Canal Connectivity Synchronizes Pollutant Loading and Cyanobacteria Shifts in Hydrologically Independent Urban Lakes

Urban lakes provide key ecosystem services in densely populated, highly developed areas. However, runoff-driven pollutant loading can diminish these services and contribute to enduring patterns of eutrophication. Here, we compared the paleolimnological records of two hydrologically independent urban lakes with distinct local land use legacies (i.e., industrial, residential) to link large-scale, chronic urbanization with long-term water quality outcomes. We measured metals, nutrients, and photosynthetic pigments (e.g., chlorophylls, carotenoids) on 210 Pb-dated sediment cores to infer environmental change over the last ∼150 years. In both lakes, canal construction coincided with synchronous metal and nutrient increases. These canal-associated inputs were subsequently sustained by regional population growth and development, while local land use and individual lake management minimally affected water quality trajectories. Specifically, population growth and watershed impervious surface expansion coincided with increasing N/P and a 1990s-era shift toward cyanobacterial dominance contemporaneous across both lakes. Collectively, these results demonstrate how hydrologic connectivity can enrich and homogenize pollutant loading regimes, while long-term stoichiometric imbalances influence phycological state shifts, complicating lake management and restoration in urban settings.