Will they be back? A framework to guide rare macrophyte conservation decisions in lakes

Shallow lake restoration typically focusses on the re-establishment of macrophytes. The likelihood of a species returning to a site is contingent on dispersal, proximity to propagule sources, and the on-site propagule-bank viability. We explore the potential of palaeoecological records in combination with botanical surveys and distribution maps, to ascertain the loss of three submerged macrophytes (Littorella uniflora, Najas flexilis, and Elatine hydropiper) from, respectively, two lakes (Barton Broad, Norfolk and Esthwaite Water, Cumbria) and one lake landscape (Greater Glasgow, Scotland). We discuss re-establishment likelihood when accounting for species' autoecology and current water-chemistry conditions. L. uniflora is widespread in the United Kingdom but absent locally in Norfolk without known seed bank, hence is unlikely to naturally recolonise Barton Broad. Furthermore, current conditions are unsuitable for this species suggesting that nutrient reduction is required prior to translocation. N. flexilis is extinct in Cumbria and the long distances involved (>100 km) for recolonisation of Esthwaite Water suggest that spatial dispersal is unlikely, rendering the seed bank the last chance of natural recovery. Alternatively, translocation may be feasible. E. hydropiper is a nationally scarce species in the United Kingdom yet would have only a short dispersal distance (similar to 10 km) to recolonise Loch Libo, hence there being no requirement for translocation. In exploring the recovery possibilities for the three focal plant species, we develop a time-space integrated framework that can be employed to guide conservation decisions for other species, enabling a more rational use of translocations in the future, in line with international guidelines.