Illinois Natural History Survey - University of Illinois

INHS report January February 1995

An Exotic Zooplankton in Illinois

The zooplankton species Daphnia lumholtzi, which is native to Africa, Australia, and southern Asia, has been discovered in central Illinois impoundments by Natural History Survey researchers. The species was first found by Survey staff in Lake Springfield in 1992, and by late summer 1993, D. lumholtzi was the most abundant large zooplankton species in that lake. In 1994, the foreign zooplankton also was discovered in Lake Decatur, Clinton Lake, Sangchris Lake, and Lake Taylorville.

Zooplankton are microscopic animals that form the base of the food chain for many aquatic animals. Because zooplankton are a vital food supply for many newly hatched and even adult fish, the introduction of D. lumholtzi may negatively affect the fish communities of the waters it infests.

Daphnia lumholtzi is very different from other zooplankton found in Illinois. It is longer and has large spines on the head and tail. Including spine lengths, D. lumholtzi reaches almost 6 mm. Newly hatched fish consume a great deal of zooplankton, but they typically do not consume zooplankton exceeding 2 mm due to mouth size limitations. Because of their larger mouth size, juvenile and adult fish may more easily consume D. lumholtzi and, in fact, they may actually prefer them over smaller native zooplankton.

The first discovery of D. lumholtzi in the United States was in a Texas impoundment in 1991. Researchers speculate that the species was accidentally introduced when two fish species, Nile perch and tiliapa, both obtained from Africa, were stocked into the reservoir. Since 1991, D. lumholtzi has spread at an alarming rate over a large portion of the southeastern United States. In addition to Illinois, it is currently found in Texas, Missouri, Tennessee, and North Carolina.

Daphnia lumholtzi (Drawing courtesy of "Oceanography and Limnology.") at left.

To date, all waters in Illinois found infested with the foreign zooplankton have been impoundments. Discharge leaving impoundments over spillways may further the spread of D. lumholtzi.

To determine possible early effects of D. lumholtzi in Lake Springfield, Survey staff compared the zooplankton community of Lake Springfield in 1992 and 1993 to those of Pierce Lake and Lake George in northern Illinois during the same time period. They found that while the zooplankton communities in Pierce Lake and Lake George in 1992 were similar to the 1993 communities, there was a dramatic decline in all types of native zooplankton in Lake Springfield from 1992 to1993. Daphnia lumholtzi increased in abundance during the same time period.

Unlike zooplankton native to Illinois, which are abundant in April and May, D. lumholtzi was found in highest abundance (2.6/L) as water temperature peaked in late July 1993. Because the foreign species is native to warmer climes, it is assumed that it is better adapted to warmer temperatures than native zooplankton species.

If the extensive head and tail spines of D. lumholtzi deter fish predation, the species could lead to a major restructuring of the zooplankton community. Initial research at Lake Springfield suggests that this may be occurring, but additional data are needed before a definite conclusion can be made. The dramatic decline in zooplankton in Lake Springfield between 1992 and 1993 may be due in part to the invasion of D. lumholtzi. The relatively low number of the species (2.6/L) in 1993 seems insufficient to cause the decline. But there are numerous ways that an exotic species might have negative effects, hence a large population of D. lumholtzi may not be necessary to have an effect on the zooplankton community.

In general, exotic species like D. lumholtzi lack natural biological controls such as predators, competitors, and parasites that are found in their native environments. As a result, there can be shifts in food webs, economic losses, and the extinction or extirpation of native species. However, if fish consume D. lumholtzi, the foreign zooplankton may provide an abundant food source for planktivorous fish during late summer when the abundance of other zooplankton is low. Given the unpredictable effects of the addition of D. lumholtzi to the aquatic communities of Illinois, further study of this exotic is necessary.

Cynthia S. Kolar, James C. Boase, and David H. Wahl, Center for Aquatic Ecology.

Charlie Warwick, editor