Groundwater drift monitoring as a tool to assess the spatial distribution of groundwater species into karst aquifers

Groundwater drift collected continuously at perennial outlets of karst aquifers has been examined for its potential in assessing the spatial distribution of the resident species. To this end, we have continuously monitored the groundwater drifted at the Mazzoccolo spring (Western Aurunci karst aquifer, Central Italy) over 2 years. Concurrently to the biological monitoring, major ion geochemistry and turbidity were investigated on the same schedule. We found that the hydrochemistry did not govern the distribution pattern of the dominant taxon (Copepoda) into the Western Aurunci aquifer. In contrast, copepod drift showed clear differences in the number of individuals between the high and low water periods because of the "piston effect," which is very frequent in karst and fractured aquifers due to recharge from rainfalls. According to the results of this study, groundwater species do not inhabit all sectors of a karst aquifer. They avoid the fast-flowing conduits where groundwater rapidly flows, while colonizing the more inertial aquifer sectors, such as the slow-flowing conduits.