Environmental factors controlling the presence and distribution of Achnanthidium delmontii in Mediterranean rivers (NW-Italy)

This study aims at investigating the environmental factors that drive the presence and proliferation of the invasive diatom Achnanthidium delmontii in some Mediterranean rivers of Liguria (NW-Italy). Achnanthidium delmontii was detected for the first time in Italy in 2013 and has rapidly spread throughout Liguria since then, becoming the dominant taxon in several sites. In this paper, we analysed chemical data and hydromorphological features to define the ecological preferences of this species. Through a statistical model we showed that the presence of A. delmontii was strongly influenced by its occurrence in upper sections of the watershed or previous sampling campaigns; once established in a certain site the species remains. Moreover, its abundance was significantly related to high percentage of agricultural surrounding land use, alkaline pH and medium-high water temperatures. For the first time, we also observed that the abundance of A. delmontii is positively correlated with hydrological disturbance. Indeed, extreme water scarcity events during summer, the presence of dams along the watercourse, and significant water abstractions, are positively correlated with its proliferation. Conversely, the presence of A. delmontii seems to be negatively affected by hydropeaking events. Our results proved that the conservation of a good habitat integrity and heterogeneity, coupled with the maintenance of a natural flow, represent important defence strategies against the establishment and proliferation of this invasive diatom.