FOOD LIMITATION IN LAKE-DWELLING LEECHES - FIELD EXPERIMENTS

1. The leeches, Erpobdella octoculata, Glossiphonia complattata and He lobdella stagnalis, and their prey were manipulated inside experimenta l enclosures, matched by controls, erected on the stony shore of an eu trophic, English lake. 2. Five separate manipulations were performed: the addition of tubificid oligochaetes, the addition of snails, the in crease in numbers of G. complanata and H. stagnalis, the increase in n umbers of E. octoculata, and the reduction in numbers of all three lee ch species. All invertebrates were sampled, using trays buried in the substratum, in early March, prior to leech reproduction, in July and i n late October/early November when breeding had ceased. Leech manipula tions were executed immediately after the first sampling of invertebra tes, when food addition, at fortnightly intervals, also commenced. 3. The addition of tubificids resulted in significantly greater numbers a nd biomasses of E. octoculata and H. stagnalis in the experimental com pared to the control sites in the autumn sample, i.e. after breeding. The same was true for G. complanata when snails were added. When the n umbers of leeches were either decreased or increased, no significant d ifferences in numbers or biomass of the manipulated species were obtai ned in the autumn. 4. Elevation or readjustment of leech numbers appea red to be achieved through changes in juvenile mortality and to a less er extent through fecundity. 5. Current results, together with previou s findings, provide evidence for food limitation, with a high mortalit y of recruited young (up to 98%) which is linked to food supply. There is also some evidence for interspecific competition for food, though the occurrence of food refuges, the temporal separation of breeding pe aks, and differences in feeding behaviour, involving body size and for aging strategies, between the glossiphoniids reduce the intensity of i nteractions.