Species-specific differential responses to climate warming that alter phenologies can dramatically affect community function. Using time series data from a Northwest Atlantic estuary, we show that the phenology of a top zooplankton predator, the ctenophore Mnemiopsis leidyi, has shifted more in response to climate warming over the past 50 years than that of its major prey species, the copepod Acartia tonsa. Before climatic warming, A. tonsa was the dominant secondary producer in the estuary and

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... its main period of production occurred before the seasonal appearance of M. leidyi. However, since 2000, the seasonal peak abundances of the two species have overlapped, intensifying the predator-prey relationship and resulting in the near extirpation of the once-abundant copepod from the estuary. We propose that the physical mechanism driving the different phenological responses of the two species is differential spring warming of the winter refugia of these two species. Substantial amplification of warming patterns in shallow embayments affects overwintering M. leidyi but has little influence over seasonal excystment of overwintering A. tonsa eggs located in the broader, deeper regions of the Bay. In this way, large-scale climatic changes are expressed as local-scale temperature variations that differentially affect alternate planktonic life histories to produce novel spring population growth dynamics of the two species and, ultimately, a new summer planktonic community dynamic in Narragansett Bay.