Nitrogen isotopes in macroalgae as a forensic tool to determine nitrogen pollution to the oceans

Overview

Tracking nitrogenous pollution sources, transport and sinks in dynamic coastal and estuarine waters presents a significant challenge, conventionally requiring significant water sample collection and understanding of complex flow patterns. Stable isotope ratios are an excellent tool to discern, or ascertain, biological, ecological and environmental processes. The modern nitrogen cycle has been heavily influenced by human activity. Waste products, such as sewage and fish farm effluent, are normally more enriched in 15N than seawater (Vizzini and Mazzola, 2004), whereas agricultural waste products are normally more depleted in 15N (Heaton, 1986). This has led to the application of using nitrogen isotope ratios of marine sediments, marine organisms and macroalgae to monitor nitrogen pollution/contamination (e.g., Savage 2005). Nitrogen isotope ratios can also be measured in dissolved inorganic nitrogen isotopes taken directly from the water (Deutsch et al. 2006; Korth et al. 2014). However, dissolved inorganic nitrogen isotopes is analytically more time-consuming and costly. To address this difficulty, nitrogen isotope ratios in macroalgal tissues have been utilized to discern sources of excess nutrients (Costanzo et al. 2001, 2005; Dailer et al. 2012). Gröcke et al. (2017) indicate that the translocation of macroalgae with isotopically distinct signatures could be used as a rapid, cost-efficient method for nitrogen biomonitoring in estuarine environments.

Coastal and estuarine eutrophication is a widespread problem across the United Kingdom and often has multiple sources. The Environment Agency use macroalgae extent and biomass as an indicator of eutrophication; there are quite a number of UK estuaries that fail to meet Water Framework Directive standards due to excessive macroalgae growth. Macroalgae blooms have numerous deleterious effects on coastal environments; for example, smothering delicate intertidal seagrass habitats and saltmarsh environments causing bleaching and plant die back, which ultimately leads to coastal erosion. However, understanding the sources of nutrients contributing to the problem is extremely difficult and time consuming, especially when nutrient inputs are entering dynamic coastal ecosystems. Our method allows for an innovative, cost-effective tool to determine the nutrient source and therefore direct efforts and put measures in place to reduce specific nutrient inputs, and environmental improvements.

This studentship will generate a modern database of nitrogen isotopes in macroalgae from coastal marine environments in the UK. Specific estuaries and coastal regions will be assessed annually to determine and understand seasonal variability. Laboratory studies will be employed to make isotopically-labelled macroalgae that will be subsequently employed in nitrogen-sensitive locations being investigated by the Environment Agency and Natural England. These investigations will be used to pin-point nitrogen sources and develop local plans and procedures for remediating /reducing the nitrogen loading.

Methodology

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 Environment Agency and consist of fieldwork around the UK. Stable isotope analysis is fundamental to this project and the student will become adept at operating a stable isotope mass spectrometer by the end of the PhD.

Project Timeline

Year 1

Year 1: Fieldwork will be undertaken in the first 4 months of the studentship in order to generate a modern collection of macroalgae from around the UK coast during the winter months. This will consist of multiple fieldtrips that are relatively short in duration (e.g. 1–2 weeks). The macroalgae will be analysed for stable isotope ratios in the Stable Isotope Biogeochemistry Laboratory at Durham University.

Year 2

Year 2: Experimental studies will involve the collection and incubation of macroalgae growing tips in different environmental conditions and seawater compositions. In addition, several locations will be selected with the CASE partner to monitor nitrogen concentrations and isotopic compositions of seawater and macroalgae to determine seasonal variability. During this year, the sites of specific interest will be selected for environmental assessment using the translocation of isotopically-labelled macroalgae.

Year 3

Year 3: CASE partner locations will continue to be monitored using macroalgae present at the location and translocated macroalgae that is isotopically produced in the laboratory. Publications, reports and presentations will be finalised during this year in order to complete the studentship by the end of year three.

Year 3.5

If required, the student will be finishing off papers and their PhD thesis during this period.

Training
& Skills

Stable isotope analysis, stable isotope mass spectrometry, seawater analysis, inductively coupled plasma mass spectrometry, environmental growth chambers, analytical chemistry.

References & further reading

Costanzo, S.D., O’Donohue, M.J., Dennison, W.C., Loneragan, N.R., Thomas, M., 2001. A new approach for detecting and mapping sewage impacts. Marine Pollution Bulletin 42:149–156.
Costanzo, S.D., Udy, J., Longstaff, B., Jones, A., 2005. Using nitrogen stable isotoperatios (15N) of macroalgae to determine the effectiveness of sewage upgrades: changes in the extent of sewage plumes over four years in Moreton Bay, Australia. Marine Pollution Bulletin 51:212–217.
Dailer, M.L., Smith, J.E., Smith, C.M., 2012. Responses of bloom forming and non- bloom forming macroalgae to nutrient enrichment in Hawai‘i, USA. Harmful Algae 17:111–125.
Deutsch, B., Mewes, M., Liskow, I., Voss, M., 2006. Quantification of diffuse nitrate inputs to a small river system using stable isotopes of oxygen and nitrogen in nitrate. Organic Geochemistry 37:1333–1342.
Gröcke, D.R., Racionero Gómez, B., Marschalek, J.W., Greenwell, H.C. (2017). Translocation of isotopically distinct macroalgae: a route to low-cost biomonitoring? Chemosphere 184:1175–1185.
Heaton, T.H.E., 1986. Isotopic studies of nitrogen pollution in the hydrosphere and atmosphere: a review. Chemical Geology 59:87–102.
Korth, F., Deutsch, B., Frey, C., Moros, C., Voss, M., 2014. Nitrate source identification in the Baltic Sea using its isotopic ratios in combination with a Bayesian isotope mixing model. Biogeosciences 11:4913–4924.
Savage, C., 2005. Tracing the influence of sewage nitrogen in a coastal ecosystem using stable nitrogen isotopes. Ambio 34:145–150.
Vizzini, S., Mazzola, A., 2004. Stable isotope evidence for the environmental impact of a land-based fish farm in the western Mediterranean. Marine Pollution 49:61–70.

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