Long term changes in North Adriatic soft-bottom polychaete assemblages following a dystrophic crisis

In the Northern Adriatic Sea, the combination of natural and anthropogenic processes causes periodic hypoxia and anoxia, which can cause mass mortality of benthic organisms. Here, we report the results of a 20-year monitoring programme carried out after an anoxic event in 1989 that caused benthic mass mortality over an area of about 1200 km2. The recovery dynamics of the polychaete fauna was followed at three stations (including one sampled a year before the anoxic event). Samples (0.1 m2 Van Veen grab, 2-mm sieve) were collected from 1989 to 1994 and from 2003 to 2008. A total of 6519 specimens belonging to 186 species were identified. The analyses of univariate biodiversity indexes highlighted higher temporal variation of assemblage diversity in the period following the dystrophic crisis, followed by higher stability in the next decade. PERMANOVA suggested that the highest component of assemblage variation belonged to the factor year. However, differences across years varied in magnitude, depending on stations and were portrayed by non-metric multidimensional scaling plots of each year’s centroids for each of the three stations. SIMPER analysis identified the species most characterizing the assemblages in each year for each of the three stations. Our results highlight the importance of analysing long-term data sets in order to understand assemblage dynamics following strong disturbance events. Assuming the 1989 anoxia as the event determining the biodiversity change, and considering our outcomes, which suggest that the assemblages still show a pattern of non-random changes through years, our data indicate that the polychaete fauna is still recovering without having reached a pattern of among-years stability. Focusing on the biology of the species characterizing the different periods might help in understanding the ecological processes that have determined the observed pattern.