Twice each day, tides ebb and flow through a maze of sandy channels, mudflats, and mangrove forests that flank the 88 islands and islets of Guinea-Bissau’s Bijag\u00f3s Archipelago<\/a> (Arquip\u00e9lago dos Bijag\u00f3s in Portuguese). Seen from above, the process leads to stark changes to the landscape: around low tide, intertidal<\/a> mudflats and sandflats emerge from the sea, causing islands to grow significantly<\/a> before shrinking again hours later.<\/p>\n The perpetual rhythm of the tides sustains outpourings of marine life in an archipelago that, as of 2025, was inscribed as a UNESCO World Heritage site<\/a>. The site protects the only active deltaic archipelago on Africa’s Atlantic coast, a place where tides, river sediments, coastal upwelling, and coastal currents come together to shape unusually productive and biodiverse island ecosystems. \u00a0<\/p>\n UNESCO estimates that the islands support some 870,000<\/a> migratory shorebirds, making this one of the most important feeding areas for birds in West Africa along the East Atlantic Flyway<\/a>. Hundreds of species of birds dine on a potpourri of marine worms, crustaceans, mollusks, and small fish found on mudflats exposed by low tides. During high tides, manatees, dolphins, and schools of fish move closer to the islands, pushing deeper into the mangrove forests<\/a> that ring them, and tens of thousands<\/a> of sea turtles swim inland to sandy beaches as they hunt for nesting sites.<\/p>\n A huge population of green sea turtles nests on the tiny island of Poil\u00e3o, part of the Jo\u00e3o Vieira and Poil\u00e3o Marine National Park<\/a>. After hatching, young turtles make perilous nighttime dashes to the water, often pursued by crabs, lizards, and birds. Once they reach the water, baby sea turtles face an array of predators, including jacks, barracudas, groupers, and snappers that patrol shallow waters as well as tuna, mackerel, sharks, and rays in deeper waters. According to some estimates, less than 1 percent<\/a> of green sea turtle hatchlings survive to adulthood.\u00a0 <\/p>\n A 2025 analysis<\/a> of the region’s tides explored why the archipelago has some of the largest tidal ranges in West Africa. The researchers concluded that the region’s wide, shallow shelf and the estuary\u2019s geometry combine to create a tidal range of up to 7 meters (23 feet), compared to about 1 meter (3 feet) in many other parts of the West African coast. The scientists used altimetry data from the NASA\/CNES TOPEX\/Poseidon<\/a>, Jason-1<\/a>, and Jason-2<\/a> satellites to help validate their findings.\u00a0<\/p>\n NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the\u00a0U.S. Geological Survey<\/a>.\u00a0Story by Adam Voiland.<\/em><\/p>\n![\"A](\"https:\/\/assets.science.nasa.gov\/dynamicimage\/assets\/science\/esd\/eo\/images\/iotd\/2026\/a-tide-fueled-trove-of-biodiversity-in-guinea-bissau\/bijagosarchipelago_oli_20251128_lrg.jpg?w=7236&h=6030&fit=clip&crop=faces%2Cfocalpoint\")
<\/figure>\n<\/div><\/div><\/div><\/div><\/div>\n\n