Bleached Elkhorn coral under a shade in Dry Tortugas National Park, Pulaski Shoal

Shown here is a colony of the threatened Elkhorn coral, Acropora palmata, that has become "bleached," that is, lost all its algal symbionts (also called zooxanthellae) because of the summer 2023 ocean-heat wave. The coral is attached to a cement block as part of the U.S. Geological Survey’s Coral Assessment Network (USGS-CAN) that provides data on coral-growth (calcification) rates throughout the western Atlantic. Data like these are collected to document seasonal and spatial patterns in coral growth that correlate with ocean conditions and are used to guide the management and restoration of coral species that have experienced population declines across the region. Pictured here is one of calcification stations located in Dry Tortugas National Park. When photographed on August 15, 2023, this coral was showing areas that were completely bleached but still alive (areas of the colony that are white), areas where the coral tissue still contained some symbionts (brownish areas on the undersides of coral branches), and areas that were already dead (area of colony in foreground encrusting the cement block showing growth of filamentous seaweed starting to grow on the empty coral skeleton). The shade structure could help to reduce light stress that is problematic when corals are in a bleached state (see https://www.usgs.gov/media/images/coral-shading-experiment-during-a-bleaching-event for more information).