Gamma rus oceanicus and the isopod Jaera sp. are examples of the former and the 

 amphipods Corophium lacustre and Gamma rus tigrinus and the isopod Cyathura 

 polita are examples of the latter. 



The number of species at a particular location depends on the season, 

 substratum, water column stability, pollutional stress, productivity, and 

 salinity. In silt-clay sediments in shallow areas of the Sheepscot estuary 

 Hanks (1964) found 104 species of invertebrates and Larsen (1979) later found 

 94 species. Over 450 species were found in the deeper, sandier portions of 

 the estuary (Larsen and Doggett 1978b). This translates to 19 species per 

 station in the shallow silt-clay areas and 51 species per station in the 

 deeper sandy areas. Comparative data are not available for other Maine 

 estuaries or for coastal marine waters. Only 27 species of invertebrates were 

 identified by Haefner (1967) in the Penobscot estuary, but it is not known 

 whether this low diversity of species is a natural condition or the result of 

 pollution. 



It appears that the Sheepscot estuary supports a diverse fauna. It is not 

 known whether this is typical of Maine estuaries or what factors are important 

 in maintaining this diversity, i.e., limited seasonal temperature and salinity 

 variation, high productivity, habitat diversity, etc. Densities of organisms 

 vary throughout estuaries in response to several ecological factors. Larsen 

 (1979) reports only 771 individuals/m in the shallow silt-clay areas of the 

 lower Sheepscot River estuary, whereas a mean of 4928 individuals/m2 was found 

 in a more comprehensive study comparing a mixture of shallow and deep water 

 areas by Larsen and Doggett (1978b). Highest densities of individuals (over 

 20,000 /m ) were found in the area where species diversity was lowest (upper 

 estuary; see figures 5-39 and 5-40). In these areas of reduced competition 

 and predation, the few species that are able cope with variable conditions are 

 able to proliferate. 



The Sheepscot Estuary is productive as well as diverse relative to other 

 temperate and boreal estuaries (table 5-6). It is not known if the same is 

 true of other Maine estuaries. 



Biomass (the weight of living animals) values for invertebrates in Maine are 

 available in Larsen' s report (1979) of his shallow water survey. This survey 

 covers a low density area. The values (mean at all stations =3.18 g/m^ ash- 

 free dry2weight; range = 0.59 to 12.4 g/m^) compare closely with the 0.1 to 

 4.0 g/m range reported in Chesapeake Bay by Mountford and coworkers (1977) 

 and the mean value of 4.25 g/m^ found by O'Connor (1972) in stations in 



Moriches Bay, New York. European values also fall in the same general range; 

 Raymont (1947) 4 g/m^; Buchanan and Warwick (1974) 3.98 g/m^ in 80 m mud, and 

 3.7 g/m (dry weight) in a shallow sandy sea loch (Mclntyre and Eleftherious 

 1968). 



Biomass levels in higher density areas (e.g., Sheepscot River and Mystic 

 River, Massachusetts; table 5-6) are undoubtedly considerably higher than 

 those listed above, especially in the species minimum area, where the 

 organisms are larger as well as more abundant. This is indicative of a high 

 secondary productivity and suggests that the potential exists for the support 

 of large populations on even higher trophic levels. 



5-85 



10-80 



