Although nitrogen stable isotope ratio (delta N-15) in macroalgae is widely used as a bioindicator of anthropogenic nitrogen inputs to the coastal zone, recent studies suggest the possible role of macroalgae metabolism in delta N-15 variability. Simultaneous determinations of delta N-15 of dissolved inorganic nitrogen (DIN) along the land sea continuum, inter-species variability of delta N-15 and its sensitivity to environmental factors are necessary to confirm the efficiency of macroalgae delta N-15 in monitoring nitrogen origin in mixed-use watersheds. In this study, delta N-15 of annual and perennial macroalgae (Ulva sp., Enterommpha sp., Fucus vesiculosus and Fucus serratus) are compared to delta N-15-DIN along the Charente Estuary, after characterizing delta N-15 of the three main DIN sources (i.e. cultivated area, pasture, sewage treatment plant outlet). During late winter and spring, when human activities produce high DIN inputs, DIN sources exhibit distinct delta N-15 signals in nitrate (NO3-) and ammonium (NH4+): cultivated area (+6.5 +/- 0.6 parts per thousand and +9.0 +/- 11.0 parts per thousand), pasture (+9.2 +/- 1.8 parts per thousand and +12.4 parts per thousand) and sewage treatment plant discharge (+16.9 +/- 8.7 parts per thousand and +25.4 +/- 5.9 parts per thousand). While sources show distinct delta N-15 NO3- in this multiple source catchment, the overall mixture of NO3- sources generally >95% DIN leads to low variations of delta N-15 NO3- at the mouth of the estuary (+7.7 to +/- 8.4 parts per thousand). Even if estuarine delta N-15 NO3- values are not significantly different from pristine continental and oceanic site (+7.3 parts per thousand and +/- 7.4 parts per thousand), macroalgae delta N-15 values are generally higher at the mouth of the estuary. This highlights high anthropogenic DIN inputs in the estuary, and enhanced contribution of N-15-depleted NH4+ in oceanic waters. Although seasonal variations in delta N-15 NO3- are low, the same temporal trends in macroalgae delta N-15 values at estuarine and oceanic sites, and inter-species differences in delta N-15 values, suggest that macroalgae delta N-15 values might be modified by the metabolic response of macroalgae to environmental parameters (e.g., temperature, light, DIN concentrations). Differences between annual and perennial macroalgae indicate both a higher integration time of perennial compared to annual macroalgae and the possible role of passive versus active uptake mechanisms. Further studies are required to characterize the sensitivity of macroalgae fractionation to variable environmental conditions and uptake mechanisms.