analysis also documented this gradual shift. The fauna assemblage in the 

 sound at Duck is not unique. The same species occur in similar community 

 groups in other estuarine systems (Diaz, 1977). 



b. Comparison between 1976 and 1981 . Matta (1977) surveyed the Duck 

 sound in 1975-76. The present study occupied the same transects and stations 

 as Matta and also employed the same methodologies. This facilitates the 5- 

 year comparison between the two data sets. Matta's (1977) data will be 

 continually referred to as the early study or the 1976 conditions. 



The water temperature appeared higher in 1976, but the only month where 

 samples were collected in 1976 and 1981 was July. In July 1976 the water 

 temperature was 7° Celsius higher than the sampling date for 1981. This is 

 well within the monthly range of temperature that is expected for that period 

 of the year. Salinity was lower in 1976, being about 2 parts per thousand in 

 October 1975 and March 1976, and about 3 parts per thousand in May and July 

 1976. In 1981 salinity went from about 3 parts per thousand in November 1980 

 to 8 parts per thousand in July 1981. This is a substantial increase in 

 salinity considering the sensitivity of the low salinity fauna to changes 

 in salinity (Leppakoski, 1975; and Diaz, 1977). The overall depth at the 

 transects increased from 1976 to 1981 by about 0.25 meter at many sites. 

 The sediment organic and carbonate content were slightly lower than in 1976, 

 but since 1981 values were very low, this is probably not an important change 

 to the fauna. The mean grain size was similar between studies, except in May 

 1976, when there appeared to be exactly a 1-phi decrease in grain size at 

 all sites sampled. By July 1976 the mean grain size had increased from 2.3 

 to 2.5 phi, the approximate model value for both studies. Sorting was similar, 

 but skewness and kurtosis were greater in 1976. The major change in the 

 physical environment between 1976 and 1981 was the increased salinity. The 

 other changes (depth, skewness, and kurtosis) probably did not have any 

 biological significance. 



In comparing the fauna (only annelids, mollusks, crustaceans, and 

 chironomids) the qualitative similarity (Jaccard's coefficient) is 0.73, 

 which is a moderate overall resemblance of the fauna from 1976 to 1981. 

 If the comparison is made on the major taxonomic level, then the similarity 

 between studies is 0.73 for annelids, 0.40 for mollusks, and 0.80 for 

 crustaceans (Table 28) . Chironomids cannot be compared since they were 

 not speciated in 1976. All polychaetes from 1976 occurred in 1981, plus two 

 other species {Polydora ligni and Streblospio benedicti) . Among the 

 oligochaetes there seemed to be a complete change in species. Matta (1977) 

 reported finding Peloscolex sp. and Lumbriculus sp. In the 2 to 3 parts per 

 thousand salinity range, which existed in 1976, the only oligochaetes known 

 to commonly occur are tubificids in the genus Limnodrilus and Tubificodies 

 (= Peloscolex) heterochaetus (Diaz, 1980). Most Lumbriculus species are 

 robust worms that resemble earthworms and prefer high organic sediments. 

 The samples from the sound are not the correct habitat for Lumbriculus sp. 

 Considering the possible differences between the 1976 and 1981 identifications, 

 oligochaetes will be compared only as a higher taxon. The only mollusk in 

 common between 1976 and 1981 was Rangia cuneata. Two other freshwater forms 

 (Physa sp. and Ferrissia sp.) disappeared and Macoma sp. , an estuarine species 



41 



