Understanding the habitat and decline of Najas flexilis (Willd.) Rostk. & Schmidt in the UK using ecology and paleoecology

Najas flexilis is a rare macrophyte, protected under international legislation. This study aims to understand its contemporary decline in the UK, using a combination of spatial analyses and paleoecological reconstructions. Using a specially developed snorkel survey protocol, data on N. flexilis abundance, aquatic plant abundance, water depth, sediment characteristics, and basic water chemistry parameters were collected at 23 sites in Scotland. In combination with existing data, this showed that N. flexilis grew in different habitats with different vegetative communities in different lakes. Because N. flexilis relies on carbon dioxide for photosynthesis, alkalinity and competition from bicarbonate-utilising plants were influential variables. In acid-circumneutral sites, N. flexilis was found in low abundances, likely limited by acidity. In mildly alkaline, clear-water lakes, N. flexilis grew in high abundances. In more base-rich sites, N. flexilis was only found in the deepest reaches of the photic zone, below the minimum light tolerance levels of other plants. One significant competitor with N. flexilis was the invasive species Elodea canadensis, which can tolerate low light conditions and uses bicarbonate for photosynthesis. Analyses suggested that N. flexilis is well represented in sediment cores. Historical records and macrofossil analyses suggest that N. flexilis has declined during the Anthropocene. Maximum abundances of N. flexilis macrofossils were associated with indicators of circumneutral-alkaline lakes with clear water. The decline of N. flexilis at Esthwaite Water aligned with the eutrophication of the site. At Loch of Craiglush, a more acid-circumneutral site, there was no evidence for nutrient enrichment within the lake, but all hydrologically connected sites had suffered from mild eutrophication. The loss of N. flexilis here also coincided with the expansion of E. canadensis. Upon consideration of both the ecological and paleoecological investigations, it is concluded that the biggest threats to N. flexilis in the UK are eutrophication and invasive species invasion