The cluster analyses indicate a complex pattern of change through time. 

 There was a shifting of the community structure that was in part due to the 

 seasonality and increasing salinity. Station groups 1 and 2 represent the 

 dominant fauna throughout the seasons. They are also the stations where 

 species patterns did not seem to vary. If these two station groups are then 

 considered constants, there is a progression of the other four station groups 

 through time. Station group 5 was most typical of November, group 4 was 

 typical of January, group 5 was typical of April, and group 6 was typical 

 of July (Table 26) . The faunal changes that occurred at these four station 

 groups coincide best with the salinity. However, station group 4 does seem 

 to reflect the severe water conditions of low abundance and species 

 diversity. 



IV. DISCUSSION 



1. Ocean Beach . 



a. Present conditions. The physical environment at the Duck ocean beach 

 is typical of the high energy, sand beaches found along exposed ocean coasts. 

 The sediment is graded from coarse sand in the swash zone to finer sand 30 

 to 60 meters from shore. The bottom is very unstable, creating a physically 

 dominated environment. The fauna inhabiting these high energy beaches, which 

 are very extensive environments along the Atlantic and Gulf of Mexico coasts, 

 while not diverse, has been remarkably successful in coping with unstable 

 substrate caused by high kinetic wave energy (Croker, 1967; Dexter, 1976; 

 Holland and Dean, 1977; Shelton and Robertson, 1981). Emerita spp., Donax 

 spp., and haustorid amphipods are three dominant groups of species that 

 inhabit ocean beaches all around the world (Pearse, Humm, and Warton, 1942; 

 Dahl, 1952; Dexter, 1972, 1974, 1976; Diaz, 1980). These three taxa were 

 also dominant at Duck, accounting for 33 percent of all taxa and 71 percent 

 of all occurrences (Table 10). 



Emerita talpoida was the only species at Duck, North Carolina, to be 

 common in the swash zone. It also exhibited a seasonal migration off the 

 beach to deeper water in the winter and back to the beach in spring and summer. 

 While the Emerita talpoida was not separated by sex, its pattern of recruit- 

 ment seemed to follow the sequence as described by Diaz (1980) and Bowman 

 (1982) for the Duck, North Carolina, beach and for Bogue Banks, North Carolina, 

 populations. At Duck, North Carolina, the highest numbers of immature 

 individuals, summer of 1980 recruits, occurred in November 1980. In January 

 1981 they were not present in the sampling area probably having moved 

 farther offshore than the transects. This offshore migration was documented 

 by Bowman (1982). By April they were moving back to the beach and increasing 

 in size. In July there were numbers of large ovigerous females. 



The Donax spp., which may reach densities as high as 60,000 per square 

 meter (for comparison 1,392 in 0.023 per square meter) (Mikkelsen, 1981) on 

 other beaches, was not consistently dominant at Duck. This species only 

 occurred in any great number in November 1980 and then only subtidally and 

 not intertidally as most other Donax spp. populations (Pearse, Humm, and 

 Warton, 1942). The subtidal occurrence of Donax spp. at Duck may be related 

 to the coarse grain size and the steep slope of the beach (Edgren, 1959) . 



35 



