The unconsolidated bottoms of estuaries are submerged glacial deposits, 

 glacial deposits presently undergoing modification by existing marine 

 processes, deposits originating from present day marine processes (Swift et 

 al. 1971), and/or deposits originating from the fresh water inflow. In 

 general, unconsolidated bottoms in the estuarine system are similar to the 

 sand and mud bottoms discussed in the marine system (chapter 4) . These 

 bottoms consist of sediments ranging from pebbly gravel to mud, depending upon 

 the velocity of associated currents. Some channels, where channel bottom 

 velocities exceed 150 to 200 cm/sec, contain gravel (Timson, in preparation ) 

 but generally gravelly sand predominates. These channels result either from 

 headward erosion of erosion gullies in tidal flats or from modification of 

 ancestral riverine channels that were submerged during the Holocene 

 Transgression (Kraft et al. 1974). 



Macroalgal populations are poorly developed on subtidal unconsolidated bottoms 

 due to lack of suitable substrata for attachment. At greater depths or in 

 other areas where water movement is greatly reduced large stable stones may 

 support minor populations of small plants. Flux of organic matter on 

 unconsolidated bottoms is determined mainly by impact of detritus and 

 organisms from the water column. 



The vast majority of research on estuarine benthic invertebrates in Maine has 

 focused on unconsolidated bottoms in the Sheepscot and Penobscot River 

 estuaries. Studies on the Sheepscot include Stickney (1959), Hanks (1961, 

 1964), Dean and Ewart (1978), Larsen and Doggett (1978b), and Larsen (1979). 

 The benthos of the Penobscot has been examined by Haefner (1967), Dean (1970), 

 Ayer (1971), and Shorey (1973). The Sheepscot studies are the most 

 comprehensive . 



From the mouth of the estuary toward upstream, the salinity becomes 

 progressively lower and the fluctuation of salinity and temperature increases 

 until fresh water is reached. These changes produce a natural stress gradient 

 for the fauna and species with limited tolerances are restricted to the 

 oceanic end of the estuary. The more tolerant a marine species is, the 

 further it can penetrate towards fresh water. As a result of these 

 hydrographic conditions, fewer species are present in the upper estuary. This 

 was illustrated for a German estuary by Remane (1934) in figure 5-39. The 

 number of species is lowest at the upper end of the estuary, before fresh 

 water is encountered (salinity = 5 to 10 ppt) and increases both toward the 

 ocean and toward fresh water as environmental conditions become more stable. 

 However, the brackish water invertebrate species of marine origin that can 

 live at the reduced salinities in the middle reaches of estuaries flourish 

 because of the absence of severe competition and predation by more specialized 

 marine species. The species diversity and abundance of benthic invertebrates 

 in subtidal unconsolidated bottoms in Maine estuaries are described below. 



In the study of the Sheepscot estuary along an estuarine salinity gradient 

 from oceanic salinity to tidal fresh water Larsen and Doggett (1978b) found 

 species numbers to approximate the shape of Remane 's classic illustration 

 (figure 5-39 and 5-40). Differences between the two illustrations are caused 

 by abrupt hydrographic changes in the Sheepscot and the limited sampling in 

 the riverine (freshwater) system in the Sheepscot gradient study. Most 

 individuals found in the area of the "species minimum" in the upper Sheepscot 

 are members of typical intertidal or estuarine species. The amphipod 



5-84 



