorded standing mortalities of all three taxa were reported in 2015; standing mortality during El Niño was double that otherwise observed. A significant correlation was found between some of the environmental variables analyzed (salinity, temperature) and increased standing mortalities in copepods and decapods. We found differences between the percentages of live/dead organisms between the years analyzed and also between the taxonomic groups involved (copepods 17%, decapods 21%), which underlines the importance of including mortality analyses in ecological studies of zooplankton.