Isotopically‐labelled macroalgae: a new method for determining sources of excess nitrogen pollution.

RATIONALE: Stable nitrogen isotope ratios (d15N values) can be used to discern sources of excess nitrogen pollution in water. The d15N values of nitrate in water often do not reflect the true d15N source owing to high temporal variation, and analytical costs associated with obtaining δ15N values from water nitrate are high. To find alternative solutions we isotopically‐labelled macroalgae (i.e., seaweed) beyond natural variation as a new method for determining sources of excess nitrogen pollution in seawater. METHODS: Fucus vesiculosus (bladder wrack) non‐fertile tips were collected from Easington Colliery, County Durham, and cultured in two isotopically‐enriched solutions containing ammonium sulphate with d15N values of 170 ± 5 ‰ and –60 ± 3 ‰ for a period of 19 days. The macroalgae were cultured in separate opened glass jars in an incubator with set temperature (11°C) and light (125 μmol photons m‐2s‐2 on a light/dark rhythm of 16/8 hours). The oven‐dried tips were analysed for d15N over the 19‐day experiment. RESULTS: The macroalgae tips incorporated the isotopically‐enriched solutions rapidly, reaching 50% of the isotopically‐enriched seawater after ~11 days for the 15N solution and ~15 days for the 14N solution. d15N values were incorporated more into the torn base of the macroalgae tips than into the middle and apex region. CONCLUSIONS: F. vesiculosus rapidly incorporates the isotopic ratio of the artificial seawater solution to which it is translocated. The lab‐developed isotopically‐labelled macroalgae can be manufactured to generate “un‐natural” d15N values for translocation into coastal environments. This approach can provide an efficient, low‐cost alternative to current analytical methods to determine and monitor nitrogen pollution