The reference curves described below for the dissolved oxygen and water clarity 
criteria are based on empirical, biologically-based field data where possible. Where 
no corroborating field data exist, a normal distribution curve representing approxi¬ 
mately 10 percent exceedance is used (see Figure VI-18). Appendix H contains 
supporting analyses and detailed descriptions of the methodologies used for defining 
these reference curves, as well as the list of reference locations. 
REFERENCE CURVES FOR DISSOLVED OXYGEN CRITERIA 
Reference curves for dissolved oxygen are intended to represent the spatial and 
temporal distribution of dissolved oxygen concentrations in areas supporting healthy 
species and communities the criteria were established to protect. The deep-water 
designated use, for example, contained the necessary water quality and biological 
source data collected over similar temporal and spatial scales. When such data were 
not available at the scales necessary to establish quantitative relationships between 
the criteria parameter and measured living resource community health, surrogate 
measures of biological and habitat conditions were explored. Ideally, each set of 
designated use-based dissolved oxygen criteria should have a separate, individually 
derived reference curve. However, satisfactory synoptic water quality and biological 
indices data or surrogate measures of habitat condition were found only for the open- 
water fish and shellfish and deep-water designated uses and were tested only against 
the 30-day mean criteria for those uses. 
Migratory Fish Spawning and Nursery Dissolved Oxygen 
Criteria Reference Curve 
Current Chesapeake Bay water quality monitoring in migratory fish spawning and 
nursery habitats is limited to midchannel stations. There also are insufficient 
spawning success fisheries-independent data available to identify biologically-based 
reference sites for these criteria. In addition, the criteria duration components for this 
designated use are an instantaneous minimum and 7-day mean, and methodologies 
to translate less frequently monitored dissolved oxygen measurements into these 
time steps have not been finalized. 
An attainment curve for exploratory purposes was created for the February-May 
spawning period, using a 30-day criterion of 6 mg liter' 1 and reference sites identified 
using nitrogen, phosphorus, chlorophyll a and total suspended solids as parameters 
(Figure VI-15). Attainment was very close to 100 percent. Until more data are 
collected to assess the attainment of the 7-day mean and instantaneous minimum 
criteria in the migratory fish spawning and nursery designated use, however, the open- 
water dissolved oxygen criteria reference curve should be applied (Figure VI-16). 
Open-Water Dissolved Oxygen Criteria Reference Curve 
In the absence of a Chesapeake Bay open-water fish community index of biotic 
integrity, reference Chesapeake Bay Program segments with ‘good’ water quality 
chapter vi 
Recommended Implementation Procedures 
