62 
CHLOROPHYLL A CRITERIA ASSESSMENT 
PROCEDURES 
CHLOROPHYLL A CRITERIA REFERENCE CURVE 
To assess attainment of the State adopted numerical chlorophyll a concentration- 
based criteria, it was necessary to establish a reference curve for use in the CFD 
criteria attainment assessment process (U.S. EPA 2003). In the case of chlorophyll 
a criteria where a biologically-based reference curve is not available, EPA recom¬ 
mends the states use of the default reference curve described in Chapter 2 (see Figure 
II-4 and Equation 1). 
CHLOROPHYLL A CRITERIA ASSESSMENT 
A criterion threshold is a concentration that should rarely be exceeded by a “popu¬ 
lation” of concentration data exhibiting healthy levels. The state-adopted 
concentration-based chlorophyll a criteria values are threshold concentrations that 
should only be exceeded infrequently since a low number of naturally occurring 
exceedances occur even in a healthy phytoplankton population. The assessment of 
chlorophyll a criteria attainment, therefore, should use the CFD-based assessment 
method described in Chapter 2 that applies the default reference curve. These 
Chesapeake Bay chlorophyll a criteria apply only to those seasons and salinity-based 
habitats for which they were defined to protect against applicable human health 
and aquatic life impairments. Each season—spring (March 1-May 31) and summer 
(July 1-September 30)—should be assessed separately to evaluate chlorophyll a 
criteria attainment. 
Assessments of seasonal mean chlorophyll a criteria should be based on seasonal 
averages of interpolated data sets. To calculate the seasonal averages, each interpo¬ 
lated cruise within a season should be averaged on a point-by-point basis in 
matching interpolator grid cells. Spatial violation rates should be calculated for each 
seasonally aggregated interpolation in an assessment period. For example, for a 
summer open-water seasonal chlorophyll a criteria assessment of a three-year 
assessment period, three seasonal average interpolations representing each season 
(Year 1 Summer, Year 2 summer, Year 3 summer) should be used. 
LITERATURE CITED 
U.S. Environmental Protection Agency. 2003. Ambient Water Quality Criteria for Dissolved 
Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries. 
EPA 903-R-03-002. Region III Chesapeake Bay Program Office, Annapolis, MD. 
chapter vi 
Chlorophyll a Criteria Assessment Procedures 
