Functional response and potential predatory impact of Tortanus dextrilobatus, a carnivorous copepod recently introduced to the San Francisco Estuary

Despite a dramatic increase in the introduction of non-indigenous estuarine zooplankton in recent decades, the trophic implications of such introductions have rarely been quantified. Here we investigate predation rates of Tortanus dextrilobatus, a carnivorous copepod recently introduced to the San Francisco Estuary that achieves peak abundances in excess of 1000 ind. m(-3). The functional response of T, dextrilobatus feeding upon 2 copepod prey types-the non-indigenous cyclopoid Oithona davisae, and the 'native' calanoid Acartia (Acartiura) sp.-was described by a type II functional response (Ivlev function) at 2 experimental temperatures. In 3 of these 4 treatments, 90 % I-max, was achieved within a naturally occurring range of prey densities. Taxon-specific seasonal size variation was identified, and carbon-based consumption values were used to determine temperature-dependent rates of predation on both prey types. These empirically derived consumption rates and temperature dependence values were then applied to broadscale surveys (1997 to 1999) of zooplankton community composition in order to estimate the predatory impact of T. dextrilobatus upon prey populations in the San Francisco Estuary. Predatory impact estimates (% population consumed d(-1)) greater than 1 % occurred on a regular basis when T. dextrilobatus was abundant, with maxima exceeding 20, 65, and 25% for O. davisae, Acartia (Acartiiura) sp. and all Copepoda, respectively. These observations support the hypothesis that non-indigenous invertebrate zooplanktivores can play a significant role, at least seasonally or episodically, in the secondary production dynamics of aquatic ecosystems.