or, in some instances, survive (Bertrand 1977). Water currents carry the 

 zooplankton and determine their distribution. 



Benthic invertebrates . Each benthic invertebrate species has certain 

 physical and chemical requirements to survive. Chief among these are suitable 

 temperatures, salinities, and substrata. Currents directly and indirectly 

 affect benthic populations also. 



The difference in water temperatures and temperature ranges between the 

 eastern regions of the characterization area (5 and 6) and the western regions 

 (1 to 4; see "Spacial Variability of Coastal Waters" above) is a major factor 

 in determining which benthic invertebrate species inhabit these areas. 

 Although it is not distinct and includes much overlap, a zoogeographic 

 boundary (species' range limit) appears to exist in region 4. Many 

 invertebrates in the northeast (regions 5 and 6) have northern affinities and 

 do not occur or have reduced occurrences west of region 5. In the southwest 

 (regions 1 to 4) many species have southern affinities and are not found or 

 rarely are found northeast of region 4. 



Many of the species with southern affinities need a certain minimum 

 temperature to spawn and tend to occur only where these temperatures are 

 reached with sufficient frequency. Maine has warm pockets of water, such as 

 the upper Sheepscot estuary (region 2) , where the summer water temperatures 

 are high enough to support species such as the American oyster and xanthid 

 crabs, which have their centers of abundance much farther to the south. 



Less variable environmental conditions in the eastern regions (5 and 6) may 

 allow for the development of diverse benthic communities. Eastern Maine and 

 the Quoddy region are the most environmentally stable areas in the 

 Northeastern United States. 



Except at the mouths of large estuaries (e.g., Kennebec, region 2 and 

 Penobscot, region 4), where freshwater discharge is present, salinity does not 

 appreciably affect marine benthic communities. The effect in areas of runoff 

 is described in chapter 5, "The Estuarine System." 



Substrate requirements vary with species. Some species can live on a variety 

 of substrates, while others are quite specific in their requirements. Bedrock 

 and boulders are suitable habitats for animals that must attach to a stable 

 surface. Sedimentary environments are inhabited by animals that burrow into 

 the substrate. 



Water currents supply food to the benthos through the transport of 

 phytoplankton or detritus. They likewise supply respiratory gases, remove 

 some waste material and act as a dispersing medium for pelagic larvae. Areas 

 with currents often have increased productivity, probably because of the 

 increased food supply (i.e., more phytoplankton passing by). Large numbers of 

 mussels and associated animals are usually found in these high energy areas. 



Benthic invertebrates living in the intertidal zone also have interactions 

 with physical factors that are unique to life in this zone. Tides, the most 

 important factor, bring life-supporting materials to the animals. They also 

 remove wastes and disperse larvae. 



4-56 



