Authorities in four coastal regions on both sides of the Atlantic have restricted swimming after separate environmental hazards were detected in the water, underscoring growing concerns over the impact of warming seas and marine pollution on public safety. In Florida, scientists at Florida Gulf Coast University reported that toxic blue-green algae, or cyanobacteria, has returned to rivers and estuaries, reviving memories of the state’s 2018 bloom that blanketed waterways and released airborne toxins. Researchers warn that a single microscopic spore can regenerate a colony and note that an estimated 50,000 metric tons of legacy phosphorus in Lake Okeechobee continue to fuel outbreaks despite recent mitigation work. Across the Atlantic, Northern Ireland’s Department of Agriculture, Environment and Rural Affairs issued a red-level “Advice Against Bathing” for Benone Beach after tests confirmed high concentrations of the same type of algae. The agency said it has logged more than 100 detections across the region this year and will keep monitoring affected sites. Spain’s Guardamar municipality temporarily hoisted red flags after lifeguards recovered two Glaucus atlanticus— a striking but venomous sea slug nicknamed the “blue dragon”— on Vivers Beach. The mollusc stores stinging cells from prey such as Portuguese man-of-war, and local officials warned that contact can cause intense pain, nausea and, in severe cases, systemic reactions. In Gloucester, Massachusetts, Good Harbor Beach closed for a second consecutive day as lifeguards confirmed a shark sighting just offshore. The city said swimming will remain suspended until further notice while patrols assess the area. Marine biologists link the rising frequency of such incidents to nutrient-rich runoff, invasive species and higher sea‐surface temperatures, conditions that favour harmful algal blooms and expand the range of tropical organisms. Coastal managers are urging tighter pollution controls and real-time monitoring to reduce health risks as the late-summer tourism season peaks.