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1. Executive summaryThe Somerset Levels and Moors (SLMs) are one of the UK’s most extensive and ecologically significant lowland wetland systems, supporting internationally important bird populations and nationally important assemblages of wetland plants and invertebrates. However, the ecological condition of the SLMs Ramsar and underpinning SSSIs has deteriorated in recent decades, driven primarily by phosphorus (P) enrichment. In 2021, Natural England downgraded the SLMs to “unfavourable declining”, citing persistent eutrophication, floating plant dominance, and loss of submerged macrophytes as key symptoms.This review synthesises current evidence and provides critical overviews for the following themes: (1) The Somerset Levels and Moors (SLMs) Ramsar, (2) The fate and behaviour of legacy phosphorous in wetlands, (3) Options for increasing the rate of nutrient export from wetlands, (4) Factors affecting ecological resilience in the face of high nutrient exposure. The findings are intended to support PR24/PR29 planning by Wessex Water, Natural England, and the Environment Agency, and to inform ongoing academic research.1.1. Nutrient sources and legacy accumulationDecades of agricultural surpluses and wastewater discharges have created exceptionally large stores of legacy P within the SLMs. Substance Flow Analysis (Withers et al., 2023) shows agricultural surpluses of 1.6–6.1 kg P/ha/yr and wastewater discharges of 0.55–0.97 kg P/ha/yr, with agricultural soils now containing approximately 2 tonnes of plant-available surplus-derived P per ha accumulated over 150 years. There are also huge quantities of Total P within the SLMs that are not represented in agricultural surplus calculations. Comber et al. (2023) estimate 41,212 tonnes of Total P stored in the top 20 cm of peat within the West Sedgemoor SSSI catchment. Legacy stores now dominate catchment P dynamics, with legacy P contributing >80% of annual river P flux (Withers et al, 2023).1.2. Processes driving P mobility and ecological declineLegacy P is mobilised through a combination of redox‑driven release from peat, disturbance of P‑rich sediments, storm‑driven runoff, and rapid recycling through floating vegetation. Peat porewaters contain 0.95-4.2 mg P/L, far exceeding ecological thresholds (Comber et al, 2023).iiiSeasonal hydrology amplifies these pressures: winter storms mobilise soil P, while summer stagnation concentrates nutrients and favours Lemna proliferation. These processes underpin widespread hyper‑eutrophic conditions, oxygen depletion, and the loss of submerged macrophyte communities.1.3. Options for increasing nutrient exportReturning the SLMs to favourable condition will require active P export, not just reductions in external inputs. A range of nature‑based and management interventions show promise:•Paludiculture (e.g., Short Rotation Coppice Willow, Typha) can export P through harvested biomass while supporting peat‑forming conditions.•Constructed wetlands and small‑form hydrological features (Robotham et al., 2023) can intercept and retain P before it enters SSSI ditch systems.•Duckweed harvesting can remove meaningful quantities of P during peak growth (Crocker et al., 2023).•Manure export and negative P balances can reduce soil P (Bailey et al., 2015).•Topsoil removal (Zak et al., 2017) is highly effective in targeted locations with severely degraded peat but risks increasing CO2 emissions.•Sediment extraction removes P‑rich ditch sediments but requires careful disposal to avoid re‑mobilisation.No single intervention is sufficient; instead, a portfolio of spatially targeted measures is required.1.4. Ecological resilience under high nutrient exposureResilience in ditch systems depends on breaking eutrophic feedback loops, restoring structural and biological complexity, and stabilising hydrology. High water levels during spring and early summer support submerged macrophytes, while avoiding prolonged stagnation reduces floating plant dominance. Tiered ditch profiles, rotational vegetation management which facilitates succession, and improved throughflow can all enhance resilience, provided they are balanced against other needs such as conveyance. Ditch and wetland management trials for measures such as raised water tables and widened / tiered ditches have been increasingly being implemented across theivSLMs and its protected areas, and as these case studies develop there will be greater clarity on the effectives for such strategies in anticipation of PR29 planning.1.5. Overall implicationsThe SLMs are now a nutrient‑rich, hydrologically modified landscape in which legacy P stores drive ecological decline to a greater degree than current inputs. Nutrient enrichment and floating‑plant dominance were the named parameters in the downgrading of the SSSI and Ramsar areas to ‘Unfavourable – Declining’ by Natural England and the Environment Agency in 2021 (Natural England, 2021).As nutrient enrichment is named parameter responsible for the current condition status of protected areas, meaningful recovery must firstly be centred around the removal and mitigation of bioavailable P, aiming towards CSMG targets. Present findings indicate that, in order to achieve this, reductions in both agricultural and Water Recycling Centre (WRC) inputs will be required, alongside nature‑based P removal or export methods, such as those outlined in Section 4.However, recovery cannot be achieved through nutrient management alone, and wetland and ditch management practices must be implemented which promote ecological resilience. Additional parameters considered under CSM guidelines are essential for favourable condition and should not be ignored; these parameters, detailed in table A1 (Appendix), consider factors such as the stability of water levels, structural complexity of ditch profiles, and diverse submerged and emergent macrophyte communities typical of middle‑succession ditches (JNCC, 2005). Action across the protected areas and the wider SLMs therefore needs to be coordinated, targeted, and considerate of the mechanisms and techniques outlined in this review.

Publication Date

2026-01-01

Funding

Wessex Water ASF funding

Keywords

Somerset Levels and Moors, Ramsar wetlands, legacy phosphorous, wetland management, nature based solutions, nutrient cycling, ecological resilience, eutrophication

Creative Commons License

Creative Commons Attribution-No Derivative Works 4.0 International License
This work is licensed under a Creative Commons Attribution-No Derivative Works 4.0 International License.

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