Determining nutrient fluxes and sources driving lowland drinking water reservoir ecosystem response

Abstract

Nutrient enrichment is widely recognised as one of the principal stressors impacting freshwater ecosystems. Managing nutrient sources is vital to restore ecosystem function. However, diffuse water pollution from agriculture presents a challenge in identifying nutrient sources inhibiting
the implementation of targeted management practices. To face this challenge, a comprehensive understanding of the nutrient chemistries involved is needed. Therefore, this study aims to investigate nutrient fluxes to link sources from land to stream to determine key contributing source areas within agricultural landscapes and to identify the origins of nutrients in these areas. Nitrogen speciation, phosphorus fractionation, and dissolved organic carbon dynamics were characterised over 22 months using high-resolution temporal and spatial monitoring in an agricultural headwater catchment. The high-resolution approach captured sharp flux variations, enabling nutrient hotspots to be identified. Key differences in nutrient flux rates depended on catchment characteristics such as land use, geology and hydrology. A faecal lipid biomarker approach using sterols and bile acids combined with high-resolution mass spectrometry allowed the targeted identification of anthropogenic sources impacting the catchment. Riverine suspended sediments and bed sediments were analysed to understand the contributing sources of faecal contamination. Ruminants were identified as the dominant faecal source for all sites, while humans were a secondary source. Variations in catchment characteristics impacted the lipid composition of suspended sediments within watercourses. Bed sediment composition was strongly associated with ruminant indicators, showing the catchment's long history with livestock-related pollution.
This research demonstrates that high-resolution monitoring of the full nitrogen and phosphorus pool provides a comprehensive understanding of nutrient fluxes necessary to locate hotspots, linking sources from land to stream. The complementary pairing of faecal sterols and bile acids
enables accurate identification of specific nutrient sources contaminating freshwaters. This multi-faceted approach is essential to inform management practices to bring nutrient enrichment under control within complicated agricultural landscapes.

Date of Award	18 Jun 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Richard P Evershed (Supervisor) & Penny J Johnes (Supervisor)