From hydrographic forcing to biogeochemical control: spatiotemporal patterns of eutrophication in a coastal marine ecosystem

Coastal eutrophication shapes ecosystem functioning in productive marine environments under natural and anthropogenic forcing. This study evaluated the spatiotemporal variability of eutrophication and associated biogeochemical processes in the La Libertad coastal system (northern Peru) during 2022–2025. Physicochemical and biological variables were obtained from the Copernicus Marine Environment Monitoring Service (CMEMS) to analyse nutrient dynamics, primary productivity, oxygen saturation, and carbonate-system variability across four monitoring points. Trophic status was assessed using a chlorophyll-based trophic state index (TSIchl), while multivariate patterns were explored through principal component analysis (PCA). Results revealed recurrent mesotrophic-to-eutrophic transitions, with stronger trophic intensification during 2024–2025. Nutrient enrichment, particularly PO₄³⁻ acting alongside NO₃⁻, showed consistent positive coupling with chlorophyll-a and phytoplankton biomass, confirming the dominance of bottom-up control. Elevated productivity was consistently associated with lower oxygen saturation and pH, indicating intensified remineralisation and stronger metabolic coupling. A clear spatial gradient emerged, with northern points more strongly influenced by hydrographic variability and southern points exhibiting tighter biogeochemical coupling. PCA confirmed this transition, with PC1 explaining 55.6–67.9% of total variance across points and consistently defining the dominant biogeochemical axis, whereas PC2 (16.0–23.9%) reflected secondary hydrographic modulation. These findings highlight increasing ecosystem sensitivity to nutrient loading under climatic and anthropogenic pressure.