Figure VI-20. Water clarity criteria reference curve for defining criteria attainment in 
mesohaline/polyhaline shallow-water bay grass designated use habitats. 
REFERENCE CURVES FOR CHLOROPHYLL A CRITERIA 
As states derive numerical regional and local specific chlorophyll a criteria, they 
should either derive biologically-based reference curves that reflect the ‘allowable’ 
exceedances of local impairments or apply the normal distribution curve representing 
approximately 10 percent ‘allowable’ exceedance in time and space (see Figure VI-18). 
The cumulative frequency distributions derived from the subset of Chesapeake Bay 
water quality monitoring program chlorophyll a data associated with the ‘Better’ and 
‘Best,’ and sometimes ‘Mixed_Better Light’ water quality categories closely 
matched the normal distribution curve in both spring and summer (figures VI-21 and 
VI-22). These categories formed the basis for characterizing the Chesapeake Bay 
phytoplankton reference community (see Chapter V and Appendix F for details). The 
cumulative frequency distributions were derived from applying the 95th percentiles 
of chlorophyll a values occurring in these categories (see Table V-6). In figures 
VI-21 and VI-22, respectively, the cumulative frequency distributions of spring 
(March-May) and summer (July-September) chlorophyll a concentration exceeding 
the 95th percentile phytoplankton reference community values (a) are overlaid with 
the normal distribution curve (b). The normal distribution curve matches well with 
both seasonal biological-based cumulative frequency distributions, providing further 
justification for applying the normal distribution curve as a chlorophyll a criteria 
reference curve in the absence of a directly derived biological reference curve. 
REFERENCE CURVE IMPLEMENTATION 
As the states adopt the Chesapeake Bay criteria and concomitant procedures into 
their water quality standards, they may decide to: 1) allow for no criteria exceedance, 
2) select the normal distribution curve representing approximately 10 percent 
allowable criteria exceedance or 3) apply a biological reference curve. The first two 
chapter vi • Recommended Implementation Procedures 
