The Lincoln Memorial reflecting pool in Washington DC has hardly been out of the news since April when President Trump vowed to have it renovated, and painted ‘American flag blue’ by 4 July. Despite the pool being stripped, cleaned, coated and refilled, within days the algae that has plagued it for decades was back. To find out why these blooms happen, what makes them so difficult to tackle and what Trump could try next, Ian Sample hears from co-host Madeleine Finlay, and from Dr Linda May, a freshwater ecologist at the UK Centre for Ecology & Hydrology.. Help support our independent journalism at theguardian.com/sciencepod