| 31 Jul 2026
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.
This manuscript constitutes a technically rigorous, conceptually coherent, and scientifically relevant contribution to the characterisation of coastal biogeochemical variability within an under-observed sector of the northern Humboldt upwelling system. Its principal merit lies in the identification of a recurrent and spatially differentiated covariance structure among NO₃⁻, dissolved inorganic carbon, total alkalinity, surface pCO₂, biological indicators, and O₂, based on spatially co-located monthly marine fields. The analytical framework is methodologically transparent, reproducible, and appropriately constrained. The authors consistently distinguish statistical covariation from mechanistic interpretation and source attribution, which is essential given the observational design and the absence of direct measurements of terrestrial nitrogen fluxes. The manuscript is logically organised, written in precise and technically mature English, and supported by graphical material that communicates the spatial configuration, seasonal behaviour, and multivariable correlation patterns effectively. The public availability of the dataset and supplementary material further reinforces the reproducibility and scientific integrity of the study. I found no methodological deficiency that invalidates the reported seasonal or sectoral patterns.
These revisions do not modify the principal findings or conclusions. They would strengthen the methodological traceability, metrological precision, and process-based interpretation of an otherwise credible and technically mature contribution.