This section deals primarily with the 20-raeter canyon station, which 

 received most of the dredged material. It describes the effect of dumping 

 on an assemblage adapted to conditions of natural sediment movement and the 

 details of recovery. 



Stations (Fig. 1) were marked with permanent buoy systems (Slattery 

 and Oliver, 1972). Major disposal occurred in August 1971. Twelve 

 samples of eight cores each were taken between July 1971 and April 1973. 

 Two predisposal samples of eight replicate cores each were taken on 14 

 July 1971, the first postdisposal samples on 6 September 1971, and the 

 last two samples on 4 April 1973. Sample replication in July and April 

 is presented as a measure of spatial variation within the 20-meter sta- 

 tion. Sampling dates are shown in Figure 12. 



The benthic assemblages at the 20-meter station were dominated by two 

 species of capitellid polychaetes, Heteromastus fi-tobradhus and Cccpitetla 

 oapitata. The C. capitata were mostly large adults. Heteromastus 

 filobrachus were commonly found in the longshore submarine canyon by 

 Hartman (1963) . Both animals are deposit feeders that do not maintain a 

 permanent burrow system. Permanent tube or burrow dwellers present were 

 the polycheates, M. saaoulata and N. elegans. Mollusks and crustaceans 

 were scarce (one or two per core). Thus, the benthos consisted primarily 

 of an active burrowing deposit-feeding assemblage of polychaetes. 



Approximately 0.26 meter of dredged material was deposited at the 20- 

 meter station. Fine sands were dredged and dumped first; muds were last. 

 Median particle size of the bottom sediment changed from 0.212 millimeter 

 before disposal to 0.111 millimeter soon after disposal (September 1971), 

 total carbon from 0.13 to 0.64 percent by weight, sorting from 0.51 to 

 0.97, and silt from 07.8 to 22.2 percent. In November 1971, 0.26 meter 

 of sand moved over the station from shallow water. Most of the finer 

 dredged material was transported to deeper water, but some was covered by 

 sand. A mud layer of varying thickness was present below this. The layer 

 was not observed 2 months later. Changes in topography caused by the 

 disposal probably allowed more sand than usual to move over the station. 

 Sediment parameters changed little during the remainder of study. Average 

 median particle size was 0.125 millimeter, total carbon 0.12 percent, sort- 

 ing 0.45, and silt 8.5 percent. Bottom surface level was relatively stable 

 during the 1972-73 winter. 



Disposal caused approximately a 60-percent reduction in the number of 

 individuals. No large C. oapitata were found in September 1971, but a 

 number of small individuals had settled. By October 1971, the density of 

 small C. cap-itata was 200 per core. Cap-itella capitata also settled in 

 large numbers at the dredged harbor station in October 1971 and in the 

 settling jars in October 1972. In November 1971, they were found in dense 

 patches that caused the high variation in the data that month; by January 

 1972 they were gone. 



Ca:pitella oapitata is a well-known opportunistic species which appears 

 in disturbed areas where competition is low (Hutchinson, 1951; Margalef, 



43 



