Timing and size of blooms of the ctenophore Mnemiopsis leidyi in relation to temperature in Narragansett Bay, RI

The ctenophore Mnemiopsis leidyi is at the northern extreme of its geograph ic range in Narragansett Bay, an estuary on the northeast coast of the Unit ed States. Blooms have typically been observed in late summer and fall acco rding to records from 1950 to 1979. We document an expansion of the seasona l range of this important planktonic predator to include springtime blooms during the 1980s and 1990s. This shift to an earlier seasonal maximum is as sociated with increasing water temperature in Narragansett Bay. Temperature s in spring have risen, on average, 2 degreesC from 1950 to 1999 with warm years being associated with the positive phase of the North Atlantic Oscill ation. During 1999, M. leidyi appeared earlier in spring and was more abund ant than during any previous year for which records are available. Changes in the seasonal pattern and abundance of this predator are likely to have i mportant effects on planktonic ecosystem dynamics of Narragansett Bay. Thes e include reduction of zooplankton abundance in spring followed by increase s in size and frequency of summer phytoplankton blooms. Earlier blooms of M . leidyi may also reduce survival of eggs and larvae of fish because, as in 1999, they coincide with the period of peak spawning.