Dynamics of surface water/groundwater ecotones in a karstic aquifer

SUMMARY.

• 1 Interactions between surface waters and groundwaters were analysed by studying degrees of similarity between a surface stream and groundwater in a karstic system.
• 2 Exchange of water and solutes (‘abiotic interactions’) and living organisms (‘biotic interactions’) were quantified by characteristic parameters measured at the outlet of the karstic system during hydrological changes across the year as well as during a flood.
• 3 The ratio of bicarbonate to calcium described abiotic interactions. In the seasonal study, biotic interactions were estimated from the proportion of epigean organisms in the communities sampled at the outlet. These interactions during the flood were estimated by comparing numbers of Cladocera, epigean organisms, with numbers of Niphargus virei, hypogean organisms.
• 4 The location of the ecotone created by interactions between the surface stream and the karstic system showed marked spatial fluctuations according to the prevailing hydrology. Interactions were strong during high-water periods whereas they were negligible during low-water. Similar fluctuations occurred within a few hours during the flood.
• 5 Results are compared with those from fluvial systems to discuss the importance of the dynamics of these surface/underground ecotones in the functioning of lotie ecosystems.