Online first articles

Periods of thermal stratification in deep dams can result in increased liberation of ammonia and phosphates at the sediment-water interface, which can cause nutrient enrichment in the downstream river section. We studied the recovery gradient of a thermally altered, regulated, unpolluted stream and its dependency on the main environmental variables. Macroinvertebrate assemblages were assessed in the spring-summer and autumn periods, at three sampling sites in a 1.25 km river reach, downstream from the dam, for 4 years (2019-2022). Water samples were collected from all riverine sites and one site in the dam. Nutrients were measured and hydrological alteration indices were calculated for the longitudinal gradient survey. We aimed to unravel the main environmental variables linked to the dam-induced hydro-morphological pressure affecting macroinvertebrate communities. Complex interactions between nutrients and the main physical parameters were identified as responsible for the observed trends. The analysis revealed natural-born hypolimnetic ammonia as the most contributing parameter to the biomonitoring metrics reflecting the status of macroinvertebrates. Ammonia enters the river system mainly during the dam thermal stratification periods (spring-summer) and as a result, macroinvertebrate communities had the highest recovery rates in the autumn. In this season the stream can achieve good ecological status even as near as 25 m below the dam wall if ammonia concentrations are low. Therefore, seasonal fluctuations of parameters such as dissolved oxygen and temperature in the hypolimnion are affecting invertebrate communities.