phytoplankton populations, June is either springlike, summerlike or uniquely 
different from either season. 
At present, the recommended method for assessing numerical chlorophyll a criteria 
attainment is to interpolate monthly chlorophyll a concentrations for each surface 
interpolator cell from the available fixed stations. The interpolator cells can be asso¬ 
ciated with the proper segment and salinity zone, so that each cell’s chlorophyll a 
concentration can be compared to the proper chlorophyll a criterion value. The cell 
area is then accumulated in the fail or pass tally for each Chesapeake Bay Program 
segment for each month. The cumulative frequency distribution curve resulting from 
the monthly percent attainment measures over the spring or summer seasons and the 
three-year attainment period is then compared statistically to the reference curve to 
determine the degree of attainment/nonattainment. If the curves are significantly 
different, then the segment/designated use is considered out of attainment, and 
failing by the amount defined by the area between the two curves. 
River Input and Flow Considerations 
States responsible for measuring chlorophyll a criteria attainment near the fall lines 
where major free-flowing rivers enter tidal waters should recognize the strong influ¬ 
ences of intra- and interannual flows on conditions in the adjacent tidal-fresh 
habitats. In addition to their upstream contributions of chlorophyll a, the flow levels 
of waters directly entering the tidal-fresh reaches of these rivers greatly influence the 
resulting tidal habitat chlorophyll a concentrations. The decadal scale record of 
water quality monitoring data provides the states and other users with a wealth of 
information from which to understand the relative influence of flow conditions on 
observed chlorophyll a criteria attainment. The EPA recommends recognition within 
states’ water quality standards of the influence of river flow conditions on chloro¬ 
phyll a concentrations, particularly in the tidal reaches just below the major fall 
lines. Management actions directed toward chlorophyll a criteria attainment in these 
tidal reaches should also reflect the long-term flow conditions and influences on 
local water quality. 
EVALUATION OF CHESAPEAKE BAY 
WATER QUALITY MODEL OUTPUT 
The Chesapeake Bay Program has developed what have become standard estuarine 
modeling tools, including a watershed model (Donigian et al. 1994; Linker et al. 
1996, 2000), airshed model (Shin and Carmichael 1992; Appleton 1995, 1996), 
estuarine hydrodynamic model (Wang and Johnson 2000), estuarine water quality 
model (Cerco 1993, 1995a, 1995b; Thomann et al. 1994; Cerco and Meyers 2000; 
Cerco 2000; Cerco and Moore 2001; Cerco et al. 2002) and estuarine sediment 
diagenesis model (Di Toro 2001). Together these linked simulations provide a 
system to estimate dissolved oxygen, water clarity and chlorophyll a in 35 major 
segments of the Chesapeake Bay and its tidal tributaries. The same criteria 
chapter vi 
Recommended Implementation Procedures 
