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

Rationale: Stable nitrogen isotope ratios (δ N) can be used to discern sources of excess nitrogen pollution in water. The δ N values of nitrate in water often do not reflect the true δ N source value owing to high temporal variation, and there are high analytical costs associated with obtaining δ 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 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 s on a light/dark rhythm of 16 h/8 h). The oven-dried tips were analysed for δ 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 N-enriched solution and ca 15 days for the N-enriched solution. δ 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 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.