Succession of Two Acartia Species in Estuaries1

In New England and Middle Atlantic estuaries Acartia clausi, a winter—spring copepod, and A. tonsa, a summer—fall form, alternate cycles of dominance. Succession from clausi to tonsa during spring starts near the head of an embayment and spreads seaward. Concurrent observation in Narragansett Bay, Raritan Bay, and the York River plus an additional program in two Rhode Island lagoons indicated that this pattern of species replacement was not solely due to summer warming. Between 11 and 18°C, intermediate temperatures for reproduction of both species, tonsa was favored because it appeared less sensitive to brackish water than clausi.