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.