Microhabitat level analysis of macroinvertebrate communities helps explaining the impact of different stressors during a catastrophic event in Lambro river

On February 23rd 2010, several thousand tons of fuel oil and other hydrocarbons were poured into the sewerage system of Villasanta (MB), causing the spill of about 3000 m3 of them into the Lambro River and the interruption of the purification activities at the local Wastewater Treatment Plant (WWTP). The effects on the benthic invertebrate communities were analyzed, comparing data collected before and after the accident. The lack of detail of the data collected before the spill and the concurrent presence of various stressors led to the necessity of more detailed analyses, based on a experimental design built over space (upstream and downstream the spill). The ANCOVA and multivariate analysis of the shifts occurred to several diversity, taxonomical and functional metrics in macroinvertebrate assemblages at microhabitat level allowed identifying effects due to the pre-existent impairment, the spill of untreated wastewater, and the oil. The de-oxygenation of water due to the WWTP breakdown affected the taxa richness and caused the disappearance of the most specialized functional groups (e.g. filterers and grazers), especially in riffle areas. The deposition of oil in finer sediments resulted in a considerable reduction of densities in tolerant taxa, thus, paradoxically, increasing Shannon Index values in samples collected downstream the spill. Overall, the greatest short-term damage to invertebrate biodiversity seemed to be caused by the spill of untreated wastewater.