| Summary: | Marine copepods are fundamental to biogeochemical carbon cycling and energy transfer through pelagic food webs. The rapid rate at which oceans are changing makes it increasingly important that we understand how copepod populations interact with their environment. My thesis addresses this by combining field and laboratory studies to examine the direct and indirect effects of temperature on the cyclopoid copepod Oithona similis, a key component of marine ecosystems across the globe. To begin, I used time series data from English Channel station L4 to compare the population dynamics of O. similis with those of the contrasting calanoid copepod Calanus helgolandicus. A major result from this comparison was that seasonal trends in population density and egg production were decoupled in both species due to trait-based differences in how the environment impacts these key life history events. To gain a more thorough understanding of O. similis population dynamics, I then supplemented time series analysis with high-resolution sampling. With this approach, I found that O. similis has maintained stable population densities at L4 over the last 30 years, despite high climatic variability. My finding contrasts with the strong decline in copepod densities across the North Atlantic in recent decades. To test the extent to which populations respond differently to increased temperature, I conducted laboratory experiments on O. similis from L4 and the Southern Ocean. From these experiments, I found only partial support for the hypothesis that temperature responses are determined by the level of climatic variability experienced in situ, and evidence that food availability is a more important driver. Finally, I performed a global meta-analysis exploring how temperature effects on body size differ among populations. Strong variability in temperature-size responses was discovered both within and among populations, particularly those from high latitudes. My thesis provides new insight into how key life history events of population ...
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