COMPARATIVE STUDIES OF SUBTIDAL VEGETATION IN THE 

 VIRGIN ISLANDS AND THE NEW ENGLAND COASTLINES 



Arthur C. Mathieson, Richard A. Fralick, 

 Richie Burns and William Flahive* 

 Department of Botany and the Jackson Estuarine Laboratory 

 of the University of New Hampshire, Durham, N. H. 



ABSTRACT 



A comparison of the species composition and vertical distribu- 

 tion of algae in New England and the Virgin Islands is given. 

 The depths at which maximum species numbers were recorded in 

 the Virgin Islands corresponded to depths of minimal species 

 numbers in New England. Light is considered to be a major fac- 

 tor causing the differential displacement of the two floras, while 

 availability of solid substrate is of secondary importance. 

 The subtidal flora of New England is primarily composed of red 

 and brown algae, while green and red algae dominate in the 

 Virgin Islands. 



Fish grazing is a major factor influencing the species diversity 

 and biomass of algae in shallow subtidal waters in the Virgin 

 Islands. A diminutive flora is available in the vicinity (depth) 

 of a coral reef, with its extensive populations of herbivorous 

 fish, and there is a significant increase in plant material with 

 increased linear distance (to 117-165m) from a reef. Reduced 

 grazing pressure was apparent at about 30m from a reef, and it 

 caused a corresponding increase in plant materials. A series 

 of cage experiments was conducted to exclude herbivorous fish 

 grazing. Although the experiments were of limited duration 

 (19 days) and the mesh screen employed (1.3cm2) was too large 

 to exclude many of the smaller fish, an increased diversity and 

 abundance of plants were recorded under two cages. Transplant 

 studies of selected plants were made from the sea grass beds to 

 the coral reefs near the habitat. Many of the plants were 

 rapidly eaten (particularly Halimeda incrassata ) by the reef 

 fish, and there was a general correlation between the linear 

 distribution of algae on the sea grass beds and the feeding 

 preferences of fish. Thus, selective grazing of plants may 

 occur. Even so, all of the species present from 2.4-30m from 

 a reef were exposed to heavy grazing pressure, and they exhibited 

 either an increase in size or abundance with increased linear 

 distance from a reef. 



*Richie Burns and William Flahive were support personnel for the 

 botanical studies of Mission 4-50. 



VI-106 



