Optically constrained trophic state patterns under seasonal hydrological forcing in a high-Andean drinking-water lake (Peruvian Andes)

Carhuallanqui Ibarra S.D.; Ñaupari Molina R.; Ozejo Lopez A.; Robles Carrión E.; Aspur Barrientos J.C.; Durand De La O O.K. · Frontiers in Water (2026) · DOI: 10.3389/frwa.2026.1789812

Resumen

High-Andean lakes used as drinking-water sources are increasingly exposed to water-quality deterioration (particularly optical conditions) under strong seasonal hydrological forcing. This study evaluates trophic patterns in Pampaccocha Lagoon (Peruvian Andes) by distinguishing trophic classification (categorical outcomes from Carlson's Trophic State Index, TSI) from trophic expression (biological response, assessed primarily through chlorophyll-a), and by identifying the main statistical correlates structuring spatiotemporal variability across stations, depths, and hydrological periods. Water quality monitoring was conducted through eight sampling campaigns spanning wet and dry hydrological periods (48 observations). Physicochemical variables, nutrients, chlorophyll-a, biochemical oxygen demand (BOD5), dissolved oxygen, and Secchi depth were analyzed. Seasonal differences were tested using Mann–Whitney U-tests, trophic status was evaluated using component-wise Carlson's TSI as an indicator-based trophic assessment, and principal component analysis (PCA) was applied to explore multivariate structure. Median TSI_mean values indicated a persistent low trophic classification (oligotrophic to low-mesotrophic range) in both wet and dry periods (wet: 35.7, IQR 4.67; dry: 34.9, IQR 3.49). TSI components diverged markedly: Secchi-depth–derived values indicated eutrophic conditions (TSI_SD ≈ 56–57), chlorophyll-a-derived values indicated oligotrophic conditions (TSI_Chl ≈ 26–27), and TP-derived values remained within the oligotrophic range (TSI_TP ≈ 18–23), demonstrating strong decoupling among trophic indicators. Nutrient concentrations and chlorophyll-a did not differ significantly between hydrological periods (p > 0.05), whereas water transparency showed a pronounced seasonal contrast (p < 0.001; wet median 1.85 m vs. dry median 1.55 m). PCA explained 60.79% of total variance (PC1 40.33%, PC2 20.46%), indicating environmental organization along gradients integrating nitrogen forms, mineralization/ionic conditions, and optical–biological responses. Overall, Pampaccocha Lagoon exhibits a persistently low trophic classification with variable trophic expression governed primarily by optical and physical controls. The eutrophic signal detected by TSI is therefore best interpreted as a Secchi-depth-dominated index response associated with transparency variability rather than evidence of nutrient-driven biological eutrophication in the mechanistic sense. These findings highlight the importance of integrating optical indicators and multivariate diagnostics when assessing and managing high-Andean drinking-water sources under seasonal hydrological forcing. Copyright

Instituciones: Universidad Nacional Autónoma de Huanta