Understanding the natural variability of diatom assemblages in springs of the Adamello-Brenta Nature Park (south-eastern Alps) on a temporal scale

It is thought that long-term monitoring is an essential tool through which conservationists and managers (i) are alerted when the system departs from the natural state, (ii) can check their environmental policy, and (iii) can detect disturbance effects. However, while long-term studies are growing in number, the lack of information on the background rates of natural changes could lead to a biased interpretation of results. In this study we analyzed the diatom composition of yearly samples (14 –16 yrs) in four springs with the following goals: (1) to estimate the consequences of sampling-related processes on the species composition and relative abundance; (2) to determine the form of the assemblages’ variation, evaluating whether the assemblages can be predicted by cyclic, directional, or stochastic changes; (3) to test if, and to what extent, the diatom variability (variation in species composition, relative abundance and diversity) on a temporal scale is driven by the environment; (4) to evaluate the degree of temporal concordance among the diatom assemblages inhabiting four springs. Results showed that because the effect of sampling was inversely proportional to the species abundances, the presence of the least abundant species over the years were mainly a result of chance. The analyses allowed us to discriminate between stochastic and directional patterns, revealing the ongoing changes in two out of four springs. Because the environmental variables did not explain a significant portion of this variability, other hypotheses are put forward. The assemblages’ dynamics of species composition over time were significantly synchronous in two out of six couples of springs, and regardless of the environment. This result can suggest that internal, within springs, drivers may be more important than extrinsic forces operating over regional spatial scales. Overall, these results provide a benchmark of diatom variability over time and in natural conditions delimiting the “limits of acceptable changes”.