Water Year 1960. Data were drawn 

 from the Chesapeake Bay Institute 

 Salinity Atlas (Stroup and Lynn 

 1963). Setting the chemical baseline 

 mostly involved nutrients. Data were 

 drawn from USGS files and from large 

 scale data banks from studies con- 

 ducted at various periods during the 

 1960's and 1970's. Biological data 

 were similarly selected from a com- 

 posite of studies during the 20-year 

 period, 1960 to 1980. An attempt was 

 made to put this data into a context 

 which defines the "health and pro- 

 ductivity of the estuary;" however, 

 after a discussion with other bay 

 researchers, it became clear that 

 these concepts could be defined only 

 relative to the baseline and not in 

 an absolute sense. 



SPECIES SELECTION 



Characterization of biological 

 impact in an estuarine system must 

 involve species at all major trophic 

 levels. However, the Chesapeake Bay 

 is estimated to contain over 2,500 

 species. Clearly some limitation is 

 necessary. 



A multi-step process was em- 

 ployed to select species which are 

 both important to the Chesapeake Bay 

 ecosystem and/or are sensitive to low 

 flow effects (Figure 2). Some organ- 

 isms which are not sensitive to flow 

 changes are too integral a part of 

 the ecological system not to be con- 

 sidered. 



From the immense universe of bay 

 species, a list of 167 candidate 

 study species was selected by assess- 

 ing from the literature the relative 

 vulnerability of any portion of the 

 species' life history to habitat 

 alteration and other criteria. These 

 were then reviewed by the anchor team 

 and Corps Steering Committee. A 

 second screening reduced the list to 

 81 species, based on availability of 



detailed literature on stress toler- 

 ance and ecosystem importance. The 

 final screening to 57 species was 

 conducted through use of comparison 

 matrices which compiled the sensi- 

 tivity of each species or any vul- 

 nerable life stage to specific habi- 

 tat alterations (i.e. salinity, food, 

 circulation, and substrate). 



The amount and quality of avail- 

 able data, the economic or social 

 value, and the competitive and pre- 

 datory or trophic relationships were 

 compiled from available literature 

 and discussions with researchers. A 

 weighted ranking system was then 

 employed to identify the most 

 important, most sensitive and best 

 researched of the study species. 

 These species were then used for 

 determination of tolerances, distri- 

 bution mapping, and conceptual and 

 simulation modeling. 



Chesapeake Bay, although a 

 relatively shallow estuary, supports 

 a large variety of species in various 

 habitats. These habitats range from 

 deepwater pelagic zones to beds of 

 submerged or emergent aquatic vege- 

 tation (Figure 3). In order to 

 explore the relationships by which 

 organism distributions are controlled 

 by environmental parameters, a class- 

 ification for habitat types was de- 

 fined. There have been numerous at- 

 tempts at estuarine habitat classi- 

 fication (Cowardin et al. 1977) for 

 various purposes. Since the low flow 

 study is mainly focused on salinity, 

 this was chosen as the major variable 

 in the habitat classification system 

 used. We employed a minor modifi- 

 cation of the Venice System 

 (Symposium on the Classification of 

 Brackish Waters 1959) in which the 

 mesohaline category was divided 

 into upper-and-lower mesohaline as 

 follows (Figure 4): 



131 



