Spatial and temporal distribution of benthic stages of Cyclops vicinus and Chaoborus flavicans in relation to abiotic factors and benthic fauna

The horizontal distributions of the benthic stages of Chaoborus flavicans and Cyclops vicinus were studied in a eutrophic stratified lake in the Massif-Central (France) over one year, at 5 stations from the shore to the centre of the lake. Their distribution was investigated in relation to temperature, dissolved oxygen, sediment grain-size and other benthic organisms. The dominant taxa of the benthic fauna of Lake Aydat were dipterans, crustaceans and oligochaetes and their distributions were independent of the grain size. In contrast to chironomids which preferentially inhabited the sublittoral zone, chaoborids and crustaceans were more numerous in the profundal zone. The sediment-dwelling oligochaetes remained numerous in both zones, according to the season. The fourth copepodite stages of Cyclops and Tubifex are tolerant to low oxygen concentrations in contrast to the fourth instar larvae of Chaoborus whose distribution was positively correlated with oxygen. The guts of these dipteran larvae were found to be empty and we assumed that, in contrast to the chironomids and oligochaetes, the resting stages of Cyclops vicinus and the benthic stages of Chaoborus flavicans did not use benthic resources. The former are activated at the autumn overturn, while the latter escape from the bottom at the start of the spring oxygen depletion. This suggests that physical factors are largely responsible for their reactivation. Both animals suffered of the effects of starvation and probably lost weight. The reactivation of the copepod at the autumn overturn would be facilitated by fluid mechanical disturbance. In addition, after the spring overturn, a small increase in temperature near 4 °C would be a reliable environmental signal for the dipteran. Food limitation does not occur, invertebrate predation pressure seems to be negligible and the predation by fish on the macrobenthic fauna and by chance on the meiofauna, clearly remains limited in both space and time.