Cattle-grazing effects on diatom assemblages and physical and chemical parameters in peat cores from Alpine mires

Besides being the most important terrestrial carbon sink, peatlands are valuable ecosystems for biodiversity conservation. In Alpine peatlands overgrazing is a main problem for habitat integrity and biodiversity due to increasing nutrient (N) inputs. Seven 50-cm deep cores were sampled from several mires located in the Adamello-Brenta Nature Park (Italy) along a grazing-induced disturbance gradient. Main physical and chemical features of peat were determined, and taxonomy and ecological characterization of subfossil diatom algae carried out. Diatoms were identified and counted (400 valves) at x1000 magnification. The absolute abundances (n-valves/g-peat-dw) were also calculated. Exploratory data analysis was done with Non-Metric Multidimensional scaling. To test for possible differences among the grazing gradient, a Permutational Multivariate Analysis of Variance was used. Grazing-gradient indicator species were identified with IndVal. Data underlined that grazing influence was limited to the top 20 cm, and resulted in an increasing of density (up to 2×), N concentration (up to 3×) and a lower gravimetric water content (up to 50%). No significant differences were observed below 30 cm of depth. In total, 190 diatom taxa were recorded throughout the 7 cores, and several of them are included in threat categories of the Red List for central Europe (e.g., Cymbopleura valaiseana, Eunotia hexaglyphis, E. triodon). Data also showed that diatom tafocoenoses respond to cattle-grazing-driven shifts in trophic status. In particular, high intensity grazed sites had higher species richness, because of moderately-sensitive, opportunistic mire species, that can become more competitive in the presence of increased nutrients.