Mortality and recruitment in two copepod populations in a subarctic oligotrophic reservoir and the influence of environmental forcing

Abstract

We describe the stage-specific mortality, survival and recruitment patterns for a cyclopoid and a calanoid copepod through the use of vertical life table analysis. Over a 2-year period in Lake Saint-Jean, Québec, Canada, we show that during the growing season, Diacyclops bicuspidatus thomasi (May-June) had its highest daily mortality rate during metamorphosis (N6-C1), while Leptodiaptomus ashlandi (June-August) had its highest daily mortality rates during the naupliar stages. Patterns in daily recruitment rates differed between the species with the early stages of L. ashlandi experiencing a greater population loss during the growing season. Recruitment of D. bicuspidatus thomasi was controlled by reproduction, while recruitment of L. ashlandi was governed by reproduction and mortality/survival although also influenced by seasonal patterns of phytoplankton biomass. A multiple regression model suggested that cannibalism influenced the mortality of the early stages of D. bicuspidatus thomasi during their growing season. Temperature, competition and cannibalism influenced the mortality of the early stages of L. ashlandi. However, there are interannual differences likely due to interannual difference in seasonal patterns of phytoplankton biomass. Our study emphasizes the importance of interactions between bottom-up and top-down controls governing copepod population dynamics.