Virginia authorities for the 2008 303d/305b listing analyses that were collected from 
the Virginia Chesapeake Bay benthic monitoring program and the Alliance for the 
Chesapeake Bay’s (ACB) Virginia volunteer monitoring program. 
PYCNOCLINE DEFINITION AND BOUNDARIES 
REVISING DESIGNATED USES BOUNDARIES WITH ENHANCED 
PYCNOCLINE DEFINITION PROCEDURE 
In U.S. EPA (2003a) Ambient Water Quality Criteria for Dissolved Oxygen , Water 
Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries (Regional 
Criteria Guidance), EPA identified five habitats (or designated uses) providing a 
context for adequately protective Chesapeake Bay water quality criteria. Water 
quality criteria and assessment procedures were developed for dissolved oxygen, 
water clarity and chlorophyll a, published (U.S. EPA 2003a, 2004a,b, 2007a,b), and 
have progressively been adopted into State water quality standards regulations. The 
five designated uses were 1) migratory fish spawning and nursery designated use, 2) 
shallow-water bay grass designated use, 3) open-water fish and shellfish designated 
use, 4) deep-water seasonal fish and shellfish designated use and 5) deep-channel 
seasonal refuge designated use (U.S. EPA 2003b). EPA published Technical Support 
Document for Identification of Chesapeake Bay Designated Uses and Attainability 
(U.S. EPA 2003b, 2004b) which provided further information on the development 
and geographical extent of the designated uses to which the criteria may apply. 
Refinements to boundary definitions involving open water, deep water and deep 
channel have been developed, as described below, to standardize layer definitions. 
CALCULATION OF UPPER AND LOWER PYCNOCLINES FOR 
DISSOLVED OXYGEN DESIGNATED USE CRITERIA ASSESSMENT 
Vertical stratification is foremost among the physical factors affecting dissolved 
oxygen concentrations in some parts of Chesapeake Bay and its tidal tributaries. For 
the purposes of water quality criteria attainment assessment, three layers are defined 
for designated use assessments: 1) an upper mixed layer above the upper pycnocline 
boundary; 2) deep water layer constrained by the upper and lower pycnocline bound¬ 
aries; and 3) the lower mixed layer below the lower pycnocline boundary (U.S. EPA 
2003a, 2003b). The depths of the upper and lower mixed layers are used to deter¬ 
mine designated use boundaries for the dissolved oxygen assessment. In segments 
where deep water and deep channel habitats are applicable, deep channel is defined 
as the lower mixed layer, open water is defined as the upper mixed layer, and deep 
water is the interpycnocline layer between the upper and lower mixed layers. 
Temperature (°C) and salinity (ppt) are used to calculate density which, in turn, is used 
to calculate pycnocline boundaries. Density is calculated using the method described in: 
Algorithms for computation of fundamental properties of seawater. Endorsed 
by UNESCO/SCOR/ICES/IAPSO Joint Panel on Oceanographic Tables and 
Standards and SCOR Working Group 51. Fofonoff, N P; Millard, R C Jr. 
UNESCO technical papers in marine science. Paris , no. 44, pp. 53. 1983. 
chapter iii 
Refinements to Procedures for Assessing Chesapeake Bay Dissolved Oxygen Criteria 
