Investigating effects of the macroalga Caulerpa prolifera on transplant success of the seagrass Halodule wrightii to improve restoration

Introduction Seagrass restoration efforts have been motivated by global decreases in coverage associated with anthropogenic influences on the environment. In the Indian River Lagoon (IRL), FL, USA, seagrass declined by 75% between 2011 and 2021. During this time, abundance of the native macroalga Caulerpa prolifera increased substantially in this system. As restoration efforts began in the IRL, it became apparent that varying abundance of C. prolifera could impact the recovery of seagrass. Halodule wrightii is the most abundant seagrass in the IRL and often targeted for restoration use. Methods Experiments were done using outdoor, recirculating-water mesocosms to test the effects of C. prolifera abundance on planted H. wrightii . Results and Discussion Results showed a competitive effect on outcomes of H. wrightii transplants when C. prolifera occurrence is 90%. Within the context of restoration, this indicates that newly transplanted fragments of H. wrightii could struggle to establish themselves due to resource deficiencies in areas with high coverage (90% occurrence) of C. prolifera . Mixed results with medium coverage of C. prolifera (65% occurrence) and no effect in areas with low coverage (25% occurrence) indicate a potential transition range above 65% occurrence and below 90% occurrence of C. prolifera where consistent effects of competition begin to appear on H. wrightii growth. Therefore, the success of transplanted H. wrightii fragments into low-to-medium C. prolifera abundance is not likely to be limited by competition with this native macroalga.