Long-term Macrobentos Responses to Environmental Flows and Water Quality improvement along River Jarama

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The Jarama River, due to its proximity to Madrid, has been a highly regulated and polluted for decades; we evaluated the effects of changing multi-pressures over time and along the fluvial continuum. Changes in macrobenthic composition and structure in response to alterations in river habitat have been analyzed from two macroinvertebrate sampling campaigns carried out 42 years apart at 11 locations along the Jarama River. Results show that the macrobenthic communities follow a similar pattern along the river continuum in both periods. Richness and diversity were higher at mountain reaches without pollution and unregulated. The minimum values are reached at San Fernando de Henares, although in the lower part of the river, after receiving all the pollution discharges from the Manzanares River, there are no macrobenthos since the Jarama bottoms are anoxic sludge.Comparing the communities at the same localities we have detected clear temporal changes. In the upper zone, the total number of taxa and diversity decreased, but the fauna richness is maintained. In the section just below large reservoirs we find an increase in macrobenthos richness. In all the medium sections there is a recovery in all the macrobenthos parameters, but this improvement does not reach good ecological status, due to the high concentrations of ammonia. In the lower section, no changes are detected because the accumulated sludge is still present.