Seasonality of the copepods Acartia hudsonica and Acartia tonsa in Narragansett Bay, RI, USA during a period of climate change

Seasonality of species living at the boundaries of biogeographic zones may be more sensitive to climate change than in other regions. This is apparently the case for the ctenophore, Mnemiopsis leidyi, in Narragansett Bay, RI, which is the historical northern boundary of its distribution in the Northwest Atlantic. Seasonal advancement of population pulses of this ctenophore correlates with an increase in average annual temperatures of 1.2 °C over the last ∼50 years. Do other zooplankton in Narragansett Bay show evidence of altered phenologies? Here we examine patterns of seasonal succession of the copepod congeners, Acartia tonsa and Acartia hudsonica, for evidence of alteration over the period 1950–2004. A warming trend might be expected to limit springtime abundance of A. hudsonica, a temperate–boreal species that produces resting eggs in response to warm weather. Conversely, increasing temperatures could favor the summer dominant, A. tonsa, over its congener, allowing a shift to earlier appearance in spring, thus preserving the predator-free window that has previously allowed it a period of high production prior to ctenophore population pulses in late summer. Contrary to these predictions we found that A. hudsonica has become the dominant copepod of the congener pair. There has been no seasonal advancement of populations of A. tonsa, whose numbers have plummeted due to intensification of the predator–prey interaction with M. leidyi. In contrast, advancement of seasonal appearance of A. hudsonica is evident in sustained population increases earlier in spring (March rather than in May), although, as predicted, there is curtailment of its distribution in late spring. This latter shift is likely exacerbated by ctenophore predation. This study demonstrates the complexity of predicting individual species responses to climatic warming, even for species with well-known patterns of seasonal and geographic distribution.