In situ light data were averaged by burst, and burst means were paired with measurements of surface light at the same time to calculate the percent transmission to 1-m depth (TPAR-19). These values underestimate water column light extinction when the sun is low in the sky and light is strongly reflected from the seawater surface and, therefore, daily transmission calculations used averages of several measurements bracketing noon. At these times, the sun was approximately overhead, and mo

In situ light data were averaged by burst, and burst means were paired with measurements of surface light at the same time to calculate the percent transmission to 1-m depth (TPAR-19). These values underestimate water column light extinction when the sun is low in the sky and light is strongly reflected from the seawater surface and, therefore, daily transmission calculations used averages of several measurements bracketing noon. At these times, the sun was approximately overhead, and most light entered the water rather than being reflected. In the first and second deployments, TPAR-19 was based on measurements at 10:30 hrs, 12:00 hrs and 13:30 hrs; in the fourth deployment, TPAR-19 was based on measurements at 11:00 hrs, 12:00 hrs, and 13:00hrs; in the fifth deployment, TPAR-19 was based on measurements at 11:45 hrs, 12:45 hrs and 13:45 hrs; and in the sixth deployment, TPAR-19 was based on measurements at11:00 hrs and 13:00 hrs. Daily TPAR-19 values were summarized by month as the maximum, minimum, mean, and SE, and their frequency distributions explored by season. Light intensities at 19.1-m depth were also used to calculate the diffuse attenuation coefficient for PAR (Kd-PAR) using the equation representing the Beer-Lambert Law:

Ed (Z) = Ed (O-) e-Kd × Z

where Ed(Z) is the downwelling irradiance at Z m depth, Ed(O-) is downwelling irradiance just below the surface of the seawater, and Kd is the diffuse attenuation coefficient for downwelling irradiance. Ed(O-) was approximated from surface PPFD, which shows ~96 percent transmission through the air-sea interface when sun altitudes are high (greater than 46 deg) and wind speeds are low (less than 5 m s-1) (Gregg and Carder 1990). This method of calculating Kd is prone to larger variance that the more standard regression approach using downwelling irradiances measured in quick succession at multiple depths (Kirk 2011),but it allows a time-series of Kd values to be obtained using modest equipment resources. Calculations of the possible magnitude of this variance suggested Kd would vary +/- 5 percent if Ed(Z) varied +/- 10 percent. Assuming seawater in Great Lameshur Bay is vertically homogeneous with regard to the factors affecting downwelling irradiance, Kd-PAR can be used to calculate PPFD by depth, although assigning causation to variation in Kd-PAR is problematic due to its summative origin in the scatter and absorption of water, dissolved pigments, photosynthetic biota, and inanimate particulate matter (Kirk 2011). Surface light was integrated over each day after excluding values less than 3 μmol quanta m-2 s-1 (i.e.,effectively darkness), and averaged by month to characterize the daily availability ofPAR (mol quanta m-2 s-1). Finally, to evaluate the biological implications of Kd-PAR for St.John, equivalent values for seawater were compiled from the literature.

The objective of this paper is to describe the underwater light environment on a coral reef over scales of time that have relevance to understanding biological processes mediating coral reef community dynamics. While it is beyond the scope of this study to comprehensively explore such effects, or to develop a bio-physical model with which they can be integrated, it is valuable to a consider a simple case in which variation in underwater light intensity could affect reef corals. We develop this case to consider the effects of light on the energetic status of a symbiotic coral through the balance between gross photosynthesis and aerobic respiration as described in one study using the common Caribbean coral Porites porites from 10-m depth on the fore reef of DiscoveryBay, Jamaica (Edmunds 1986). We present the outcome of these calculations, which use published values for aerobic respiration and the hyperbolic tangent function relating light intensity to gross photosynthesis, with the present study generating the light intensities inserted into this function. Daily gross photosynthesis for P. porites was equated to the 24 h aerobic respiratory demand (Edmunds and Davies 1986), and with a currency of Joules, gross photosynthesis was calculated using the hyperbolic tangent describing photosynthesis as a function of light intensity (with 5-min resolution), and dark aerobic respiration over 24 h (see Table 2 in Edmunds and Davies 1986). In situ daily light was calculated as 12 h sine curves with a maximum irradiance corresponding to the maximum surface irradiance attenuated using Kd (determined empirically in the present study) to the value at 10-m depth. With this method, the quotient p12h grosss/R 24h provides a rough indication of the capacity for photosynthesis by endosymbiotic Symbiodinium algae to meet the daily energy requirements of aerobic respiration; values greather than or equal to 1 suggest energy surplus to the daily needs might be produced. While use of the quotient p12h grosss/R 24h to evaluate the energetic status of corals has important limitations, and has been superseded by more sophisticated and accurate approaches (Lesser 2013), in the present case it served as an effective measure of the relative impacts of differing light regimes on coral energetics.

BCO-DMO Data Processing Notes:

-Changed month names to be full names instead of abbreviations
-Added underscores to column names
-Combined rainfall and surface light tables from "fig 1" in paper