Cyanobacterial resilience and productivity decoupling from declining phosphorus during a four-year whole-lake...

Urban eutrophic lakes in subtropical regions represent a persistent management challenge due to their strong internal feedbacks that resist rehabilitation. We report a four-year whole-lake intervention evaluating the effectiveness of artificial aeration in a 10 m-deep, eutrophic urban lake in Uruguay, using a Before-After Control-Impact (BACI) design with a comparable reference lake. Despite continuous operation, aeration did not alter the seasonal warm-monomictic regime nor eliminate summer deep-layer anoxia. However, it partially weakened thermal stratification and drove a sustained, statistically significant decline in surface total phosphorus (∼18 μg P L^-1 yr^-1), shifting the TP-based trophic state index from hypereutrophic to eutrophic. In contrast, chlorophyll-a remained hypereutrophic throughout, revealing a marked decoupling between nutrient availability and phytoplankton productivity. Persistent cyanobacterial dominance, rising phosphorus-use efficiency, and cycles of intense bloom growth, collapse, and rapid recovery indicate physiological adjustment to declining phosphorus rather than a proportional loss of biomass. The reference lake showed no equivalent directional changes, supporting the attribution of these trends to aeration. The intervention altered specific processes-thermal stability and internal phosphorus release-without shifting the dominant feedbacks sustaining eutrophy, consistent with "pathological resilience", wherein internal mechanisms perpetuate a degraded state despite external perturbation. Aeration alone proved insufficient to trigger structural reorganization within four years, underscoring the need for integrated, adaptive rehabilitation strategies in urban lakes with strong self-sustaining eutrophic feedbacks.