Officials suggest at least part of the bloom is "residual" after water lines sat dormant for weeks. Dark paint can warm shallow water and encourage algae; fully eliminating it may require draining and treating the pool, a move that raises concerns about wildlife such as ducks if chemicals are used. Similar blooms followed a 2012 renovation, when crews had to drain and recalibrate treatment systems.
An Interior Department spokesperson praised the repair work and described the current algae as a routine post-renovation issue.