Investigating the Ecology and Thermal Biology of the Exotic Cladoceran Daphnia lumholtzi

Abstract

Daphnia lumholtzi is a microcrustacean with tropical origins in Africa and parts of Asia and Australia. The species was found in US reservoirs approximately thirty years ago where it is thought to have been introduced with fish imported from Africa. D. lumholtzi apparently colonized reservoirs by expanding during the summer when water temperatures rise and native Daphnia populations decline, suggesting that the species would only thrive in warm reservoirs during the summer months. However, D. lumholtzi has spread throughout North America and continues to expand into new areas. D. lumholtzi has been studied mostly in reservoirs, and less is known about the species’ establishment in other environments. In 2002, D. lumholtzi was discovered in the Mobile-Tensaw Delta (MTD), providing an opportunity to study its distribution in a unique environment. In this study, surveys in the MTD and in Weeks Bay National Estuarine Research Reserve (WBNERR) found that D. lumholtzi occurred in low abundance in the upper MTD but was not detected in the lower delta or in WBNERR, where salinity levels are higher. In the MTD, D. lumholtzi was more likely to be present in the fall compared to native Daphnia. Laboratory experiments showed that there is no difference in the upper thermal tolerance limits between D. lumholtzi and the native Daphnia ambigua collected from the MTD. These studies show that D. lumholtzi populations are currently present in freshwater areas of the MTD and have likely been established since at least 2002. There are seasonal differences in the distributions of D. lumholtzi and native Daphnia, but these do not appear to be solely due to differences in their thermal tolerances. To better understand D. lumholtzi invasion biology in the MTD, laboratory studies are needed to characterize the effects of high and low temperatures on life history traits of D. lumholtzi and native Daphnia from this area. Field and laboratory studies are needed to determine the effects of salinity on D. lumholtzi distribution in coastal environments.