Hydroacoustic surveys detect and quantify extensive beds of the invasive charophyte Nitellopsis obtusa in Irish lakes.

Nitellopsis obtusa is a large invasive freshwater macroalgae that forms extensive, monotypic beds which have altered habitat and impacted littoral communities in lakes across North America. N. obtusa is native to Eurasia including the UK but is considered to be cryptogenic and likely introduced in Ireland, where it was first recorded in 2016. Recent evidence of its spread prompted this hydroacoustic survey which investigated the distribution and abundance of N. obtusa in two large interconnected lakes, Loughs Ree and Derg, within the Shannon River system. Using a consumer grade depth finder and a simplistic approach, we propose an accessible method of hydroacoustic surveying for macrophytes such as N. obtusa which form dense, often monotypic beds. Nitellopsis obtusa beds quantified within three bays in Lough Ree covered 254.4 hectares with a total biovolume of ~ 2.7 million m3, indicating that it may be one of the most aggressive macrophytes introduced to Ireland. The hydroacoustic survey methodology proved to be cost and time efficient. Measurements of plant height and lake depth gathered from the hydroacoustic survey were comparable to diver gathered measurements which provides confidence in the methodology. The rapid expansion of N. obtusa within Loughs Ree and Derg is a concern for other alkaline waterbodies in Ireland, especially marl lake systems, and warrants monitoring. There is no feasible method to eliminate the extensive populations of N. obtusa in Lough Ree and therefore management interventions should aim to limit the spread of this species to other waterbodies. [ABSTRACT FROM AUTHOR]