Modelled Seasonal Nitrate, Carbon and Oxygen Covariability in Coastal Receiving Waters of Lambayeque, Peru (2022–2025)

Agricultural drainage discharges into coastal waters whose nutrient concentrations also respond to ocean circulation and internal biogeochemical cycling. We analysed 48 monthly Copernicus Marine surface records per sector from January 2022 to December 2025 along the Lambayeque coast, Peru. All nitrate, carbon and oxygen relationships derive from one numerical modelling framework without independent contemporaneous local validation. Two distinct model series represented northern and southern receiving-water sectors adjacent to four drain outlets. Nitrate averaged 10.672 ± 2.623 and 12.210 ± 2.906 mmol m−3, respectively, and remained higher in the south in all 48 paired months. Both sectors showed enrichment during May–October. After removing calendar-month means, nitrate remained positively correlated with dissolved inorganic carbon (r = 0.812 north and 0.908 south) and inversely correlated with oxygen (r = −0.678 and −0.784), with nominal month-adjusted p < 0.001 for all four associations. Block-bootstrap intervals and resampling with re-estimated seasonal means retained these directions. Reproduced four-point agricultural relationships supplied ancillary context. The results identify persistent receiving-water contrasts and covariance beyond the mean seasonal cycle within the product. They support coordinated drainage and coastal monitoring rather than source attribution.