in 738 samples of macroalgae collected during one year across the Urías lagoon system (Mazatlán, Sinaloa, Mexico) were analyzed in order to study the N sources. The  15 N-macroalgae premise is that they register and integrate the variability of N and may provide time-integrated information about N pollution. To validate this premise, we applied two models to predict the isotopic variability of N available in the water column ( 15 N-DIN): one is based on a physical mixing balance of N sources (simple model), and the second includes the effects of the biogeochemical processes on the signals (coupled model, CM). Both models showed significant R 2 , but CM better explained the  15 N-DIN variability. The relation between δ 15 N-DIN CM and δ 15 N-macroalgae also showed a good fit, although lower than expected and with higher dispersion. The multiple N sources and co-occurring biogeochemical reactions, which produce a mixture of chemical species of N and of their isotopic compositions, as well as the high diversity of species (45) explained the elevated variability and lack of fit. Even so,  15 N in macroalgae is the most useful available tool to quantify the relative contributions of N to ecosystems from different sources. Based on Bayesian isotopic mixing models, the main anthropogenic sources of N to the Urías lagoonal system were domestic effluents (50%), atmospheric deposition (24%), and seafood processing plant effluents (21%), and to a lesser extent, aquaculture effluents (3%), agriculture runoff (1%), and oceanic N (1%).Key words: δ 15 N, eutrophication, biogeochemical cycle, NLOAD model, MixSIR. RESUMEN. Las concentraciones de las formas químicas de nitrógeno (N) y su composición isotópica en la columna de agua y las señales deδ 15 N en 738 muestras de macroalgas recolectadas durante un año en el sistema lagunar de Urías (Mazatlán, Sinaloa, México) fueron analizadas para estimar las fuentes de N. La premisa sobre el uso de  15 N en macroalgas es que las macroalgas asimilan 15 N en proporción a la disponibilidad y reflejan la fuente de N de una manera predecible. Para validar esta premisa, se aplicaron dos modelos para la predicción de la variabilidad isotópica del N en el agua ( 15 N-NID): el primero se basa en una mezcla física de las fuentes de N (modelo simple), y el segundo integra los efectos de procesos biogeoquímicos sobre sus señales isotópicas (modelo acoplado, MA). Ambos modelos mostraron valores de R 2 significativos, pero el MA explicó mejor la variabilidad del δ 15 N-NID. La relación entre δ 15 N-NID MA y δ 15 N-macroalgas también mostró un buen ajuste, aunque menor al esperado y con una alta dispersión. Las múltiples fuentes de N y reacciones biogeoquímicas concurrentes, que producen una mezcla compleja de especies químicas de N y de sus composiciones isotópicas, además de la alta diversidad de especies (45), explicaron la elevada variabilidad y la falta de ajuste. Aun así, la técnica de  15 N en macroalgas es la herramienta más útil que existe para cuantificar las contribuciones relativas...

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In this work we aimed to determine if different sources of anthropogenic impact of different water bodies may affect the nitrogen (N) and phosphorus (P) contents of macroalgae biomass. To this end, samples of water and macroalgae were collected every second month in Urías lagoon (UR) and in Altata-Ensenada del Pabellón (AL and EP) lagoonal system of NW Mexico. The order of concentrations of dissolved N was UR> EP>AL (P<0.001), that of reactive P was EP>UR>AL (P < 0.001). The N content of the macroalgal biomass of the three lagoons was not significantly different, but the P content was significantly (P <0.001) lower in UR than in EP and AL. The N: P ratios in algal biomass seemed to indicate that N was below optimum values for growth in AL and EP, while P was below optimum in UR. However, the algae N and P contents were consistently high in the three lagoons, indicating adequate concentrations of external nutrients for optimum growth.