Atnpelisca spp. 



These suspension-feeding amphipods inhabit 

 sediments from fme sands to muds and are com- 

 mon to shallow subtidal communities from the 

 Gulf of St. Lawrence to the Gulf of Mexico. 

 Atnpelisca spp. (including Ampelisca ahdita, A. 

 vadorwn, A . verrilli) are common members of 

 topographic depressions of the Middle Atlantic 

 Continental Shelf, and do not enhance materials 

 exchange because they live at the sediment surface 

 and actually tend to reduce sediment resuspension 

 and hence geochemical exchange (Boesch 1979). 



Ampelisca spp. were dominant components of 

 the IN community during 1986-87, and were also 

 present in previous years, although densities were 

 typically low ( < 10/core) and large year-to-year 

 fluctuations in density were common (low BIV's). 

 In 1986 and 1987, quarterly densities of this taxon 

 ranged from 2 - 228/core (Fig. 12 S). Except for 

 March 1986, abundances over the last two years 

 were consistently higher than those obtained from 

 March 1979 - June 1984 and of similar magnitude 

 to those obtained from September 1984 through 

 June 1985. 



Species diversity values during 1986 and 1987 

 generally fell within the range established by pre- 

 vious studies (NLISCO 1987). In 1986 and 1987, 

 diversity was generally lower than 1985, a year 

 during which relatively higher species number and 

 evenness occurred. At three of the monitoring 

 stations (GN, JC, and IN), lower H' and J over 

 the last two years were attributable to the large 

 increase in the abundance of Mediomastus 

 ambiseta and Leptochcirus pinguis. Diversity at 

 EF was within the range observed in previous 

 years, but density and number of species in 1986 

 and 1987 were low compared to 1984 and 1985. 



Cluster Analysis 



Cluster analysis of annual subtidal species abun- 

 dances (Fig. 13) showed two major station/groups: 

 Group I included all Intake collections and Group 

 II contained all remaining samples. The low sim- 

 ilarity of IN to other sampling stations is due to 

 large differences in species composition and abun- 

 dance. For example, the infaunal communities 

 collected in 1986 and 1987 at IN included high 

 numbers of ampelisciid amphipods, species which 

 are present, but are not usually among the nu- 

 merical dominants at other stations. 



Over the eight year sampling period, a significant 

 increasing trend has occurred in the abundance 

 of these species principally due to the large in- 

 creases observed in each of the last three years. 

 Annual mean abundances in 1986 and 1987 were 

 significantly higher than those obtained from 

 1980-1984. However, neither year was significantly 

 different from 1985. 



Species Diversity 



Mean species diversity (IF) of subtidal commu- 

 nities during 1986 and 1987 ranged from 2.9 - 4..3, 

 evenness (J) from 0.5 - 0.7 and total number of 

 species (S) from 46 - 82 (Table 6). In both 

 sampling years, highest diversity and evenness and 

 lowest number of individuals were collected at 

 EF. Highest species number occurred at GN in 

 both sampling years. 



The IN grouping was further divided into a 

 group containing 1984-87 collections and one in- 

 cluding 1980-8.3 collections. This separation re- 

 flected a change in the species composition which 

 occurred after 1983. The 1984-87 collections 

 shared similarly high densities of amphipods, 

 molluscs, Mediomastus ambiseta and Owenia 

 fusiformis, and overall lower numbers of other 

 annelids, particularly oligochaetes. 



Group II included all other station/years which 

 separated into spatial groups (Subgroups A, B, 

 C). The 1986 and 1987 collections at GN, EF 

 and JC formed couplets within each of their re- 

 spective subgroups which then linked to remaining 

 years. The lower similarity between 1986-87 col- 

 lections and those of other years reflected not 

 only interannual shifts in population abundance 

 but changes in dominance structure as well. 



108 



