Aquatic bird populations as possible indicators of seasonal nutrient flow at Ichkeul Lake, Tunisia

Abstract

Lake Ichkeul is fed in autumn and winter by 7 main freshwater oueds (rivers) which create an overflow towards the mediterranean sea. Conversely, seawater enters the lake after the end of the rainy season. Thus, strong inverse variations occur twice a year both in water depth and salinity level, with a major hydraulic flow to the sea between October and January.

Waterbirds exploit the lake (ca 50 species belonging to 13 families) with a marked seasonal variation in specific richness and diversity; in winter, population size (200 000 ducks, coots and geese) and biomass are much higher: 92.2% of the trophic impact occurs between October and March, because of both numbers and size of the wintering birds. The bird community is phytophagous for 96.2% of its annual biomass.

The data, related to the hydraulic regime of the lake, support the hypothesis of an energetic input (N and P) which is mostly linked to the freshwater flow and an export brought about by migrating birds and the harvesting of fish. Nutrients would be a limiting factor for both plant and animal communities, Ichkeul being an atypic (rather oligotrophic) mediterranean lake. New dams around the lake (for drinking and irrigation water) are changing drastically this energetic balance and will lead to an important loss of the main characteristics of the lake, making Ichkeul an ordinary man-managed wetland.