Cyanobacterial Resilience and Productivity Decoupling During Phosphorus Decline in a Four-Year Whole-Lake Aeration Experiment

Urban eutrophic lakes in subtropical regions represent a persistent management challenge due to their strong internal feedbacks and resistance to rehabilitation. We report a four-year whole-lake experiment 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 hypolimnetic 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 at hypereutrophic levels throughout the study period, revealing a marked decoupling between nutrient availability and phytoplankton productivity. Persistent cyanobacterial dominance, rising phosphorus-use efficiency, and episodes of intense bloom growth followed by collapse and rapid recovery indicate that the phytoplankton community adjusted physiologically to declining phosphorus rather than retreating. The reference lake showed no equivalent directional changes, supporting the attribution of observed trends to aeration. The intervention modified specific processes — internal phosphorus release and thermal stability — but did not shift the dominant feedbacks sustaining eutrophy. These findings are consistent with the concept of pathological resilience, wherein internal mechanisms perpetuate a degraded state even under external perturbation. Aeration alone proved insufficient to trigger a structural reorganization of the system within four years, underscoring the need for integrated, adaptive rehabilitation strategies in urban lakes with strong self-sustaining eutrophic feedbacks.