attainment percentages for each reference Chesapeake Bay Program segment were 
pooled in their respective low and higher salinity groups and plotted as cumulative 
frequency distribution curves (figures VI-19 and VI-20). Appendix H contains the 
reference curves generated using the more recent 1995-2000 data. All these water 
clarity criteria reference curves were derived using data spanning decadal scales, 
capturing the full range of wet, dry and average hydrologic conditions. 
The derived water clarity criteria reference curves reflect findings published in the 
scientific literature for Chesapeake Bay species that indicate that underwater plants 
can survive reduced light conditions for periods of days to weeks. Field and labora¬ 
tory experiments indicated that lower salinity species were more tolerant of longer 
periods of reduced light conditions (Rybicki et al. 2002) compared with species 
inhabiting higher salinity waters (Goldsborough and Kemp 1988). These salinity 
regime differences also are reflected in the different shapes of the derived reference 
curves. The lower salinity reference curve allows for more exceedances over time 
and space than are allowed for by the higher salinity reference curve (figures VI-19 
and VI-20, respectively). 
It should be noted that the water clarity criteria were derived, in part, on the basis of 
underwater bay grass growing season medians (Batiuk et al. 1992, 2000), but 
attainment is measured on a monthly basis over the growing season (see “Devel¬ 
oping the Cumulative Frequency Distribution,” p. 154, for details). Appendix H also 
shows water clarity reference curves based strictly on growing season median 
assessments. 
o Percentage of Area/Volume Exceeding the Criteria 
Figure VI-19. Water clarity criteria reference curve for defining criteria attainment in 
tidal-fresh/oligohaline shallow-water bay grass designated use habitats. 
chapter vi • Recommended Implementation Procedures 
