SLIMY-FRUITED STONEWORT AND TASSEL STONEWORT IN SUFFOLK PONDS JULIET HAWKINS
Monitoring of newly restored farm ponds in several Suffolk parishes resulted in the recent discovery of two rare spring-fruiting stoneworts in 2019 (Hawkins 2019):
• The Slimy-Fruited Stonewort Nitella capillaris thought to have been Extinct until re-discovered in ponds in North-East Suffolk in 2019, and
• The Tassel Stonewort Tolypella intricata, with an Endangered status, recorded in ponds in North-East Suffolk in 2019 and previously known in two Suffolk sites; found scattered in various southern counties.
As a follow-up to two years of monitoring to find out more about the stoneworts’ persistence, I carried out further studies in 2023, funded by Natural England’s Species Recovery Project:
1. Third year monitoring the persistence of N. capillaris (Nc) and T. intricata (Ti) in the 12 ponds where one or both species were recorded (Hawkins 2023a).
2. Surveying for Nc and Ti in 53 ancient farm ponds and 14 ghost ponds (previously infilled) mostly restored in 2022, and a few more restored in 2021, to find additional sites and get a better understanding of their ecology (Hawkins 2023b).
The key findings are that:
• One new site for Nc, the largest yet, was discovered in a pond on Great Common in Ilketshall St Andrew 1.2km away from a very small, seasonally dry, restored ghost pond, that was known to host both species (see Fig. 1)).
• In 2023 Nc and Ti were persisting abundantly in only one out of 12 ponds where they had previously been found – a ghost pond in grassland in Ilketshall St Andrew. Whilst it was not surveyed in spring 2019, following its early November 2018 restoration, it is assumed that the stoneworts fruited the year following restoration, irrespective of a cattle disturbance incident in summer 2020 and, as such, it is the only pond where both species are apparent, fruiting, in their fourth
Figure 1. This small, shallow, ghost pond in Ilketshall St Andrew, which dries out annually by July, supports both the Slimy-Fruited Stonewort and the Tassel Stonewort in good numbers. Both species have persisted for four years following restoration in 2018, the longest period known for any of the 12 ponds supporting one or other of the species.
consecutive year following restoration. This 2023 study has shown that neither species have persisted in a fruiting state beyond two or three years elsewhere, although both may sporadically fruit undetected in ponds that have significant disturbance such as cattle poaching.
Some new conclusions were drawn and management recommendations updated in the light of these pond restoration observations.
The known distribution of Nc in Suffolk (and the UK) is still confined to seven ponds in three parishes within 15km of each other in North-East Suffolk.
Both stoneworts appear to thrive in water depths of 10-50cm in areas that are newly disturbed from restoration or livestock (Ti has been found in water deeper than this). This limits them to the bare substrate of small areas of most Suffolk farm ponds, or large shallow ponds. Many restorations do not re-create the shallow margins of ancient ponds where these stoneworts may have thrived. However, we do not know if these stoneworts were more widespread in the past or have always been rare.
Timing of pond restoration may be critical. Nc and Ti have both been recorded in ponds that were restored or disturbed between August and early November which
may give these autumn-germinating species a head-start over more competitive spring-germinating stoneworts such Chara vulgaris. So far, neither stonewort has been found in ponds cleaned out between mid-November and March. There is no data for April/May restorations.
Water depth in the same year following restoration may be significant. If a pond is restored in what may be the optimum timeframe for Nc and Ti to germinate, but if the subsequent water depth is not optimum, then the stoneworts may ‘miss’ their critical window of opportunity to germinate and get a head-start over Chara stonewort species or other macrophytes the following year.
Both stoneworts appear to thrive in ponds which dry out annually or occasionally, which tend to be inherently shallower. Drying out may lead to the cracking of the clay substrate; and a concentration of poaching and browsing of competitive macrophytes by livestock as the water recedes - thus restricting other competitive macrophytes or stoneworts such as C. vulgaris
Most ponds with Nc and/or Ti have no, or low levels of filamentous green algae. If the presence of the green algae is taken as a proxy for high nutrient levels, it implies these rare stoneworts may be less able to compete in these situations. Thus, ponds receiving agricultural inflow/runoff are likely to be unsuitable - a large proportion of ponds in an intensively farmed county like Suffolk will have unfavourable conditions. Ponds at the top of a slope, off-ditch, or surrounded by semi-natural habitat such as ancient grassland or woodland, are more likely to support Nc and Ti. This too has implications for the choice of ponds to target for restoration projects.
The intensity and timing of shade may be important. Both species appear to grow in places where there is some tree or shrub shade, or nearby low-level shade from Juncus, for some or all of the day. Both species appear to tolerate, or thrive with, some localised shading by floating leaves of Glyceria fluitans, Ranunculus aquatilis agg. and Callitriche agg. However, it would appear that where species such as Callitriche densely grow in the entire water column, they out-compete both stoneworts. Elsewhere, Nc has appeared to tolerate three years growing amongst Mentha and Typha where they are in partial shade for some of the day. A newly discovered site shows Nc growing in open water, where Typha reaches very dense levels and is periodically removed (Hawkins 2023a & b).
