dispersal capacities (Hutchinson, 1951; Margalef, 1963; Odum, 1969; 

 Connell, 1972). In the second phase, the community dominant was estab- 

 lished; i.e., Phoronopsis. Dominant species have a longer life cycle, 

 larger size, and superior competitive abilities (Margalef, 1963; Odum 

 1969; Connell, 1972) . 



Whether the colonized phoronid patch is a stable assemblage or merely 

 a stage in a long-term sequence of recolonization is unknown. The dominant 

 deposit feeders present before dredging and adjacent to the patch might 

 invade and successfully exclude the tube dwellers. There is a spectrum of 

 possibilities, and as mentioned earlier, it is just as likely that the 

 patchy nature of the slough bottom increases the number of possible se- 

 quences of recolonization and stable end products. In Buzzards Bay and 

 Cape Cod Bay, Massachusetts, studies have shown the persistent separation 

 of communities of deposit-feeding and suspension-feeding animals (Rhoads 

 and Young, 1970, 1971; Levington, 1972). Maintenance of this separation 

 is apparently due to sediment characteristics and the behavior and eco- 

 logical requirements of the two trophic groups. In this study, the sedi- 

 ments do not differ, but the adults of one trophic group may prevent 

 recruitment of the other. 



The similarity between the predredging and control conditions 1.5 

 years later is remarkable. It appeared to be a very stable deposit- 

 feeding assemblage, although there were changes in the species composi- 

 tion. In contrast, the recolonized harbor area was completely different, 

 in terms of species number, composition, number of individuals, species 

 diversity, evenness, and trophic dominance. 



There is a need for experimental studies to determine the importance 

 of physical phenomena, life history characteristics, and animal inter- 

 actions in benthic communities. If this information was available, the 

 relationship between community structure (spatial heterogeneity) and 

 temporal changes, including recolonization, could be adequately studied 

 for marine ecosystems. 



VII. SUMMARY AND CONCLUSIONS 



The benthic assemblages differed with changes in amount of sediment 

 movement or substrate stability. Many animals were characteristic of the 

 relatively stable submarine ridges but few inhabited the unstable terrace 

 slopes. Even fewer animals were found in channeled areas, and their num- 

 bers decreased with increasing sediment movement and accumulation. Accu- 

 mulation of sediment directly buried animals and seaweed. Burying the 

 animals and algae raises the decomposition rate, thereby decreasing 

 available oxygen and increasing hydrogen sulphide concentrations to toxic 

 levels. Sediment movement was the most important environmental factor 

 affecting the distribution of macroinvertebrates at the control area and 

 in the canyon. 



Dredging removed 60 percent of the original population of benthic 

 animals. After 1.5 years, the number of individuals was low but the 

 species diversity and evenness indexes were higher than before dredging. 



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