419 



There was no evidence of a biological impact that could be attributed to the 

 accumulation of drilling fluids and cuttings in sediments around the rig site in Block 410. 

 Some species of polychaetes and amphipods showed increases in abundance in November 

 and February followed by a decline in May. Others showed declines in November followed 

 by a large increase in February. Gravid females and recently hatched young of Ampelisca 

 agassizi were abundant in February at Station 18. 



Seasonal densities of several species at Station 13, the Mud Patch, showed an 

 increase from July to November or February, followed by a sharp decline in May. These 

 population fluctuations probably represent a sequence of normal seasonal settlement and 

 mortality patterns. 



6.5 Population Patterns: Site-Specific Stations 



All site-specific stations and sampling dates could be clustered at once using 

 NESS at 200 individuals. The clearest separation occurred between Station 5-29 and the 

 rest of the site-specific stations. This was due in part to the fact that at Station 5-29, 

 where sediments were finer-grained, the amphipod Ampelisca agassizi was more abundant 

 and the amphipods Erichthonius rubricornis and Unciola inermis were less abundant than 

 at the other site-specific stations, regardless of season. With the exception of Station 5- 

 28, the station farthest east or upstream of the drill site, all site-specific stations showed 

 some seasonal changes in community characteristics. 



Bothner et al. (1982) reported increases in barium concentrations in bulk 

 sediment samples from several site-specific stations between the first and fourth cruises. 

 Drilling actually started in Block 312 on December 8, 1981, shortly after Cruise M2 and 

 continued to June 1982, shortly after Cruise Mft. As much as 900 metric tons of drilling 

 fluids and 1,000 tons of drill cutting may have been discharged during this time. The 

 ' major increase in sediment barium concentration occurred between the February and May 

 cruises. Drill cuttings were observed in the gravel fraction of sediments at Station 5-1. 



In an effort to determine if these accumulations had a biological impact, 

 abundances of several species were compared over the four seasons at those near-field 

 stations showing the largest increment in sediment barium concentration (Stations 5-8, 5- 

 2 and 5-1), those downcurrent stations showing moderate increments in barium (Stations 

 5-10 and 5-25), and upcurrent stations where there was no evidence of drilling fluid or 

 cuttings accumulation (Stations 5-28 and Regional Station 2). At stations near the rig 

 site, there was a decrease in the number of individuals per sample from July to November, 



