The realised niche of the invasive charophyte Nitellopsis obtusa in Ireland and its impact on macrophyte community structure

Nitellopsis obtusa is a charophyte native to Eurasia which has become invasive within North American lakes in recent decades. It is not considered native to Ireland where it was first recorded in the Shannon River system in 2016. Recent surveys showed N. obtusa occupying large areas of Lough Ree and suggested that its distribution in Lough Derg is expanding. Thus it has been deemed invasive, although there is still much uncertainty about its distribution, habitat preferences and impacts on native macrophytes in an Irish context, which this thesis aimed to investigate. Snorkel surveys found that N. obtusa was present across 11 of 32 transects in Lough Ree and 21 of 32 transects in Lough Derg. Boosted regression tree models showed that depth was the most important predictor of N. obtusa presence with it occupying a deep water niche from 2 to 5 m depth. Few species had strong positive associations with N. obtusa due to its dense growth form and heights which exceeded 2 m. It was almost always the deepest growing macrophyte along transects where it was present, and accounted for up to 81.3% of total macrophyte abundance along individual transects, and so its invasion represents a shift from angiosperm to charophyte dominance within the deeper water of the eutrophicated lakes studied. While native macrophyte richness decreased as N. obtusa cover increased on a localised scale, on a larger scale transects with N. obtusa did not have a significantly lower abundance or diversity of native macrophyte species than those where it was absent. N. obtusa beds differ structurally from the angiosperm beds common within deeper water of the invaded lakes, suggesting that its expansion could be impacting fish and invertebrate habitat and the wider biogeochemical cycling of the lakes. Its expansion coincides with increasing water clarity within the lakes, and previous studies on the effects of charophyte proliferation on clear water states suggests that N. obtusa could be partially driving these increases, but shallow lakes are complex systems and further research is required to fully understand its impacts. N. obtusa is very likely to continue to spread in Ireland which has many calcareous eutrophicated waterbodies suitable for N. obtusa invasion. Pondweed lakes and degraded marl lakes with alkalinity over 100 mg L-1 CaCO3 and total phosphorus concentrations exceeding 0.15 mg per L-1 are at the highest risk of invasion. Attempts at controlling current populations are unlikely to be successful and management interventions should instead focus on raising awareness and preventing its spread.