Stable nitrogen isotopes in coastal macroalgae: Geographic and anthropogenic variability

► Anthropogenic versus upwelling nitrogen effect on macroalgal δ15N was studied. ► The influence of populations and upwelling has not been made before on macroalgal δ15N. ► Natural variability has not been taken into account in most biomonitoring studies. ► Upwelling explains most of the variability in δ15N in macroalgae.