Blue-Green Algae and the Seasonal Succession of Daphnia

The cladoceran Daphnia lumholtzi invaded North America from the Old World tropics and has since spread across much of the southern United States. This exotic cladoceran is most common during midsummer, when native Daphnia are rare and Cyanobacteria (blue-green algae) are abundant. Phytoplankton and zooplankton surveys were conducted from late spring to late summer at four lakes (Stockton Lake, Fellows Lake, McDaniel Lake, and Bull Shoals Lake) in 1999 and in McDaniel Lake in 2000. All the study lakes except one (McDaniel in 1999) showed the typical trend for eutrophic lakes with low blue-green algae in the spring, and then becoming abundant by late summer. Copepods were the dominant zooplankton throughout the summer 2000 surveys in McDaniel Lake. I hypothesized that D. lumholtzi would utilize blue-green algae more effectively than the native Daphnia species. As a test, I conducted field experiments in a eutrophic lake in May and July 2000. During this period, generic richness of the algae remained high and abundance of blue green algae increased (13.7 to 80.3%). For each experiment, I placed D. lumholtzi and two native species (Daphnia galeata mendotae and Daphnia parvula) into replicate 1.6-L experimental chambers in the lake for two hours incubation. Diets were analyzed at 400x following dissections of Daphnia guts. In May all three Daphnia species selected for edible greens. In July, both D. lumholtzi and D. parvula avoided blue-green algae, whereas D. galeata mendotae showed no significant discrimination. However, all three Daphnia species consumed about 25% blue-green algae in July. Contrary to my predictions, D. lumholtzi did not utilize a great proportion of blue-green algae than D. parvula and D. galeata mendotae. However, D. lumholtzi ingested significantly more total algae in July than the two native Daphnia species, suggesting that D. lumholtzi has less feeding inhibition when blue-green algae are present than native Daphnia