open-water, deep-water or deep-channel designated use habitats in the entire water 
column. For example, if the deep-water use was found in nonattainment at a rate of 
50 percent but only accounted for 10 percent of the total habitat of the water column, 
the management actions taken in response would differ from those taken if the deep¬ 
water use accounted for 75 percent of the total habitat. This may prove to be a useful, 
additional source of data when difficult decisions must be made. 
DEFINING THE REFERENCE CURVE 
The recommended criteria attainment assessment approach is designed to protect the 
living resources as defined by the designated uses. The criteria levels themselves 
were largely based on scientific studies performed in laboratory settings or under 
controlled field conditions. The criteria establish the level of a given habitat condi¬ 
tion that living resources need for survival. They do not account for many other 
environmental factors that could affect survival. 
Reference curves were developed to provide a scientific-based, direct measure of the 
‘allowable’ criteria exceedances. These exceedances are defined to be those that last 
a short enough time or cover a small enough area to have no adverse affects on the 
designated use. It is assumed that the designated uses can be attained even with some 
limited level of criteria exceedances and thus, the reference curves define those 
criteria exceedances deemed to be allowable—chronic in time but over small areas, 
or infrequent occurrences over large areas. Exceedances that occur over large areas 
of space and time would be expected to have significant detrimental effects on 
biological communities, which would imply nonattainment of designated uses. 
STRENGTHS AND LIMITATIONS 
Although the Chesapeake Bay and its tidal tributaries are listed as impaired water 
bodies, there are some places that have met or usually meet the Chesapeake Bay 
criteria and support healthy aquatic living resource communities. Reference curves 
derived from monitoring these areas reveal patterns of criteria attainment or 
exceedances that support the healthy community. That is, they show whether areas 
that support a relatively healthy target community: 1) never exceed the applicable 
criteria, 2) exceed the criteria frequently, but over a small area or volume, 3) exceed 
the criteria infrequently over a large area or volume or 4) exhibit some other pattern. 
The EPA recognizes that there are currently a limited number of reference sites, given 
the Chesapeake Bay’s nutrient-enriched status. In addition, there are limited data avail¬ 
able—both for criteria parameters as well as measures of the biological health of target 
communities—with adequate spatial and temporal coverage from which to develop a 
full array of biological-based reference curves. However, where sufficient data exist, 
the reference curves appear to be stable. The reference curve for the deep-water desig¬ 
nated use dissolved oxygen criteria is the most solidly grounded in data. 
chapter vi 
Recommended Implementation Procedures 
