The Effects of an Alien Invasive Alga on Sediment Retention and Resuspension in Coastal Hawaiian Waters and the Influence of Community Based Restoration

Marine macrophytes contribute to sediment stabilization and accumulation in coastal areas. Sediment accumulates and is resuspended by wave motion to remain a consistent stressor of coral reef organisms. Avrainvillea amadelpha is an alien invasive alga that has become established in Hawaiian coastal regions, especially in soft bottom habitats where it competes with seagrass and other coral reef organisms for habitat and resources. Maunalua Bay on the island of O'ahu has been impacted by poor land use and watershed management, which has contributed to substantial sediment input into the bay. The Paiko Lagoon Peninsula (PLP) at Maunalua Bay had a carbonate reef which has since been buried in sediment and the dense cover of A. amadelpha is suspected to contribute to sediment retention on this reef. Community based organizations along with several partners have identified PLP as the first location of an ambitious attempt to restore Maunalua Bay. Volunteers have manually cleared A. amadelpha from an experimental 10,355 ft² plot near PLP in hopes of freeing trapped sediment with the goal of clearing the carbonate reef. The goal of our work was to assess the role of A. amadelpha in sediment retention and resuspension and to investigate the efficacy of manual clearing in eliminating sediment. A sediment resuspender was used to suspend sediment on the surface of and between algal thalli as well as areas that have been cleared of algae and suspended sediment was measured with a turbidity meter. Areas of reef with A. amadelpha had nearly 4x more sediment than the cleared plot and areas without algae. We found that manual removal of algae was effective in freeing up and reducing sediment. The majority of sediment that had accumulated on the reef at PLP was trapped between algal holdfasts and not on the algal thalli.