OCS Study MMS 95 - Data Center

eliable data of this type ; and photo surveys generally cannot provide an exhaustivemapping of individual communities . It is therefore difficult to determine whatconstitutes a chemosynthetic community in terms of management concerns . Does,for example, a single collection of clam shells signify a lush community? Likewise,in how much detail should offshore operators be required to search in order tocertify that their operations will not impact a lush community? One approach tothis issue is to examine the variable evidence for the typical length scales andgeological dependency of well-documented chemosynthetic communities .An analysis of clustering frequencies for tube worms at GC 234 foundsignificant clustering at spatial scales of about 5, 20, and 75 m (MacDonald 1990a),which may be indicative of clustering due to formation of tube worm "bushes" at thesmaller scale, and to the size and spacing of fault zones at the larger scales .Areview of the occurrence of chemosynthetic fauna along the submarine track-linestends to confirm a characteristic size of 10 to 100 m for vestimentiferan and mytilidcommunities and 100 to 300 m for clam communities .We therefore have supportfor speculation that communities separated by less than 300 m probably share acommon hydrocarbon reservoir. Multi-channel seismic data from the GC 184/185lease blocks (Cook and D'Onfro 1991) provided information on the spacing ofreservoirs and migration pathways at this site .From these data, it appears thatcommunities separated by distances of 1 km are not supported by seepage from acommon reservoir.2.3.2 Regional PatternsChemosynthetic fauna have been found in a 700 km-long corridor between88°W and 95°W and between the 290 and 2200 m isobaths . Distribution is uniform ;the greatest number of communities were found between 91°W and 93°W betweenthe 500 and 700 m isobaths .These observations have been influenced by the limits2-6