Both stoneworts seem to fruit for 1-4 years following a major disturbance/ restoration in situations where the ponds get no ongoing disturbance from livestock. Summer disturbance of the pond by livestock, may ensure Nc and Ti can fruit sporadically at low population levels. 6/9 ancient ponds where Nc and/or Ti have been found in the North-East cluster are adjacent to a road, bridleway or footpath where livestock may have been watering and moving oospores around between ponds on their hooves. Patch-scraping, whereby 10-30% of aquatic and emergent vegetation is removed together with accumulated organic matter, may be a practical solution to mimic livestock disturbance where livestock do not have access to pond margins.
Pond restoration and grant aid
Recent restoration dates are hugely influenced by grant criteria and avoidance of harm to protected Great Crested Newts (GCNs) and not by when it is most practical to clean a pond i.e., when it is driest or traditional restoration dates. If there is a chance GCNs could be present, best practice to avoid harming them, is to wait until temperatures drop to 5°C at night (usually after mid-November). By concentrating restoration in the winter months, conservation projects may be inadvertently restricting the opportunity for the rare stoneworts to germinate at their optimum time (before mid-November).
When targeting ponds for restoration, especially with funding targeted at scarce species, priority should be given to sites which: are not vulnerable to agricultural runoff; are not vulnerable to scrubbing over and leaf fall; retain a high proportion of shallow water <50cm deep from April to June; and which have livestock disturbing the shallow margins or owners committed to ongoing management that mimics livestock poaching at the margins.
Thorough research of old maps and documents should be carried out prior to restoration to establish likely origins which may provide valuable pointers to where gentle access gradients of approximately 1:10 exist or could be re-created. Study of drainage maps will inform contractors of existing or past drainage schemes which may need breaking up to ensure dry/ghost ponds will fill and hold water but not continue to take agricultural pollutants. Baseline aquatic plant surveys, and follow-up monitoring the year after, should be encouraged or mandatory for every proposed pond restoration, especially grant-funded projects, to improve understanding of the ecology of rare stoneworts.
Areas for research
These two studies have thrown up many questions and areas for research. Management advice arising from this ongoing study in Suffolk has been based on the premise that periodic fruiting of the stoneworts in response to disturbance is required to ensure a reservoir/seedbank of long-lived oospores. Disturbance could be small scale or prolonged such as may be created by livestock or wild animals watering in a pond, or through pond restoration and patch-scraping by machine. However, these stoneworts may have evolved to spend a lot of time as oospores buried in protective organic matter. If so, might increased germination weaken the overall population? Once a pond has been ‘restored’ resulting in a flush of fruiting Nc or Ti, are they more vulnerable without increased levels of buffering? Buried under layers of deep organic matter in neglected ponds, the oospores may have been relatively protected from modern artificial fertiliser and herbicides, but once ‘exposed’ the stoneworts might become more vulnerable to agri-pollution (Hawkins, 2019; 2023a & b).
If ponds are restored, and organic matter containing oospores is removed at the ‘wrong’ time of year, with resulting dominance by other Chara species and macrophytes, will this weaken any remnant population’s resilience to fruit again if the substrate is subsequently disturbed at the optimum time?
Early monitoring of more ponds that have been restored (or patch-scraped), and ponds where livestock have access, would help us understand whether these stoneworts are really rare or simply under-recorded. Perhaps early monitoring for rare stoneworts after pond restoration could be mandatory for schemes that receive government grant aid and advisory for those involving District Level Licensing mitigation for Great Crested Newts.
Future research might also cover:
• Autumn surveys following restoration earlier in the same year as we know that the rare stoneworts germinate in the autumn
• Timing of pond restoration/management – Is August to early November best?
- Do Nc and Ti need to germinate before being swamped by other species?
• Are water level and water temperature critical for germination?
• What are the effects of substrate disturbance by livestock, wild animals or drought?
• How tolerant of shade are Nc and Ti?
• How do nutrient levels and agri-pollution affect the stoneworts?
• What can pollen analysis of pond organic matter tell us about distribution of these stoneworts?
Ongoing survey and monitoring are recommended and should be prioritised for:
• All known sites for Nc and Ti
• Clean water sites where there is considerable annual disturbance/livestock browsing
• Newly restored pond sites in the year following restoration, especially those near to ponds known to host Nc and Ti
• Ponds that have been restored/patch-scraped between August-early November
References
Hawkins, J. (2019). The discovery and conservation of rare stoneworts in Suffolk’s farmland ponds 2019. Trans. Suffolk Nat. Soc. 55: 109-125.
Hawkins, J. (2023a). Monitoring the persistence of Nitella capillaris and Tolypella intricata in Suffolk ponds 2023. Report for Species Recovery Project, Natural England.
Hawkins, J. (2023b). Surveying for Nitella capillaris and Tolypella intricata in recently restored ponds in Suffolk, 2023. Report for Species Recovery Project, Natural England.
Juliet Hawkins
Milden Hall
Milden
Brent Eeligh
Suffolk CO10 9NY
gfhawkins@btconnect.com