RATE OF BIOTURBATIONAL DESTRUCTION OP SEDIMENTARY 

 STRUCTURES AS A FUNCTION OF ENVIRONIIENT 



In order to establish the role of environmental setting a series 

 of experiments was conducted at four localities (fig. 1) : (1) 

 unvegetated sand near the mouth of the channel through the reef 

 which extends southwest from the habitat site; (2) the unvegetated 

 sand strip that borders the reef; (3) a sand substrate approx- 

 imately 30 meters from the reef, where the bottom was moderately- 

 vegetated by algae, largely of the genus Udotea ; (4) sparsely 

 vegetated sand about 50 m northeast of the habitat in the patch 

 reef area. The biota at each site was not recorded. The texture 

 of the sand at the channel mouth differs from that at the other 

 sites by containing a far greater percentage of fine sand. The 

 channel-mouth site is distinctive in the abundance of fauna living 

 in burrows within the sand. 



At each site several experiments were monitored daily. To deter- 

 mine the rate at which wave or current-generated structures were 

 disrupted by organisms, a set of eight similated ripples, 5 cm high 

 and 100 cm long and 20 cm apart were shaped by hand in thp sand and 

 left to the activity of the organisms. An area of undisturbed 

 bottom approximately 4 meters square was photographed daily to 

 establish the rate of general surface modification. To determine 

 the rate at which internal structure near the sediment surface was 

 disrupted, bedding was artificially produced in a small plot 25 cm 

 wide by 100 cm long. This bedding consisted of 2 layers of dyed 

 sand, each 2 mm thick, separated and covered by layers of indigenous 

 carbonate sand, 3 to 5 cm thick. This structured sand was cored at 

 4-day intervals with small Senckenberg boxes, which provided un- 

 disturbed sections about 15 cm wide by 5 cm deep. 



The artificial ripples at each site became progressively modified 

 and finally totally obliterated. Differences in the degree and 

 style of modification among the sites were obvious after the first 

 day. At that point, the ripples in the channel mouth were covered 

 with 20 to 30 craters and mounds 2 to 5 cm across. In contrast, 

 the sand strip and patch reef showed only one or two mounds or 

 craters each, and the algae flat only niomerous small (less than 1 cm 

 across) pock marks. 



The ripples in each plot tended to follow a similar pattern of day 

 to day modification. At first they became subdued, their crests 

 becoming rounded and less high. In a second phase, part of the 

 ripple pattern became indiscernible, and ultimately none of the 

 pattern could be discerned. An exception to this pattern occurred 

 in the patch reef site, where the ripples were nearly totally 

 obliterated by a feeding stingray on the second day of the experi- 

 ment. A second ripple plot, which replaced the first in this 

 environment followed the general pattern. 



VI-25 



