Estimation of eutrophication in Paldang Reservoir using trophic state index deviation

Abstract

Carlson’s trophic state index (TSI) evaluates trophic status using transparency (expressed as Sechhi depth – SD), chlorophyll-a (Chl), and total phosphorus (TP), while the trophic state index deviation (TSID) plot graphically integrates their variations. This study applied TSI and TSID to assess eutrophication dynamics in South Korea’s Paldang Reservoir using monthly data (2018–2022) from five sites influenced by the South Han River, the North Han River, and Gyeongan Stream. Values of TSI indicated mesotrophic to eutrophic conditions, with the shallow, Gyeongan Stream-affected site (P5) showing persistent eutrophy (TSI(Chl) = 54–67). Spatial patterns reflected tributary influence: P4 and P5 exhibited TSI(Chl) ≈ TSI(SD) > TSI(TP), suggesting algal-driven light attenuation under phosphorus limitation, while the South Han River sites (P1, P3) often had TSI(SD) > TSI(Chl) due to particulate inflows after rainfall. Plots of TSID confirmed phosphorus-limited conditions (TN:TP > 17) but revealed strong seasonal variability in Chl and SD. Summer monsoons increased TP and reduced N:P ratios, with riverine sites showing biomass suppression and lacustrine sites remaining stable. Prolonged rainfall in 2021 caused exceptional biomass fluctuations at riverine sites, including temporary oligotrophy at P1. Compared to TP alone, the TSID approach more effectively captured hydrological-biological interpretations, suggesting that controlling phosphorus load alone may not be sufficient to mitigate eutrophication. Although high-flow events temporarily reduced cyanobacterial abundance, winter cyanobacterial presence has risen, suggesting climate-driven shifts in bloom dynamics. Continuous, site-specific TSID monitoring can enhance understanding of eutrophication processes and support adaptive management of large multipurpose reservoirs under changing hydrological regimes.