It is also listed as UK priority BAP species and the current distribution is restricted to less than 100 hectares in the UK and it should therefore be classed as nationally scarce. The main stronghold for the species is in the oligotrophic lakes of central Wales and Cumbria, as well as some canals in Wales and Shropshire. Due to its deep-water habit, L. natans is easily overlooked and although this may have resulted in it being under-recorded in some locations, there is also evidence of it having been lost from some lowland sites in recent years. This purpose of this project was to assess the extent of Luronium natans within two sites in North Wales, Llyn Padarn and Llyn Cwellyn.
Objectives:The objectives of the aquatic plant surveys were to;
The Common Standard Methodology (CSM) surveys used to assess aquatic flora are generally restricted to visual (bathyscope) and grapnel methods conducted from a boat. Where Luronium natans is present, the use of a grapnel is avoided as much as possibl to prevent damage and up-rooting of plants, and therefore detection of the species can prove difficult where it occurs in deep water. Furthermore, the CSM surveys are (usually) restricted to only four 100 m sections located approximately evenly around the lake shore. Large areas of the site are therefore overlooked unless the specific purpose of the survey is to detect rare species. The survey data collected over the past 10 years in both Llyn Padarn and Llyn Cwellyn are not therefore sufficient to confirm the status of Luronium natans in terms of the whole-site distribution or population size.
The use of underwater survey techniques such as scuba or snorkelling gives a much better change of detecting Luronium natans. In the case of the study sites, we know the depth range of aquatic plants to be less than 5 m deep and therefore snorkelling provides the best means of surveying these shallow water populations without the complexities and time restrictions associated with scuba. Relatively clear water in the lakes makes visual survey to 5 m possible without exceeding 3 m dives for the snorkeler and the entire littoral zone can be assessed within a relatively short timeframe.
Snorkel surveys were carried out at both lakes in October 2014. Water temperatures were approximately 15℃ and two PADI qualified divers undertook the snorkelling surveys carried out under NRW Protected Species Licence 59401:OTH:SP:2014. When in the water, the snorkeler had very good vision of the littoral zone to a depth of approximately 2.5 m; a short dive to 2.5-3.0 m was adequate to gain a good view to 5 m. Water depth was measured by the diver at each sample location using a hand-held echo-sounder. A dive was then conducted to determine presence / absence of Luronium natans.
Both lakes have extensive populations of Luronium natans, but there is concern in Llyn Padarn that many plants were in poor condition and that algal growth may be impacting the overall depth range, and hence habitat availability, of the plants. Filamentous algae was also noted as a problem in Llyn Cwellyn, but here plants appeared to be in better condition.
At neither site was there any evidence of floating leaves or any flowering stems. Generally, both lakes are exposed and do not have any extensive areas of emergent vegetation (e.g. Carex rostrata, Phragmites australis etc.) that could offer the protection required by flowering plants of this species. Plants in both sites were restricted to vegetative reproduction.
The use of snorkelling is an excellent survey method for these clear-water sites. It has the advantage over boat-based or wading surveys (which utilise bathyscopes and grapnels) by affording much greater visual range and manoeuvrability of the surveyor whilst in the water. The ability to seek out and locate individual plants and investigate uncertainties is also greatly enhanced by being in the water. Where plants are no deeper than 5 m, this method is also considered be more efficient than scuba surveys, which have a much greater reliance on specialist equipment and limit the time the diver can spend in the water during any one period.
When practised safely by trained surveyors and boat operatives, snorkelling is recommended as the most accurate as well as cost and time efficient method for surveying submerged aquatic plants.