Ecology of a highly abundant, introduced cyclopoid copepod in a temperate estuary

The cyclopoid copepod Limnoithona tetraspina (Oithonidae) was introduced into the San Francisco Estuary (SFE) in 1993 and within a year became the most abundant copepod in the low-salinity zone. L. tetraspina makes up similar to 95% (median) of the total adult copepods in the low-salinity zone, and the biomass of adults is similar to that of 2 larger co-occurring calanoids, Pseudodiaptomus forbesi and Eurytemora affinis. The main goal of our research was to understand which food resources L. tetraspina uses in the low-salinity region of the SFE. Incubation experiments using natural water revealed feeding by L. tetraspina on mixotrophic and heterotrophic aloricate ciliates, but rarely on loricate tintinnids or diatoms. The co-occurring calanoids consumed similar prey, but also readily consumed diatoms. Capture and consumption of Strombidium spp. by L. tetraspina was confirmed visually, and experiments using cultured prey also showed that these copepods fed on motile phytoplankton but not on diatoms. Estimated grazing rates were low (median 2.3, range 0.6 to 8.3% body weight d(-1)); although these rates may be underestimates because of high concentrations of copepods in experimental containers, they are consistent with low specific egg production of females (0.3 +/- 0.2% body weight d(-1)). Low selectivity of one fish species for L. tetraspina suggests that this copepod may not be an important food resource for visually-selective fishes in the SFE. The low abundance of filter-feeding predators in this region of the estuary may be responsible for the high abundance of this cyclopoid copepod, despite its low potential population growth rate.