Evaluation of ecological flows in highly regulated rivers using the mesohabitat approach: A case study on the Nestos River, N. Greece

Preservation of a good ecological status in riverine habitats emerged as a priority for water management policies in Europe since the negative impact caused by the construction and operation of dams on the ecohydrology and habitat availability became more evident. Ecological flows, as reported in the recently published European guidance, represent a link between water and habitat EU Directives. This study presents the application of a mesohabitat simulation model (MesoHABSIM) to evaluate and quantify ecological flows in a highly regulated Mediterranean watershed (Nestos River, Northern Greece). Data collection was performed through GIS/GPS mapping surveys, hydro-morphological measurements (water depth, flow, substratum type, etc.) and electrofishing samplings at mesohabitat scale under different discharge conditions. In total, 81 hydromorphological units were surveyed and 7532 fish samples were collected to develop habitat suitability predictions. Ecological flows were calculated in the range 10–15 m3/s as the required discharge which assures the welfare and sustainability of protected fish species populations. In the lower course of the Nestos River habitat time-series indicated irrigation abstractions as a major stressor since summer was the period where habitat availability thresholds were mostly violated. Application of a revised water management plan is required for the downstream part of the Nestos River in order to maintain high ecological standards in the Natura 2000 sites of the Delta.