DISCUSSION 



Among others, Gail (1922), Gilmartin (1960), Klugh and Martin (1927) 

 and Larkum et al, (1967) have emphasized that light is a major factor 

 influencing the distribution and growth of subtidal algae, for both 

 the quantity and quality of illumination changes with increased depths. 

 Neushul (1965) has recorded shallow vegetational limits in turbid 

 (northern) locations as compared to deeper limits in clear (southern) 

 waters. Larkum et al. (1967) emphasize that light penetrates to a 

 much greater depth in clearer tropical waters than in northern coastal 

 waters, and that it is blue rather than green at such depths. A 

 comparison of the species composition and vertical distribution of algae 

 in New England and the Virgin Islands shows obvious differences. Fore- 

 most, the depths at which maximum species numbers were recorded in 

 the Virgin Islands corresponded to depths of minimaL species numbers in 

 New England. In addition, the subtidal flora of New England is 

 primarily composed of red and brown algae, while green and red algae 

 dominate in the Virgin Islands. 



Substrate availability is also an important factor limiting subtidal 

 vegetation, for it can determine both the quantity and types of 

 vegetation. In New England the deep subtidal zone (12 - 20m) is often 

 composed of a silty-sandy substrate, which restricts the potential 

 development of the flora when light is not limiting. The lower pro- 

 ductivity (biomass) and high frequency of epiphytism at the outer limits 

 of transect 2 in the Virgin Islands is probably also due to adverse 

 substrate, for it was very silty. The higher productivity of plant 

 materials on transect 1 was probably due to more suitable substrate. 



Neushul (1965) has discussed the importance of several other environ- 

 mental factors upon subtidal vegetation (the depth at which the thermo- 

 cline reaches the bottom, shading effects, tidal currents and grazing). 

 The effects of sea urchin grazing have been documented by several 

 workers (Jones &. Kain, 1967; Kain & Jones, 1966; Leighton, Jones & North, 

 1966; North, 1964), primarily in northern areas. Sea urchin grazing 

 may be extensive (particularly in areas of domestic pollution) and a 

 residual flora of crustose algae and a few hardy plants may only remain 

 (Mathieson, Dawes and Humm, 1969). 



A major factor influencing the species diversity and biomass of algae 

 in shallow subtidal waters in the Virgin Islands is fish grazing. Thus, 

 there is a diminutive flora in the vicinity (depth) of a coral reef, with 

 its abundant populations of herbivorous fish. In addition, there is a 

 conspicuous increase in plant materials with increased linear distance 

 (at least to 117 - 165m) from the reef. Limited information is available 

 in botanical literature concerning the interaction of fish grazing on 

 tropical algae (e.g., Dawson, et al.^ 1955; Gilmartin, 1960; Larkum, et 

 al., 1967). The fish studies of Hiatt and Strasburg (1960) have provided 

 a detailed account of the ecological relationships between fish and coral 



VI-127 



