Abstract
Records of occurrences of planktonic calanoid an cyclopoid copepods from estuarine locations in eastern North America reveal that, while combined numbers of calanoid plus cyclopoid species increase to some extent proceeding southward, the patterns for calanoid and cyclopoid species are different. Cyclopoid species richness is highest in the subtropics, but calanoid diversity is highest in the temperature zone. Since the species patterns are not duplicated at the family level, the diversity patterns are mainly due to distributions of closely-related species. The higher calanoid diversity in the temperate zone reflects a series of seasonal alternations of various congeneric species. These alternations are associated with seasonality of temperature oscillations over the annual cycle. The reasons for the higher diversity of cyclopoids toward the tropics remain obscure, but it appears that many cyclopoids are primarily limited to estuarine waters with greater temperature stability. Thus, estuarine species richness of calanoids is highest in physically unstable estuaries, and cyclopoids are most diverse in more physically stable ones. Due to opportunistic feeding habits of many calanoids, and probably cyclopoids as well, it appears unlikely that the latitudinal patterns of estuarine copepod species are mainly due to different degrees of diet specialization. Likewise, the unstructured nature of the pelagic habitat suggests that habitat structural complexity is not as important a determinant of diversity of marine plankton as it appears to be for marine benthos.