Competion and facilitation between seagrass and calcifying algae (Halimeda and penicillus) in the Caribbean coastal ecosystem

ABSTRACT

The Caribbean coastal ecosystem could protect shorelines from wave erosion. Key elements are the calcifying algae (Halimeda spp. and Penicillus spp.) that are main producers of carbonate sediment, and are early successional species that help to stabilise the sediment. Calcifying algae occur in mixed communities with seagrasses. Seagrass appears to both facilitate the algae and compete with them. In this experiment, four treatments were established ranging from full density to bare density (full patches of seagrass, thinned patches of seagrass (50%) with calcifying algae, patches with only algae and patches with only algae and seagrass roots), in order to explore how the presence and abundance of seagrass affects the growth and long-term persistence (abundance and recruitment) of these algae. Growth rates were not influenced by seagrass in our field experiments. However, measuring recruitment over time gives evidence that there is a balance between competition and facilitation by seagrasses. On one hand, calcifying algae get outcompeted by a lack of free space; on the other hand a facilitative effect appears that is possibly related to the seagrass providing a shelter for young recruits. An adequate mixed community of seagrass and calcifying algae could provide a climate proof Caribbean coast: seagrass might provide a high-pH refuge for calcifying algae, and calcifying algae will help seagrass meadows recover faster from storm disturbance – both protecting the shore from wave erosion.