Nitrogen (N) and carbon (C) isotopic analyses in seagrasses have been established as effective environmental tracers to assess the anthropogenic influence on seagrass meadows. The aim of this study was to determine the spatial patterns and seasonal variability in the C and N stable isotopic composition of sediments, leaves and epiphytes at a Zostera noltei meadow in an intertidal complex located at the Ría de Vigo (NW Iberian Peninsula) and to explore the mechanisms underlying such variability.Measurements of carbon and nitrogen stable isotopes, morphological features of Z. noltei and the C:N mass ratios were undertaken monthly during an annual seasonal cycle. N recycling between the seagrass and the sediment under its canopy influenced the spatial and seasonal nitrogen isotopic variability in the Z. noltei meadow. The relative contribution of nitrogen stable isotopes both in Z. noltei and the sediments under its canopy showed significant seasonal differences. The seasonal variability observed in carbon and nitrogen stable isotopes and C:N mass ratios of Z. noltei was mainly explained by the physiological mechanisms of carbon and nitrogen uptake and the differential nutrient demand of the plant depending on the growth stage.