TEMPERATURE-DEPENDENT ENERGETICS OF CHAOBORUS POPULATIONS - HYPOTHESIS FOR ANOMALOUS DISTRIBUTIONS IN THE GREAT-LAKES OF EAST-AFRICA

Chaoborus, the phantom midge (Insecta, Diptera, Chaoboridae), has a wi despread distribution, commonly occurring in lakes and ponds all over the world. In the great lakes region of East Africa Chaoborus is prese nt in Lakes Victoria, Albert, Edward, Malawi and George, but absent fr om Lakes Tanganyika, Kivu and Turkana. Tropical lakes typically have w ater temperatures in the range of 22-26 degrees C year round. Lakes Ta nganyika and Kivu have only 20% of their bottom sediments oxygenated d uring full circulation, contrary to 95-100% in all of the other lakes, excluding Lake Malawi (45%) (Hecky & Kling, 1987). Planktivorous fish . are present in all lakes (Lehman, 1995). We hypothesized that the ab sence of Chaoborus larvae from some lakes of East Africa may be the re sult of interaction among high temperatures, low oxygen levels, and fi sh predation. We developed a model to estimate energetic costs for Cha oborus larvae at temperatures greater than 14 degrees C. We hoped to s hed light on the bioenergetics of Chaoborus populations, and the possi bility that extant distributions of Chaoborus larvae are the result of energetic constraints. We found that relative respiratory and growth costs of Chaoborus larvae are highest in the early stages of developme nt. We estimated that non-feeding instar I larvae living in 25 degrees C water will starve to death in less than one day. It is possible tha t Chaoborus populations are prevented from establishing in certain are as because high energetic costs condemn young larvae to death by eithe r predation or starvation.