Isotopically labelled macroalgae: A new method for determining sources of excess nitrogen pollution

Rationale Stable nitrogen isotope ratios (δ 15 N) can be used to discern sources of excess nitrogen pollution in water. The δ 15 N values of nitrate in water often do not reflect the true δ 15 N source value owing to high temporal variation, and there are high analytical costs associated with obtaining δ 15 N values from water nitrate. 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, UK, and cultured in two isotopically enriched solutions containing ammonium sulphate with δ 15 N 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 −2 s −2 on a light/dark rhythm of 16 h/8 h). The oven‐dried tips were analysed for δ 15 N over the 19‐day experiment. Results The macroalgal tips incorporated the isotopically enriched solutions rapidly, reaching 50% of the isotopically enriched seawater after ca 11 days for the 15 N‐enriched solution and ca 15 days for the 14 N‐enriched solution. δ 15 N values were incorporated more into the torn base of the macroalgal tips than into the middle and apex regions. Conclusions F. vesiculosus rapidly incorporates the isotopic ratio of the artificial seawater solution to which it is translocated. The laboratory‐developed isotopically labelled macroalgae can be manufactured to generate ‘unnatural’ δ 15 N values for translocation into coastal environments. This approach can provide an efficient, low‐cost alternative to current analytical methods for determining and monitoring nitrogen pollution.