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

The population dynamics of an exotic cladoceran (Daphnialumholtzi Sars) and a native cladoceran (Daphniaambigua) were studied over a 12 month period in subtropical LakeOkeechobee, Florida (USA), to quantify the extent of invasion of the exoticspecies and compare ecological niches. Daphnialumholtzi accounted for up to 70% of theDaphnia assemblage during the summer months(June-August), while D.ambigua accounted for up to 97%of the Daphnia assemblage from fall to spring(October-April). The densities of the two species were inversely corelated.The exotic species was most concentrated in the shallower, warmer, northand south ends of the lake during the summer. It also was present, but atmuch lower densities, in the central lake region during the fall. Thenative species displayed a ubiquitous distribution throughout the lakeduring spring and winter, but was concentrated in the deeper, cooler,central region during the summer. Relationships of the two species withenvironmental conditions indicate that water column temperature mightaffect the seasonal and spatial distribution of the twoDaphnia species. The results also indicate thatD.lumholtzi may be filling a 'vacant' seasonal orspatial niche when conditions are unfavorable forD.ambigua.