it. For these reasons, great care was taken in determining water flow and 

 discharge rates (this information is provided in detail by Adey, et al . , 1981). 

 Our measurements of water motion demonstrate that water flowing over the reefs 

 oscillates but has a net shoreward flow. Typically, we recorded a landward 

 surge of 6-8 s duration (reaching speeds of about 0.3-0.6 m/s) and a reverse surge 

 lasting 2-4 s (reaching 0.05-0.3 m/s). Net epibenthic water flow was determined 

 from chart recordings of mean maximum currents in both directions (i.e., the 

 peaks of spikes) integrated over 1 hr intervals from Marsh-McBi rney current meter 

 data. Reversing wave surges and accompanying turbulence produced by waves breaking 

 over irregular reef surfaces mix the water flowing over the reef, particularly 

 for back reef habitats where measured rates of production are the greatest. The 

 turbulent effect of wave surges also significantly increases metabolite exchange 

 and resulting photosynthesis and growth (Hackney, et al . , in ms.). 



Epibenthic flow rates increase dramatically from fore to back reef. Means for 

 all reef transects were 1.2 + 0.066 m/min for fore reefs and 11.0 +_ 3.5 m/min for 

 back reefs. This increased flow rate results from the progressive shallowing from 

 fore reef (overall mean depth of 5.3 m) to back reef (overall mean depth of 0.6 m). 

 With a constant discharge rate, flow rate varies with depth. Since depth changes 

 daily with tidal changes, we recorded tidal heights during each sampling period. 



Discharge rates are not constant because wind-induced water movements change 

 geographically along the south shore of St. Croix due to the island's effect of 

 diminishing the strength of easterly trade winds. Specifically, in the fore 

 reef environments, Isaacs Reef to the east has the highest discharge rate (mean 

 8.1 m^/min/m, range 2.5-12.1 m^/min/m, N = 65 1 hr intervals). Robin Reef is 

 intermediate (mean 6.2 m3/min/m, range 2.8-9.8 m^/min/m, N = 86 1 hr intervals), 

 and Halfpenny Reef is the lowest (mean 5.3 m^/min/m, range 1.4-9.8 m^/min/m, N = 79 

 1 hr intervals). These discharge rates created mean back reef epibenthic flow 

 rates of 9.2, 8.8, and 15 m/min/m, respectively. Although a geological pattern 

 in exposure from east (most exposed) to west (least exposed) results in a 

 similar pattern of decreased discharge rates, the magnitude is much less than 

 the difference observed between back reef and fore reef at any one location. 



There is little support for the hypothesis that reef productivity is 

 controlled by water motion even though differences in flow rates between fore 

 and back reefs roughly correspond with differences in rates of production. If 

 reef productivity was primarily dependent upon water flow, diurnal reef produc- 

 tivity curves (figs. 4 and 5) would level off at the point where light saturation 

 occurred rather than corresponding closely with variations in light intensity 

 (discussed below). Also, for any given reef, productivity would correspond 

 with data on water currents, which we did not observe. Water motion is an 

 important factor, but the south shore of St. Croix is characterized by consis- 

 tently high wind-induced wave energy year round (see Adey, 19781b); thus, water 

 motion does not appear to limit productivity. Note that there is no relationship 

 between reef productivity (table 1) and epibenthic flow rates. 



Light 



Primary production on the reefs we studied is most likely controlled by 

 light. Evidence for this comes from the correspondence between diurnal and 

 seasonal cycles in light intensity (fig. 6) and measured rates of primary 



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