Abstract
Climate change has significant effects on freshwater ecosystems, including lakes, influencing key water quality indicators such as water temperature and dissolved oxygen concentrations (DO). This study uses a 3D hydrodynamic model (AEM3D) coupled with a simple biogeochemical module (iWQ) alongside observations to examine historical changes in Lake Taupō, a deep, oligotrophic lake and the largest freshwater lake in Australasia. Warming trends in air temperature during 1992–2022 correlate with increasing heat content and enhanced stratification, subsequently impacting DO concentrations. Both surface and bottom waters have experienced significant warming, with summer surface waters warming at a rate of 0.065 ± 0.025°C yr −1 . This warming is associated with a notable decline in summer surface DO concentrations, at a rate of −0.008 ± 0.003 mg L −1 yr −1 , driven by the reduction in solubility of oxygen. Reduced bottom DO was recorded in mid‐lake, at times dropping below a 7 mg L −1 threshold for fish habitat. Occasional peaks in observed chlorophyll‐ a coincided with elevated lake heat content, suggesting a possible influence of thermal conditions on phytoplankton productivity. Lake inflows appear to have a minimal effect on heat balance and bottom DO, compared to atmospheric conditions. The influence of global climatic phenomena is evident, as significant warming of air and surface water temperatures occurred during periods when Niño indices were ≤ −1. The interplay between rising temperatures and declining water quality underscores the complex impacts of climate change on Lake Taupō's ecosystem.