| Summary: | Zooplankton play a pivotal role in marine ecosystem by cycling carbon and energy up the food chain. The species composition of a zooplankton community can influence such energy transfer pathways in a marine ecosystem. For example, when large calanoid copepods dominate a food web, more of the energy produced by phytoplankton is transferred up the food chain and high biomass of large, lipid-rich calanoid copepods or euphausiids has been associated with higher recruitment of upper trophic levels compared to a system dominated by smaller copepod species. Thus, understanding how environmental forcing drives fluctuations in zooplankton community composition is essential to assessing how the structure of marine ecosystems may vary in the future. However, knowledge of what zooplankton life history strategies or functional traits (e.g. feeding guild) make some species more successful under specific environmental conditions remains limited. Here, using three years of field data on zooplankton abundance, biomass, and species composition from a fjord in British Columbia, Canada, I identified the zooplankton seasonal succession cycle and the controlling effects of different environmental forcing factors on the recruitment success of various zooplankton functional traits. Zooplankton succession was delayed when the spring bloom was late. Furthermore, herbivorous, ontogenetically migrating copepods with a short reproductive season, such as Calanus marshallae and Eucalanus bungii, had reduced recruitment when the spring bloom was delayed or in years of high spring temperature, as compared to omnivorous copepods with a long reproductive season. A population dynamics model was employed to determine the environmental drivers of the observed temporal variability of C. marshallae and E. bungii. Interannual differences in mortality and advection drove the observed change in the recruitment of the two copepod species. High mortality was associated with conditions of low chlorophyll biomass and high temperatures. I suggest that the success of these copepod species may depend on the presence of a low predation-high chlorophyll window in early spring.
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