417 



joining with those of another station. This occurred for each of the four sampling seasons. 

 The excellent comparability of replicate samples from the same station allows us to 

 detect small changes in benthic community characteristics. 



If the data from the six replicates for each station for each sampling period 

 are summed, it is possible to perform a cluster analysis on all the data at once. With one 

 exception the samples for the four seasons at each station cluster together before any 

 separation occurred between stations. The exception was the closely-spaced Stations 16 

 and 17 which had particularly similar community characteristics in February (M3). 



The species composition of the benthic infauna on the southern flank of 

 Georges Bank changed very little over the year and differences between sampling dates 

 were always less than differences between stations. If drilling activities had had a short- 

 term regional impact on the Bank, we might have expected to have seen some changes in 

 community characteristics at some regional stations between the first (predrilling) and 

 subsequent seasonal sampling periods. This was not the case. 



The stations tended to cluster together according to water depth, and to a 

 lesser extent according to sediment type (Figure 't). Station 15 at the top of the Bank and 

 Station 13 in the Mud Patch were the only two stations that were distinct from all t'ne 

 others. The remaining stations sorted into five groups: the eastern deep (IW-l^O m) 

 Stations 8, 16, 17, and 18; a western deep (100-250 m) grouping of Stations 6, 7, 9, and 12; 

 a low similarity fusion of the 80 m Station 11 with the 100 m Station 3; a 70-80 m 

 grouping of Stations 2 and 5; and a 60 m group of stations 1, <*, and 10. 



In Block ^10, Station 18 to the west of the rig site clearly was different from 

 Stations 16 and 17. For example, the amphipod Ampelisca agassizi was dominant at 

 Station 18 and rare at Stations 16 and 17. This can be attributed to the finer sedi-nents at 

 the former station compared to the latter stations. 



Exploratory drilling began in Block 'flO in July 1981, shortly after Cruise Ml, 

 and continued until the end of March, 1982. During this time, approximately 1,000 metric 

 tons of drill cuttings and 600-800 metric tons of drilling fluid solids were discharged. 

 Bothner et al. (1982) reported that concentrations of barium (presumably from drilling 

 fluids) in unfractionated surficial sediments at Station 16, 200 m from the rig site, 

 increased by a factor of 3.5 between 3uly, 1981 and May, 1982. Smaller increments in 

 sediment barium concentration were observed at Stations 17 and 18. Some evidence of 

 drill cuttings was observed in the gravel fraction of sediments at Station 16. The clay 

 size fraction of sediments from this station showed increases in concentrations of barium, 

 chromium, aluminum, copper, and mercury between pre- and postdrilling samples. 



12 



