Hoyos-Alayo, Walter Manuel, Leiva-Piedra, Jorge Luis, Ramirez-Juidias, Emilio, Amaro-Mellado, José Lázaro
Abstract. Coastal nitrate variability along the northern Humboldt upwelling margin reflects the combined influence of ocean circulation, biological transformation, and land-derived inputs. We examined monthly surface fields from the Copernicus Marine Service for 2022–2025 in two marine sectors adjacent to agricultural drain outlets along the Lambayeque coast, Peru. A northern sector used a 0.25° grid cell with one drain outlet and a southern sector used a 0.25° grid cell with three drain outlets. Nitrate (NO3−) was analysed with total alkalinity, dissolved inorganic carbon, surface pCO2, chlorophyll a, phytoplankton carbon, net primary production, and dissolved oxygen. Annual rice production and regional urea imports were retained only as contextual indicators of land-based nitrogen pressure. Their four-point annual relationships with marine nitrate were positive but not statistically significant and were interpreted descriptively. Surface NO3− generally increased from May to October and remained higher in the southern sector. Within-year correlations showed recurrent positive covariation of NO3− with dissolved inorganic carbon and pCO2 and inverse covariation with O2, with the most persistent multivariable structure in the southern sector. These patterns identify a recurrent nitrate-carbon-oxygen structure in an under-studied coastal segment of the Humboldt system. They do not quantify land-to-sea nitrogen flux or establish source attribution, but they show that nitrate variability in Lambayeque must be interpreted together with upwelling variability, carbonate-system behaviour, and oxygenation.