| Summary: | PhD Calanus helgolandicus is a key copepod species occurring in the North East Atlantic that is responding to oceanic warming through an expansion of its geographic range. This range extension has led to concerns about how this may affect ecosystem trophodynamics. Here I investigate the interannual variability and seasonality of C. helgolandicus, using a ~28 year time-series from the western English Channel (station L4). I focus specifically on the role of mortality, as a key life history process that is challenging to quantify and historically has received little attention. C. helgolandicus abundance remained within a narrow ~four-fold interannual envelope, which was a consequence of multiple losses that removed ~99% of the potential population. Loss of early life stages occurred through the incidence of non-viable eggs and abnormal nauplii (both higher in spring), and via predation; egg mortality rates were positively correlated with C. helgolandicus copepodite abundance and total copepod biomass, indicative of intraguild predation and cannibalism. By contrast, late-stage copepodite mortality rates were highest in autumn, and were positively related to gelatinous predator abundance and biomass (medusae, ctenophores and chaetognaths). Molecular gut-content analyses revealed that two abundant jellyfish species present during 2015 (Pleurobrachia pileus and Leuckartiara octona) both preyed on C. helgolandicus. Adult male consumptive mortality rates were ~6 times higher than that of adult females; whereas male non-consumptive rates were only ~1.5 times that of females, providing evidence that predation was the primary mortality source in males. Non-consumptive mortality rates contributed 0-54% (median of 4.5%) to total mortality and were positively related to the 72-hour maximum wind speed, implying that turbulence created during extreme weather events may increase zooplankton mortality. I conclude that C. helgolandicus population control is modulated via a series of mortality-related losses occurring through the ...
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