astreoides colonies on Curacao reefs to measured changes in the distribution of 

 underwater radiance, and who observed the expected morphological changes in 

 transplanted Montastrea annularis in computer simulations of coral growth based 

 on empirical measurements of light distribution and on simple assumptions about 

 the effects of liqht on calcification. 



As part of a larger study of the ecology and variation of the genus Porites 

 (Brakel, 1976), I collected detailed information on the colony shape and distri- 

 bution of Porites astreoides on Jamaican reefs. These data were supplemented by 

 relatively simple, but objective and repeatable, estimates of two important depth- 

 related variables, water movement and irradiance. The results presented below 

 reveal that the generally recognized depth-related morphological trend is more 

 complex than expected. 



METHODS 



Study area 



Observations and collections were made on the north coast of Jamaica at 

 Discovery Bay. The structure and zonation of the reefs of this region have been 

 described by Goreau (1959) and Goreau & Goreau (1973). I selected 7 different 

 reef habitats as study sites, encompassing a depth range of 27 m (Fig. 1). Four 

 sites were located on the exposed seaward side of the reef crest; 3 additional 



were situated in the sheltered, more turbid waters leeward of the fringing 

 At each study site the coral fauna was surveyed in quadrats along transect 



sites 

 reef, 

 lines 



perpendicular to the slope of the 

 following features were recorded: (1) 

 colony was growing; (2) the exposure of 

 from (fully exposed, open to water on 

 crevice); and (3) the shape of the colony, 

 measurements described below. 

 Irradiance estimates for each colony 

 Light measurements were made at all 



reef. For every Porites encountered, the 

 the slope of the substrate on which the 

 the colony, measured on an arbitrary scale 

 all sides) to 3 (in a deep hollow or 

 based on a series of morphological 



Fig. 1. Study sites at Discovery Bay, 

 Jamaica. Depth contours are in meters 



sites using a diver-operated Lambda Instru- 

 ments L192S underwater quantum sensor 

 coupled to an LI 85 quantum/radiometer/ 

 photometer mounted in an Ikelite 5910 

 underwater housing. In addition to down- 

 ward irradiance taken with the cosine 

 sensor pointed straight up, measurements 

 were also taken at various angles from the 

 vertical in the four cardinal directions, 

 providing data on the 3-dimensional 

 angular irradiance distribution. Results 

 relevant to this paper are summarized 

 below; complete details of the methods and 

 statistical analyses are provided else- 

 where (Brakel, 1979). Regression 

 equations were obtained to describe the 

 attenuation of light with depth. Two 

 equations were necessary, since the water 

 column in the bay (sites 1-3) had a light 

 extinction coefficient significantly 

 different from that of the fore reef 

 (sites 4-7): 



In I = 4.31 - 0.11 d (bay) 



In I = 4.20 - 0.06 d (fore reef), 



22 



