Daphnia lumholtzi and Daphnia ambigua: population comparisons of an exoticand a native cladoceran in Lake Okeechobee, Florida

The population dynamics of an exotic cladoceran (Daphnia lumholtzi Sars) an d a native cladoceran (Daphnia ambigua) were studied over a 12 month period in subtropical Lake Okeechobee, Florida (USA), to quantify the extent of i nvasion of the exotic species and compare ecological niches. Daphnia lumhol tzi accounted for up to 70% of the Daphnia assemblage during the summer mon ths (June-August), while D.ambigua accounted for up to 97% of the Daphnia a ssemblage from fall to spring (October-April). The densities of the two spe cies were inversely correlated. The exotic species was mostly concentrated in the shallower, warmer, north and south ends of the lake during the summe r. It also was present, but at much lower densities, in the central lake re gion during the fall. The native species displayed a ubiquitous distributio n throughout the lake during spring and winter, but was concentrated in the deeper, cooler, central region during the summer. Relationships of the two species with environmental conditions indicate that water column temperatu re might affect the seasonal and spatial distribution of the two Daphnia sp ecies. The results also indicate that D.lumholtzi may be filling a 'vacant' seasonal or spatial niche when conditions are unfavorable for D.ambigua.