diversity of algae in low abundance was recorded on virtually every specimen 

 regardless of season, reef, or zone. 



Cyanobacteria of 6-8 species formed a large part of the turf biomass on 

 both fore and back reefs. Heterocysted species (particularly Calothrix spp. ) 

 are far more abundant in fore reefs than back reefs, but nonheterocysted species 

 (e.g., Osci 1 latoria ) were most abundant overall. Of eukaryotic filamentous 

 al gae, the brown ( Sphacelaria spp. ) and red ( Polysiphonia spp. , Herposiphonia 

 spp., and Taenioma macrourum ) species dominate fore reef substrates. Boring 

 filaments (usual ly green algae, e.g., Ostreobium ) were also ubiquitous. The 

 predominant macroalgae are calcareous, articulated red algae (e.g., Jania 

 capi llacea and Amphiroa f ragi 1 issima ) which are found on bottom surfaces in the 

 fore reef and on both top and bottom surfaces in back reefs. Crustose corallines 

 ( Porol ithon pachydermum and Neogoniol ithon megacarpum ) were most abundant in 

 the shallow fore reefs, particularly where grazing is intense. Of the fleshy 

 macroal gae, Lobophora variegata and Acanthophora spicifera were found but they 

 were not abundant compared to the turf community. Minute macrophytes such as 

 Gel idiella trinitatensis and Gelidium pusil Turn commonly were found in the turfs 

 of the back reef. 



As shown in table 3, the differences between fore reef and back reef turf 

 community structure along the St. Croix reef track are rather consistent. 

 These numbers and the community structure of the turf species we found agree 

 with that found on a reef in Curacao by Van den Hoek, et al . (1975). With the 

 exception of Lobophora variegata (which does not appear in Van den Hoek, et 

 al . , 1975, for Curacao) and Dictyota dichotoma (which was common in Curacao but 

 not in St. Croix), the floras for Curacao and St. Croix are quite similar. There 

 are also floristic similarities for the same reef zones between the Caribbean 

 and Indo-Pacific at the genus level (Hatcher and Larkum, 1983; Sammarco, 1983). 



It is beyond the scope of this study to examine the myriad of factors 

 maintaining turf algal community structure. Most studies indicate that intensive 

 herbivory in productive environments is important for maintaining a turf 

 community (Borowitzka, 1981; Carpenter, 1981; Littler, et al . , 1983; Sammarco, 

 1983; Hatcher and Larkum, 1983). 



Primary Production: Not Nutrient Limited 



High primary productivity on reefs growing in nutrient poor water has 

 received considerable scientific attention (reviewed by Mann, 1982). The 

 resulting research has focused on the role of cyanobacteria as in situ nitrogen 

 fixing organisms (Mague and Holm-Hansen, 1975; Capone, et al . , 1977). Recently, 

 Sammarco (1983) demonstrated that fish grazing on the Great Barrier Reef can 

 cause a shift in the community structure of turf algae from filamentous reds 

 (e.g., Polysiphonia ) to minute filamentous cyanobacteria. Other studies have 

 demonstrated that, as a result of this change in community structure, nitrogen 

 fixation is significantly enhanced (Wilkinson and Sammarco, 1983). Our results 

 support their conclusions. 



Heterocysted (nostocacean) and nonheterocysted (hormogonalean) cyanobacteria 

 are abundant in turfs growing on the upper surfaces of all reefs that we 



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