Assessment of the impacts of nutrients on seagrass. Natural England Commissioned Report NECR654

Seagrass habitats (Zostera marina and Z. noltei) are vital for biodiversity, carbon storage, and coastal protection but are in decline across the UK. This paper examines how elevated nutrients, primarily nitrogen and phosphorus, affect seagrass health. Evidence shows many English seagrass beds are in unfavourable condition, with nutrient enrichment often linked to opportunistic macroalgal growth and eutrophication, particularly in Poole Harbour, Chichester Harbour, the Solent, and Essex estuaries. Stable isotope analysis (δ15N) indicates widespread influence of human and livestock-derived nitrogen. A review of the literature found that nitrate and ammonium toxicity can reduce shoot density, growth, and survival of seagrass species, with ammonium effects exacerbated by low light and organic enrichment. Environmental factors such as turbidity, temperature and salinity, and biological interactions such as macroalgae, epiphytes and bioturbation, also amplify nutrient impacts. Nutrient enrichment is a major pressure, acting alone or synergistically with other pressures. To improve conservation outcomes, detailed local monitoring of nutrients, alongside turbidity, temperature, and sediment conditions will be important. Standardised seagrass health metrics and integrated water quality data are needed to set ecologically relevant targets and guide restoration.