Spatial and temporal variations of the food web structure in a dynamic water-level lake.

The effects of water-level variations on the structural organization of a detritus source food web were studied in the littoral zone of Lake Trasimeno (C Italy). Dry leaf-packs of Phragmites australis were used, a detritus food source. The web structure changes both temporally and spatially with the initial depths of the leaf-packs in water (0, -10, -35, -50 cm). In winter (submersion season), there was a very complex detritus source web with: 1) high values of connectivity (number of taxonomical units per connectance), 2) more polyphagous predators than monophagous ones and 3) high number of competitive horizontal links. In the dry season, a minimal structure of the source web is characterized by the opposite relationships. The structure of the source web is also influenced by the permanence of detritus source in water, the bathymetrical declivity, and the ontogenetic niche shift of many aquatic insects. The space-time elasticity of the structural organization of the source web could represent a stabilizing property. -from Authors