Written evidence submission from Professor Jeremy Biggs (NIT0016)

Prof Jeremy Biggs - Freshwater Habitats Trust - supplementary written evidence for House of Lords Environment and Climate Change Committee Nitrogen Enquiry

Importance of small waters as refugia for nutrient-sensitive species

• Small freshwater habitats (ponds, small wetlands, headwater streams) generally  have correspondingly small catchments and, as a result they are more likely to be protected from nutrient pollution via surface runoff than larger waters.1 Surface runoff is generally considered the largest source of nitrogen pollution in freshwaters, although in nutrient-poor upland systems atmospheric deposition can be a significant source15.
• As a consequence of protection from nutrient (nitrogen and phosphorus) pollution, ponds and other small waters tend to collectively support more biodiversity across a landscape than large rivers and lakes – and are particularly important as refugia for nutrient-sensitive species.2,3 This is also helped by the fact that small waters are more varied one from another than larger waters. This is because they can reflect very local influences in geology, surrounding landuse, shading by trees, the effects of grazing animals and so on – these very local influences are simply averaged out in bigger rivers and lakes. For example, in the generally alkaline and naturally nutrient-rich south-east of England there are hardly any acid big rivers or lakes but there are hundred – perhaps thousands – of acid small streams and ponds fed by small areas of heathy and acid soils.
• Because large rivers and lakes have large catchments, they tend to be affected by surface-water pollution from many sources, which are extremely difficult to satisfactorily mitigate.4,5 Attempts to restore large rivers through channel physical restoration (which has attracted the name ‘rewiggling’) rarely deliver significant biodiversity benefits, probably because of the overriding effects of poor water quality.6
• By contrast, small waters (with small catchments) can be created or restored in pockets of unpolluted land to quickly ‘add clean water’ to a landscape, with significant benefits for freshwater and terrestrial biodiversity.7–11
• The best example is from the Water Friendly Farming project – a joint venture of the Environment Agency, Game & Wildlife Conservation Trust, University of York and Freshwater Habitats Trust – where the creation of 20 clean water ponds across a 10 km2 area of Leicestershire farmland – doubling pond density in the area - delivered a sustained increase in wetland plant species richness across the entire landscape, with annual censuses showing a 16% increase in wetland plant diversity, which has been consistent for 13 years (ongoing). During the same period, the richness of regionally rare wetland plants increased by 80% across the project area.8,12

Regulatory context

• As currently implemented, the Water Framework Directive excludes most smaller lakes (area < 50 hectares), virtually all c.500,000 ponds in Britain and headwater streams (catchment < 10 km2) from monitoring and management.
• Small waters have resultingly received considerably less investment than large ones. It would cost approximately £1.6 billion to double the number of ponds in England (from 250,000 to 500,000), which would drastically reduce the impacts of nitrogen pollution on freshwater habitats. We estimate that this would increase populations of half of all freshwater priority species, and raise the abundance of wetland plants across England’s landscapes by about 40%.13 By contrast, over the next 5 years, £12 billion is being spent by the water sector on reducing spills from storm overflows in England and Wales – which will deliver only modest improvements in nutrient status, to a relatively small proportion of the river network.4

Interactions between climate warming and eutrophication in freshwaters

• Interactions between the effects of eutrophication and climate warming on freshwaters are highly heterogeneous, but generally effects appear to be additive or multiplicative, with antagonistic effects, where one effect cancels out another, are less common. Obviously, this is not good.

Warming can intensify eutrophication in several ways:

• Higher net precipitation and more intense precipitation events cause increased surface runoff, resulting in increased nutrient loadings.
• Warming can enhance microbial nutrient cycling and stratification in standing waters. Resulting anoxia in the ‘hypolimnion’ (below thermocline) increases release of phosphorus and nitrogen stored in pond or lakes sediments.
• Warming-induced changes in the composition of waterside and aquatic plants and animals can also affect nutrient cycling in different and unexpected ways. Eutrophication may also increase emissions of carbon dioxide and methane from freshwaters.14

View on the current revision of the national emissions ceilings under the Gothenburg Protocol of the UNECE Air Convention

• The Gothenburg protocol is a key mechanism for tackling atmospheric nitrogen pollution, and we’ve seen considerable progress in reducing emissions of nitrogen oxides and ammonia. However, atmospheric deposition of nitrogen remains a pressure on freshwaters – particularly upland and other naturally nutrient-poor systems (for example, those in lowland heaths). Now that the statutory provisions mandating preparation/implementation of the National Air Pollution Control Programme have been revoked, the government should set new statutory targets and measures to ensure future progress.

References

1.              Davies, B. R., Biggs, J., Williams, P. J., Lee, J. T. & Thompson, S. A comparison of the catchment sizes of rivers, streams, ponds, ditches and lakes: implications for protecting aquatic biodiversity in an agricultural landscape. in Pond Conservation in Europe (eds. Oertli, B. et al.) 7–17 (Springer Netherlands, Dordrecht, 2010). doi:10.1007/978-90-481-9088-1_2.

2.              Williams, P. et al. Comparative biodiversity of rivers, streams, ditches and ponds in an agricultural landscape in Southern England. Biological Conservation 115, 329–341 (2004).

3.              Davies, B. et al. Comparative biodiversity of aquatic habitats in the European agricultural landscape. Agriculture, Ecosystems & Environment 125, 1–8 (2008).

4.              Environment Agency. WFD RBMP2 Reasons for Not Achieving Good Status. (2016).

5.              Natural England. Catchment Sensitive Farming Evaluation Report – Water Quality Phases 1 to 4 (2006-2018). https://publications.naturalengland.org.uk/publication/4538826523672576 (2019).

6.              Palmer, M. A., Menninger, H. L. & Bernhardt, E. River restoration, habitat heterogeneity and biodiversity: a failure of theory or practice? Freshwater Biology 55, 205–222 (2010).

7.              Rannap, R., Soomets-Alver, E., Vries, W. & Briggs, L. Large-scale habitat restoration—a successful tool halting the decline of threatened amphibians. in Strategies for conservation success in herpetology 275–283 (2024).

8.              Williams, P. et al. Nature based measures increase freshwater biodiversity in agricultural catchments. Biological Conservation 244, 108515 (2020).

9.              Walton, R. E., Sayer, C. D., Bennion, H. & Axmacher, J. C. Improving the pollinator pantry: Restoration and management of open farmland ponds enhances the complexity of plant-pollinator networks. Agriculture, Ecosystems & Environment 320, 107611 (2021).

10.              Lewis-Phillips, J. et al. Ponds as insect chimneys: Restoring overgrown farmland ponds benefits birds through elevated productivity of emerging aquatic insects. Biological Conservation 241, 108253 (2020).

11.              Biggs, J., von Fumetti, S. & Kelly-Quinn, M. The importance of small waterbodies for biodiversity and ecosystem services: implications for policy makers. Hydrobiologia 793, 3–39 (2017).

12.              Williams, P. Pitsford Water Friendly Farming: Wetland Plant Survey Results 2021-2024. (2024).

13.              Wildlife and Countryside Link. The Charter for Small Waters. https://www.wcl.org.uk/docs/WCL_Small_Waters_Charter_2024.pdf (2024).

14.              Meerhoff, M. et al. Feedback between climate change and eutrophication: revisiting the allied attack concept and how to strike back. Inland Waters 12, 187–204 (2022).

15. https://uk-air.defra.gov.uk/assets/documents/reports/cat13/0802201401_freshwater_umbrella_final_report_2004_2007.pdf

07/03/2025