WALTERS, L.J.*; ABGRALL, M.J.: Impact of Drift Species (Gracilaria compressa, Zoobotryon verticillatum) on the Recruitment and Fitness of Sessile Invertebrates

Mosquito Lagoon is the northern-most estuary in the Indian River Lagoon system. Here, large, dislodged masses of macroalgae and the bryozoan Zoobotryon verticillatum can be found drifting in the water at varying times throughout the year. Capture of drift in nets ranged from 0-158 g/hr. Drift algae and masses of Zoobotryon frequently become entangled around submerged hard surfaces, and thus may significantly impact the settlement and fitness of hard-bottom sessile invertebrates. We examined the impact of the common red macroalga Gracilaria compressa and the bryozoan Zoobotryon on the recruitment and survival of two dominant fouling organisms, the barnacle Balanus eburneus and the bryozoan Bugula neritina. Manipulative field experiments were designed to separate out chemical versus abrasion effects of the drift organisms. In recruitment trials, replicate surfaces were submerged as one of five treatments: caged with live drift species, caged with drift mimic, empty cage, partial cage and uncaged control. Plates were monitored every two days and reductions in recruitment were found with both live drift and mimics of both species. The impact of drift on survival of recruits in 6-week trials was next tested using the same design. Gracilaria, Gracilaria mimics and Zoobotryon significantly reduced the survival of Bugula. Survival of Balanus was only significantly reduced by live Gracilaria. Thus, when present, drift organisms can significantly alter sessile invertebrate community composition.