Factors influencing algal blooms on tropical reefs with an emphasis on herbivory, nutrients and invasive species

Algal blooms on tropical reefs have become increasingly common over the last several decades. Reduced herbivore pressure, eutrophication and the introduction of exotic species have all been posited as factors that may drive these transitions in organism dominance. Several different types of algal blooms have been identified on reefs in the Hawaiian Islands including multispecies algal assemblages, monospecific algal blooms and seasonal or ephemeral blooms. This study sought to determine the causes of these different types of algal blooms by conducting both field and laboratory experiments and quantitative field assessments.;In a factorial nutrient enrichment and herbivore exclusion experiment conducted for 6 months on the island of Hawai'i significant changes in algal biomass, community structure, sediment accumulation and mobile microinvertebrate abundance were found. Results of this study show that benthic reef communities can change rapidly in response to changes in both top down and bottom up factors.;From field assessments across the main Hawaiian Islands a total of five (Acanthophora spicifera, Hypnea musciformis, Gracilaria salicornia, Kappaphycus spp. and Avranvillea amadelpha) species of nonindigenous algae can now be considered highly successful. Detailed studies on the ecology of select nonindigenous marine algae (NIMA) have identified particular concerns and highlighted the need for management action. In summary management of these invasive species will be challenging as a result of unique ecological and physiological strategies that each NIMA possesses.;The ephemeral bloom forming native alga Cladophora sericea was studied during a bloom cycle during 2001 on the island of Maui. Results of ambient and sediment pore water sampling and algal physiological parameters suggest that ground water intrusion is occurring at this site and the alga appears to be utilizing this terrestrial based nutrient source.;Upwelling and internal tides can naturally deliver nutrient rich water into coral reef ecosystems. In an area where internal tidal upwelling occurs in the Florida Keys, one of the most common benthic reef algae Halimeda tuna reflected patterns associated with natural nutrient enrichment. This study provides evidence that rich and highly productive benthic algae may thrive in deep water coral reef environments in response to naturally elevated nutrient conditions.