Macroalgae in Tropical Marine Coastal Systems - Seaweed Biology - page 330

herbivory to associated macroalgae. The study showed that aggregations of
Turbinaria significantly enhanced species richness, especially of fleshy species
like Caulerpa serrulata , Dictyota bartayresiana, Sargassum mangarevense, Valo-
nia utricularis, and Ventricaria ventricosa. On the contrary, in coral heads without
Turbinaria, there was less algal density and these were dominated by crustose
coralline algae, filamentous algal turfs, and cyanobacteria. As a result, unprotected
coral heads without Turbinaria had the lowest diversity of all treatments.

16.4.1.4 Nutrient Enrichment

The influence of excess nutrient supply on the development of reef macroalgae is a
topic of major debate between marine researchers (Littler and Littler 2006 ; Mork
et al . 2009 ; Smith et al . 2010a ). Experimental studies have often investigated the
effect of nutrient enrichment in the presence and absence of herbivory to determine
its role in regulation of macroalgal growth and cover in tropical habitats, especially
coral reefs. The available literature is divided between studies that report a signifi-
cant effect of nutrient enrichment on enhancing growth and abundance of fleshy
macroalgae and cyanobacteria in coral reef habitats and others that report no
significant effect. For example, Littler et al . ( 2010b ) conducted a 12-month study
in a sedimentary lagoon at Carrie Bay Cay in Belize's barrier reef to compare the
enrichment effects of dissolved inorganic nitrogen (DIN) and soluble reactive
phosphorus (SRP) additions on algal community development. They found that
SRP and SRP DIN treatments stimulated the growth of cyanobacteria ( Spirulina

spp . ) and inhibited settlement and colonization of CCA. Meanwhile, the DIN
enrichments increased CCA cover, decreased cyanobacteria cover, and had no
influence on fleshy macroalgae. Sotka and Hay ( 2009 ) reported similar findings
related to fleshy macroalgae from a 20-week nutrient-enrichment experiment
conducted in the shallow fore reefs of the Florida Keys. They concluded that
herbivory is stronger than nutrient abundance in regulating macroalgae abundance
and composition on coral reefs. Their results revealed that large herbivorous fish
selectively grazed on nutrient-enriched macroalgae. On the contrary, experiments
conducted by Smith et al . ( 2010a ) on the reef slope at Puako Reef, Hawai'i found
that both fertilization and herbivory exclusion impacted macroalgae community
structure at different temporal scales. They observed that reducing herbivory led to
rapid changes in benthic communities (1 month), while the effects of nutrient
enrichment were only visible after 3-4 months. This can explain the lack of strong
nutrient-enrichment effects noted in other studies, as they may be a result of short-
term experiments. Vermeij et al . ( 2010 ) conducted experiments in Cura¸ao,
Netherlands Antilles, to examine the effects of Caribbean turf algae on neighboring
corals and interactions of these algae with nutrient enrichment and grazing. They
found that nutrient enrichment significantly enhanced the capacity of turf algae to
overgrow the coral Montrastrea annularis , since turf algae is also able to overgrow
coral at ambient nutrient concentrations when herbivory is excluded or reduced.

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