Thalassodendron ciliatum (Cymodoceaceae) in Palau: occurrence, typhoon impacts and changes over time

Abstract

Abstract The seagrass Thalassodendron ciliatum (Cymodoceaceae) is uncommon in Palau, Micronesia, but remnant beds occur in an area of less than 1 km2 on the northern tip of Velasco Reef, a sunken atoll, plus small areas on its western rim. The ridges are lengthy, consisting of rhizome mat and seagrass up to 1 m high, and 4–8 m wide forming curving elongate shapes (arcs, concentric curves, circles, patches). Most occur on a gently sloping limestone bottom 22–27 m deep, with some as shallow as 15 m on the Western Rim, and ridges sit directly on top of the limestone pavement, with only the weight of mat and its incorporated rock materials holding them in place. Photographic monitoring over time has indicated they are actively eroding on their edges, but the passage of Super Typhoon Haiyan in 2013 caused much greater damage to the beds. Small patches occur in shallow water on the east side of Babeldaob Island slightly behind the fringing reef crest and have not changed significantly over recent decades.