effects on certain estuarine species. They largely control the distribution 

 and seasonal abundance of many species. 



Jeffries (1967) has classified copepods into five groups according to their 

 ability to reproduce along a salt-concentration gradient: (1) fresh water 

 species; (2) true estuarine (brackish water) species; (3) estuarine-marine 

 species (widely distributed in an estuary and to a limited extent in open 

 coastal waters); (4) euryhaline-marine species (coastal and mouths of 

 estuaries); and (5) stenohaline marine species (open water). Representatives 

 from each group form a chain of overlapping species populations along the salt 

 gradient from fresh water to fully marine conditions. The exact relations to 

 salinity are determined in part by the specific nature of the embayment and 

 its flushing mechanisms. Correlations between salinity and species 

 distribution often are not immediately clear, because the density of a 

 planktonic population in an estuary also is influenced by conditions up and 

 downstream from the point of observation (Jeffries and Alzara 1970). 



Benthic Invertebrates 



Each benthic invertebrate species has certain physical and chemical 

 requirements for survival. Chief among these are suitable temperatures, 

 salinities, substrata, and currents. 



The differences in water temperatures and ranges of water temperature between 

 the northeastern part of the characterization area (regions 5 and 6) and the 

 southwestern regions (1 to 4; see "Hydrography," page 4-llin chapter 4) is a 

 major factor controlling which species inhabit these areas. Although much 

 overlap exists and the boundary is not distinct, a broad zoogeographic 

 boundary appears to exist in the area of 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 north 

 of region 4. 



Many of the species with southern affinities need a certain minimum 

 temperature to spawn and occur only where these temperatures are reached with 

 sufficient frequency. Maine has certain warm-water pockets, such as the upper 

 Sheepscot Estuary (region 2) , where summer water temperatures are high enough 

 to support populations of 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. 



Salinity has a profound effect on the distribution of estuarine benthic 

 invertebrates. This is discussed in detail below in the class level 

 discussion of unconsolidated bottom but, in general, species diversity 

 decreases from the mouth of the estuary upstream to fresh water. 



Substratum requirements vary among species. Some species live on a variety of 

 substrata, while others are quite specific in their requirements. Bedrock, 

 boulders, and mollusc shells, are suitable habitats for animals that must 



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