Macroalgae in tropical seascapes : regulating factors and functions in the coastal ecosystem

Abstract: Although macroalgae usually are inconspicuous on pristine coral reefs, they often thrive on reefs that are subjected to various types of anthropogenic disturbance. This thesis consists of five papers and investigates how biomass and composition of macroalgal communities on coral reefs are affected by regulating factors, such as nutrient availability, herbivory, substrate availability and hydrodynamic forces. In addition, ecological functions and potential impacts of both wild and farmed macroalgal communities are evaluated. Paper I describes a method for using macroalgal tissue nutrient concentrations as bioindicator for nutrient availability, with the possibility to map nutrient loading from larger coastal cities. Papers II and III are manipulative studies comparing top-down and bottom-up regulation of macroalgal communities, where herbivore consumption seems to be the main regulator of biomass whereas nutrient availability mainly influences community composition. Exclosure of large-bodied herbivores had a positive influence on algal biomass in both studies, and during different climatic periods. Paper III also includes the influence of hydrodynamic forces on algal community biomass and structure by comparing a reef crest and a back reef-habitat. Alterations of top-down and bottom-up regulation generally had a stronger effect within the protected back reef-habitat, suggesting that such environments may be more sensitive to anthropogenic influence. Paper IV confirms the general conclusions from papers II and III by studying macroalgal biomass and composition on reef sites with different environmental prerequisites. This study also supports the notion that herbivorous fish can suppress accumulation of macroalgal biomass if substrate availability is low, but not where coral cover is reduced and plenty of substrate is open to macroalgal colonization. The study also found a large temporal variation of macroalgal standing stock and associated nutrients at sites with low top-down regulation. Paper V evaluates potential impacts of seaweed farming on coral reefs and nutrients in the seascape by experimentally studying growth, survival and nutrient binding capacity of Eucheuma denticulatum. This study showed that seaweed farms counteract eutrophication through nutrient extraction and that the risk of farmed algae colonizing local reefs seems to be small as they were rapidly consumed. In conclusion, the studies in this thesis contribute to the understanding of macroalgal regulation and function in tropical seascapes, thereby adding to the knowledge base for coastal management.