Seasonal nitrate-carbon-oxygen coupling along the Lambayeque coast in the northern Humboldt upwelling system: association with land-based nitrogen pressure

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.