Small-Scale Spatial Variation in Light Available to Coral Reef Benthos: Quantum Irradiance Measurements from a Jamaican Reef

Multidirectional quantum irradiance measurements performed at various localities on the fringing reef at Discovery Bay. Jamaica, made it possible to characterize in detail the light micro-environment of the benthos. Waters of the open fore reef and the sheltered bay had significantly different light transmission properties, with average attenuation coefficients of 0.06 m⁻¹ and 0.11 m⁻¹, respectively, in the upper 27 m. However, the relative amount of light incident from different directions was the same for both sites, and remained constant in the depth range surveyed. Details of substrate type, slope, and exposure had a marked influence on irradiance patterns at any locality, sometimes equivalent to large differences in depth. Bottom reflectance averaged 18% for sandy substrates and 5% for surfaces of living coral. Vertical surfaces received about 25% of the irradiance of horizontal surfaces. Canopies of the branching coral Acropora cervicornis diminished irradiance immediately below by nearly half.

These results indicate that light distribution on coral reefs is very heterogeneous at the microhabitat level. This spatial heterogeneity must be taken into account in studies of benthic photosynthetic organisms.