The importance of symbiont-bearing benthic foraminifera for West Pacific carbonate beach environments

In the tropical Central and West Pacific as well as in the East Indian Ocean, symbiont-bearing benthic foraminifera often are the main producers of calcium carbonate grains deposited at beaches. The proportion of foraminiferal tests in medium-sized to coarse sand grains may range from 20% to 95%. This depends on two factors – productivity and transport – because foraminifers do not live on or in the sandy bottom near the beaches, but prefer firm substrate in high-energy environments close to the front of coral reefs. Foraminiferal tests are produced in extreme numbers on the reef crest of Sesoko Island, Okinawa, Japan (0.6 kg m− 2 year− 1), which is covered by macroalgae, or in the transition zone between the crest and the reef moat (1.3 kg m− 2 year− 1). Empty tests released by reproduction or death are entrained at the reef crest by waves and transported by currents. This transport depends on the direction and intensity of currents connecting the habitat (reef crest and transition to moat) with the deposition area (central moat, lagoon, beach). Habitat destruction and/or the interruption or diversion of water flow hinders the accumulation of foraminiferal tests at the beach. Where deepening the moat intensifies beach-parallel currents, supply to and deposition at the beach are reduced, while the remaining sand is eroded by tropical cyclones. Understanding the ecology – especially population dynamics – of larger foraminifera, in combination with factors influencing the transport of empty tests, is essential to preserve the balance between deposition and removal of sand at carbonate beaches in the West Pacific and East Indian Ocean.