Are the toxic sediments deposited at Flix reservoir affecting the Ebro river biota? Purple heron eggs and nestlings as indicators.

The Flix reservoir, in the low course of the Ebro River, contains thousands of tons of polluted sediments, accumulated from the activities of a chemical factory. An ongoing project is working toward removing these pollutants. Piscivore birds like the purple heron (Ardea purpurea) may be useful bioindicators, so eggs and nestling feathers were sampled during the 2006-2008 breeding seasons at three localities: a reference site situated upstream and two potentially affected by the toxic muds; one at the focal area and one at a distal area, the Ebro Delta. The samples were analyzed for isotopic signatures of ¹⁵N and ¹³C and concentrations of heavy metals and selenium. Baseline nitrogen signatures were higher in riverine sites than in the delta. Nitrogen together with carbon signatures adequately discriminated riverine and deltaic ecosystems. Mercury levels are highly influenced by the polluted sediments at Flix and pose potential risks for the birds, as they are among the highest ever recorded in heron species. Selenium and copper concentrations probably derive from other sources. Except for mercury, heavy metals and selenium levels were below toxic levels. Purple heron eggs and nestling feathers have demonstrated their usefulness as bioindicators for pollution in the river biota; feathers in particular show pollutant impacts on a strict local basis. A long series of study years is necessary in dynamic ecosystems such as this, so continued monitoring of the heron population at Flix is advisable to trace the effects of the toxic muds, particularly during their removal, because of the high levels of mercury detected.