Nitrogen isotopes in macroalgae as a monitor of nutrient loading around island communities


The delivery of human waste products through polluted water has a profound impact on the chemistry of the oceans, especially in coastal waters. A major waste product that is delivered to the ocean is nitrogen-based (e.g., agricultural fertilizers, livestock and human sewage/effluent), which is a nutrient to most marine organisms. Seaweed is very capable of harvesting this nitrogen from the ocean, which often lead to growth expansion and suffocating the environment of chemicals and habitat (e.g., green algae blooms, outcompeting seagrass, natural fish nurseries). Tracking nitrogenous pollution sources, transport and sinks in dynamic coastal waters presents a significant challenge. Stable isotope ratios are an excellent tool to discern, or ascertain, biological, ecological and environmental processes.

Stable nitrogen isotopes in seaweed are controlled by the nitrogen sources in the water it lives. Nutrient loading (nitrogen input) through industrial activities and effluent discharge typically has distinct nitrogen isotope ratios (nitrogen isotope values of c. 0‰ and +15‰ respectively). Hence, seaweed nitrogen isotope signatures can be used as a proxy to determine the type of nitrogen source entering the ocean. This technique has been applied in many countries, but only on small geospatial scales (e.g., Viana & Bode 2015), which allows one to identify and research specific discharge points (e.g., fish farming, Carballeira et al. 2013) — this does not allow large-scale patterns/problems to be recognised and addressed. Traditional techniques for monitoring nitrogen loading are very costly, time consuming and often inconclusive. Also, previous research has dominantly focused on large rivers and estuaries in Europe (Blicher-Mathiesen et al. 2014). In 2017, the UK spent at least £24M monitoring river and estuarine water, but only 4% of this (~£1M) was spent on conducting nitrate water analysis and in 2018 that number was only ~£0.4M per year (House of Commons Environmental Audit Committee 2018, Eleventh Report of Session 2017–19). The CASE partners are aware of issues surrounding their islands and are keen to explore techniques for monitoring. There have also been very few studies looking at seasonal variation in seaweed nitrogen isotope values (Thorsen et al. 2019). Many previous studies have taken a single snapshot of nitrogen isotopes and nutrient loading (typically, in the summer months when it is more conducive for fieldwork), and therefore have not captured the true signature and source of pollution in the region. In addition, the concept of using isoscape maps for monitoring nitrogen isotopes in seaweed and nutrient loading is novel.

This PhD project will focus on the seasonal record of nitrogen isotopes from macroalgae around the coastal environments of Jersey, the Isles of Scilly and Gotland. Nutrient loading hotspots will be identified from the nitrogen isoscape maps generated. CASE partners will be involved to assess the extent and implications of these results against currently government and environmental policy and regulations. Subsequent research will focus on using translocated isotopically-labelled macroalgae to constrain the source of the nitrogen pollution in the key hotspots at these locations. The project will involve extensive fieldwork and laboratory research. The CASE partners will help with training and logistics while doing fieldwork, including collecting samples throughout the year to produce a continuous environmental record of nitrogen isotopes and nitrate concentrations.


The student will learn fieldwork and laboratory techniques associated with seawater and macroalgae analysis related to water quality and environmental analysis. This work will be done in close collaboration with the CASE partners and consist of fieldwork around the UK. Stable isotope analysis is fundamental to this project and the student will become adept at performing stable isotope mass spectrometry and environmental analysis.

Project Timeline

Year 1

Detailed literature review on nitrogen isotopes in macroalgae, marine river systems and coastal environments. Training in stable isotope mass spectrometry. Fieldwork will commence. Communicate and meet the CASE partners.

Year 2

Fieldwork will continue throughout the year. Compilation of nitrogen isotope data from the field sites including water analysis. Growth chamber experiments will be conducted assessing nitrogen uptake with specific reference to the island locations being studied. Spend time with the CASE partners integrating isotopic and water data against the environmental policy and issues at each island setting. Start preparing any manuscripts for submission as well as any reports for CASE partners.

Year 3

Continue fieldwork and data collection. Continue working with CASE partners to produce deliverables. Conduct macroalgae translocation experiments at the island hotspots. Continue writing manuscripts and reports for CASE partners.

Year 3.5

Completion of thesis and manuscripts for publication.

& Skills

This project will provide the student with training in stable isotope mass spectrometry, environmental science, GIS and regional/national environmental policy.

References & further reading

Blicher-Mathiesen et al. 2014, Agriculture, Ecosystems & Environment 195
Carballeira et al. 2013, Journal of Applied Phycology 25
Gröcke et al. 2017, Chemosphere 184
Thorsen et al. 2019, Marine Ecology Progress Series 616
Viana & Bode 2015, Science of the Total Environment 512

Further Information

For further information or an informal discussion, please contact Dr Darren Gröcke (

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