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Penny Johnes is a British environmental scientist and professor of biogeochemistry at the University of Bristol. Her work focuses on the geochemistry of aquatic systems and the effects of food production and environmental change on inland and coastal water quality. She has advised the UK Government on nutrient enrichment in aquatic environments and its ecological consequences. She serves as Chair of the Department for Environment, Food and Rural Affairs Water Expert Advisory Group.

Johnes earned a BSc in Environmental Science at the University of Plymouth in 1986. She completed her doctorate at the University of Oxford, where she studied the effects of land use on water quality.^[1] Her doctoral research, funded by Natural Environment Research Council, examined the effects of agricultural practices on water quality in lowland catchments.^[1] It provided early evidence of the quantitative importance of dissolved organic and particulate nutrient fractions and the role of extreme high‑flow events in transporting particulate‑bound nutrients to water bodies. Johnes was a postdoctoral scientist in the at the University of Liverpool, where she understanding of nitrogen–phosphorus co‑limitation in eutrophic waters.^[2]

She was appointed to the University of Reading in 1993, where she was made a professor in freshwater science in 2002.^[2] She became director of the Aquatic Environments Research Centre and executive director of HYDRA, a research partnership linking hydrosciences researchers at universities across the South East of England. Johnes joined the University of Bristol in 2014.^[1] She has studied the role of dissolved organic matter (DOM)^[3] in biodiversity loss within freshwater ecosystems, the development of integrated management approaches to address multiple environmental stressors, and the use of robust scientific evidence to inform environmental policy.

Johnes has explained the significance of organic and particulate nitrogen and phosphorus in total nutrient loads,^[3] the influence of short-term extreme flow events on nutrient dynamics in soils, wetlands, and freshwater systems, and the importance of defining quasi-homogeneous geoclimatic units (areas of land that share similar environmental and climatic characteristics) for scaling nutrient flux models from local to national levels.^[2] Her work has advanced knowledge of pollutant sources, transport pathways, and biogeochemical processing.^[2]

She has developed advanced analytical and high‑resolution monitoring technologies to examine fine‑scale variability in nutrient fluxes.

• P M Vitousek; R Naylor; T Crews; et al. (1 June 2009). "Agriculture. Nutrient imbalances in agricultural development". Science. 324 (5934): 1519–1520. Bibcode:2009Sci...324.1519V. doi:10.1126/SCIENCE.1170261. ISSN 0036-8075. PMID 19541981. Wikidata Q46460438.
• P.J. Johnes (September 1996). "Evaluation and management of the impact of land use change on the nitrogen and phosphorus load delivered to surface waters: the export coefficient modelling approach". Journal of Hydrology. 183 (3–4): 323–349. doi:10.1016/0022-1694(95)02951-6. ISSN 0022-1694. Wikidata Q58074389.
• Robert Howarth; Dennis Swaney; Gilles Billen; et al. (February 2012). "Nitrogen fluxes from the landscape are controlled by net anthropogenic nitrogen inputs and by climate". Frontiers in Ecology and the Environment. 10 (1): 37–43. doi:10.1890/100178. ISSN 1540-9295. Wikidata Q58074320.