years of contrasting salinity regimes 

 in the upper bay. The species which 

 inhabit the bay are divided into 

 groups with certain tolerances which 

 govern their distribution (Figure 

 14), ranging from those limited to 

 marine waters, to those limited to 

 fresh water, either for their entire 

 lifetimes or for critical stages such 

 as spawning. 



The conceptual or CBEM models 

 can be used as tools to study the 

 effects of new organisms introduced 

 into a given segment of the bay or 

 tributary as salinity regimes change. 

 For instance, CBEM has been run under 

 conditions of higher salinity simu- 

 lating low flow conditions in the 

 Patuxent. The simulations involved 

 the introduction of predators such as 

 the sea nettle, which are not 

 normally present under average flow 

 conditions. Other factors such as 

 respiration changes, abundance 

 changes, etc. were also altered to 

 conform to the expected changes due 

 to low flows. The biological inter- 

 actions which were then observed were 

 checked against data from higher sa- 

 linity zones and some data from a pre- 

 vious drought period and shown to be 

 within a reasonable range of values 

 for such conditions. 



Obviously, such ecological 

 modeling has many limitations. Its 

 usefulness is limited by the validity 

 and completeness of the input data 

 base (which for many estuaries is not 

 particularly good) . It is also 

 limited by the number of simplifying 

 assumptions that must be made. How- 

 ever, we believe CBEM provides an 

 effective tool for better under- 

 standing the complex dynamic inter- 

 actions which occur under estuarine 

 conditions . 



APPLICATION OF 

 METHODOLOGIES 



In Phase II of the Biota Assess- 



ment, the tools and methods developed 

 during Phase I will be applied to 

 assessing impact of three low flow 

 scenarios based on Corps hydraulic 

 model salinity data. Organism habi- 

 tats, defined in Phase I for the 57 

 study species will be analyzed for 

 changes under each low flow scenario. 

 This will be quantified in terms of 

 "impact ratios" which are measures of 

 the change in either known or po- 

 tential habitat due to salinity 

 alterations. These ratios may be 

 either positive or negative for a 

 particular organism and represent a 

 range rather than a single number, to 

 reflect the inherent inaccuracies in 

 data and gaps in current scientific 

 knowledge of the estuary. These 

 "impact ratios" will measure the 

 direct effects of low flow conditions 

 on individual species. 



Indirect or secondary effects 

 which result from species inter- 

 actions will be expressed quantita- 

 tively or qualitatively (as possible 

 and appropriate) utilizing conceptual 

 or simulation modeling as described 

 in the previous section. Quanti- 

 fication of these secondary effects 

 is difficult, particularly in areas 

 where the data base is weak, such as 

 tributaries on the eastern shore of 

 the bay. 



SUMMARY 



Phase I of the Biota Assess- 

 ment, a portion of the Chesapeake 

 Bay Low Flow Study, has developed 

 a set of tools and methods with 

 which to assess impacts of low 

 flows. These include 



1 . Definitions of habitats 

 using uniform variables on a bay- 

 wide basis. 



2. Selection of a set of 

 study species which represent the 



146 



