Spatial Associations Between Chlorophyll-a Concentrations and Estimated Alzheimer’s and Parkinson’s Disease Patient Counts in the Nakdong River Basin, South Korea: A Geographically Weighted Regression Analysis

Chlorophyll-a monitoring characterizes algal biomass but does not directly measure cyanotoxins or human exposure. We examined ecological spatial associations between chlorophyll-a and population-weighted estimates of Alzheimer’s disease (AD) and Parkinson’s disease (PD) patient counts in the Nakdong River basin, South Korea. Data for 2010–2022 combined chlorophyll-a from a nationwide 939-site monitoring network, older-adult populations, and administrative patient counts across 33,049 1 km grids. Geographically weighted regression described local associations. Higher chlorophyll-a occurred along river reaches and downstream areas; positive local coefficients were strongest in selected inland districts, including Sangju-si and Jinju-si, and weaker in metropolitan areas. Administrative sensitivity analyses showed that association direction and magnitude depended on population adjustment and model specification. Supplementary bandwidth analyses also identified scale dependence and persistent residual spatial autocorrelation. Because the grid outcomes were constructed from administrative counts, interpolation accuracy was unquantified, and source-specific clinical definitions were unverified; these findings indicate exploratory ecological covariation rather than a toxicological effect or causal risk. The mapped contrasts may guide targeted water-quality and cyanotoxin monitoring and epidemiological validation.