The quagga mussel (Dreissena bugensis) south of the Alps: an ongoing biological invasion in Lake Garda

Abstract Over the past century, Lake Garda has been increasingly affected by the introduction of several non-indigenous species. In early 2022, the identification of Dreissena bugensis (quagga mussel) marked the first record of a naturalized population in lakes south of the Alps. To investigate the colonization patterns of this invader and its potential interaction with the established invader D. polymorpha (zebra mussel), a comprehensive field survey was conducted in 2022 across twelve stations, encompassing littoral, sublittoral, and profundal zones, during both spring and autumn. By spring 2022, quagga mussels had colonized the littoral and sublittoral zones of nearly all stations, with higher density in the northern part of the lake and a growth rate dependent on depth. Between spring and autumn, their population continued to grow, surpassing zebra mussels at all stations and expanding into the profundal zone as well. Analysis of population structure and veliger monitoring suggest that quagga mussels likely arrived in the lake in 2019. The evidence of D. bugensis already being firmly established in Lake Garda, as well as in nearby Lake Ledro, highlights the need for biosecurity protocols to prevent further invasions and the spread of established invaders to other Italian lakes.