Limnetic 



(tidal fresh water) 0.0 - 0.5 o/oo 



Oligohaline 0.5 - 5.0 o/oo 



Lower Mesohaline 5.0 -10.0 o/oo 



Upper Mesohaline 10.0 -18.0 o/oo 



Polyhaline 



Euhaline 



18.0-25.0 o/oo 

 30.0 °/oo 



In addition, we have defined cate- 

 gories of substrate, depth, season- 

 ality (as related to temperature) and 

 modifications of habitat by other 

 organisms. These categories have 

 been placed on base maps from known 

 Chesapeake Bay data bases. Appro- 

 priate combinations of categories 

 into the requirements of a particular 

 organism are then used to define that 

 organism's "potential habitat," wher- 

 ever sampling data are insufficient 

 to define a "known habitat," or area 

 where the organism has been previ- 

 ously identified (see Figure 5). 



These habitat systems then were 

 used to form the framework for inter- 

 preting information in the literature 

 on salinity tolerances or require- 

 ments for salinity, depth, substrate, 

 etc. Salinity tolerances, for in- 

 stance, were derived for each orga- 

 nism and the appropriate Venice cate- 

 gory determined. Organism distribu- 

 tion was defined by a combination of 

 known locations with "potential 

 habitat" derived from the Venice 

 categories and the other param- 

 eters discussed above. Locations 

 of the salinity contours necessary 

 were taken from seasonal data from 

 the 1960 water year, largely from 

 the Chesapeake Bay Salinity Atlas 

 (Stroup and Lynn 1963). These 

 have been plotted on 1:250,000 

 scale maps of Chesapeake Bay, as 

 have categories of substrate, 

 depth, and other habitat var- 

 iables. Using these base maps, 

 organism distributions have been 

 mapped on 1:250,000 scale mylar 



overlays for the 57 study species. 

 Examples of such maps are shown in 

 Figures 6 and 7 . 



ECOLOGICAL RELATIONSHIPS 



Low flows cause direct or 

 primary effects on species through 

 physiological responses due to 

 changes in salinity, nutrients, water 

 quality and similar factors. These 

 effects are generally either immed- 

 iate or occur over a short period of 

 time (i.e. a few days or weeks). In 

 response, species may increase or 

 decline in population, become extinct 

 in the area, or migrate to suitable 

 habitat in other parts of the estu- 

 ary, if such habitat exists. These 

 shifts in abundance or distribution 

 imply a new interplay of trophic 

 relationships, which we will term 

 here indirect or secondary effects. 

 The time span of such effects may 

 range from several days to several 

 years, or permanently in cases where 

 a new ecological equilibrium is 

 established. In order to investigate 

 these species relationships, con- 

 ceptual and mathematical models were 

 developed for Chesapeake Bay. 



The approach to modeling used in the 

 project is shown in Figure 8. Data 

 from the scientific literature and 

 related sources served as input to 

 define physiological tolerances and 

 constraints. Predator-prey inter- 

 actions were defined and basic 

 trophic interactions were charted for 

 interrelated groups (i.e. phyto- 

 plankton-zooplankton, etc.). These 

 were then integrated and formed the 

 basis for a conceptual model illus- 

 trating the major trophic interac- 

 tions and nutrient flows . The sum- 

 mary version of this conceptual model 

 is shown in Figure 9, using H.T. 

 Odum's energy language to illustrate 



132 



