Brenton-B, is not fully understood. This marked difference may be 

 related to subtle differences in relief (maximum depths in Brenton- 

 A exceeded those in Brenton-B by 3 m) , or the resolving capability 

 of the sonar system. More bedforms may have been present in 

 Brenton-A but, due to their size and/or orientation, may not have 

 been observed. Ripples, classified as forms with wavelengths <0.6 

 m, may have existed over surfaces that appeared featureless in the 

 sonographs. These forms are known to occur on the inner shelf in 

 response to oscillatory wave action. The threshold velocity for 

 formation is 10 - 20 cm/sec, depending on the grain size. Ripple 

 migration results in a reworking of the upper few to 10 cm of 

 sediment. 



With the exception of a few isolated gravel patches, the 

 surface texture at the Prudence site was a uniform, silty sand to 

 sandy silt (Table 3-8) . Seismic data and core analyses indicated 

 that this unit was greater than 8 m in thickness. Side scan data 

 suggested that there was much human activity in this area (trawl 

 marks, fish pots, etc.)- The moundlike features observed at the 

 site did not appear to be wave- or current-generated, and probably 

 were created when the site was historically used for dredged 

 material disposal. 



4 . 3 Benthic Community Structure 



A final objective of this study was to determine the 

 benthic community structure inside and outside the Brenton Reef 

 historic disposal site and compare the findings with those found 

 during the 1978 study. There appeared to be four distinct infaunal 

 communities at the Brenton Reef disposal site, related most likely 

 to gradients in sediment grain-size and organic matter. Stations 

 1 and 2 (Figure 2-4) consisted of fine-grained material derived 

 primarily from disposal operations and dominated by the amphipod A. 

 aqassizi , the bivalve N_;_ annulata , and the polychaete P. 

 quadrilobata . A transition through two different communities 

 associated with winnowed sand/ shell lag deposits was found as one 

 moves up the flanks of the mound to the mound center (stations 3- 

 8) . Community dominants of the polychaetes A. suecica, L. 

 fragilis, P. steenstrupi and T. annulosus gave way to the mobile 

 sand community dominated by the amphipod B. serrata . Moving off 

 the mound on to the ambient seafloor consisting of fine sand, the 

 highest densities of the amphipod A. aqassizi were found along with 

 N. annulata . P. steenstrupi , and the thalassinid shrimp A. 

 serratus . 



One problem with comparing results from the present 

 survey with historical data is the difference in sieve size used to 

 process the benthic samples. While the use of a 0.5 mm sieve (as 

 used in the present study) is the accepted standard today, one 

 practical drawback to using a sieve finer than 1 mm is the 

 tremendous increase in the number of individuals retained for 



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