| Blooms of gelatinous zooplankton are growing problem in many estuaries, including the upper San Francisco Estuary where at least four species of Ponto-Caspian hydrozoans have become seasonally abundant in recent decades. This dissertation examines the ecology of these non-native hydrozoans. The first chapter presents the first field-derived ecological data for the polyp stages of Blackfordia virginica, Moerisia sp., and Cordylophora caspia. Settlement data indicate a seasonal presence of B. virginica and Moerisia sp., with both distribution and abundance correlated with a combination of water quality and physical parameters. Cordylophora caspia appeared to be present beyond the time period sampled and may active in the system year-round. The ability of polyps to persist month to month was low, likely due to predation by other non-native species and competition for space. The second chapter investigated the life history and population dynamics of Moerisia sp. Polyps emerged from a resting stage during June, at a minimum temperature of approximately 14ºC. Asexual reproduction of medusae buds was positively correlated with temperature and negatively related to salinity. The production of polyp buds was positively correlated with dissolved oxygen and water transparency levels. Sexual reproduction, defined by the presence of eggs, was related to the size of medusae. Cessation of reproduction of both polyps and medusae occurred in October, when temperatures dropped below 17ºC. Chapter 3 documents feeding patterns for the medusae of Maeotias marginata and Moerisia sp., as well as their trophic interactions with declining planktivorous fish species, juvenile striped bass and threadfin shad. Both hydrozoan species fed on a variety of small crustacean prey, most notably, calanoid copepods. Feeding occurred throughout the diel cycle for both species, but was greatest during flood tide for M. marginata. The only fish larvae consumed were those of a non-native goby. Diet overlap was high between threadfin shad and hydrozoans, but low for juvenile striped bass. Because of strong spatial and temporal overlap between hydrozoans and shad, competition for zooplankton may be occurring. Our results suggest that the trophically similar delta smelt, a federally threatened species, may be seasonally at risk as well. |
