Table 5-6. Average Density of Estuarine Invertebrates in Unconsolidated 

 Sediments of Temperate and Boreal Estuaries and Bays^ 



Location 



Mean number 

 individuals/m 

 (sieve size 1mm) 



Source 



Sheepscot River Estuary, Maine USA 

 Gradient Study 



Shallow Water Study 



Mystic River, Massachusetts USA 



Moriches Bay, New York USA 



Delaware Bay, Delaware USA 



False Bay, South Africa 



Gullmarsf jord , Sweden 



Lambert Bay, South Africa 



4928 

 771 

 3000 

 1300 

 722 

 2220 

 4198 

 1153 



Larsen and Doggett 



(1978b) 

 Larsen (1979) 



Rowe et al . (1972) 



O'Connor (1972) 



Maurer et al . (1978) 



Field (1971) 



Rosenberg (1973) 



Christie (1976) 



Modified from Maurer et al. 1978 a & b. 



Seasonal variations in invertebrate density along the estuarine gradient are 

 minimal at the oceanic end, where environmental factors are most stable, and 

 increase upestuary where physical conditions vary more seasonally. The few 

 species that dominate in the upper estuary undergo extreme seasonal population 

 fluctuations. For example, near the head of the Sheepscot Estuary, the 

 crustaceans Gammarus tigrinus , Corophium lacustre , and Cyathura polita account 

 for over 90% of the individuals present. Their density is 19,603/m^ in the 

 summer. This drops to 13,l40/m2 in the fall, and to 713/m2 in the spring. 

 Spring is the time of lowest salinities and lowest temperatures. 



Larsen (1979) compared a 1973 survey in the lower Sheepscot with one done by 

 Hanks (1961 and 1964) in 1955. During the intervening 18 years significant 

 changes appeared to occur in mean density, dominant species, species 

 composition, and distribution of species within the study area (Larsen 1979). 

 It is not known if these apparent changes are associated with a long-term 

 environmental trend or if they are simply random fluctuations around a mean 

 condition. 



Most of the above discussion is based on the Sheepscot estuary as data are 

 available. As stated in the hydrography and nutrients discussions, 

 considerable difference exist among the physical characteristics of Maine 

 estuaries. Many of these characteristics, such as flow rate, tidal exchange, 

 sill depth, low tide volume, intertidal area, and suspended load, govern the 



5-87 



10-80 



