18 
c. AND IF AS >0.1, 
d. Then this depth represents the upper boundary of the pycnocline. 
8. Identify whether there is a lower mixed layer: use the same vertical profile but 
examine it from the second deepest value upward (exclude the deepest value): 
a. IF change in density (AaT) at the second deepest depth < 0.2 
b. OR IF AoT at the next depth (moving upwards, i.e. shallower) < 0.2 
c. THEN a lower mixed layer (i.e. a layer at depth where the density is not 
changing) below the pycnocline exists. 
9. If a lower mixed layer exists, then look for the lower boundary of the pycno¬ 
cline. Beginning at the second deepest value, and stepping up to the depth 
immediately below the upper pycnocline boundary, for AS and AoT values at 
each depth: 
a. IF AaT > 0.2, 
b. AND IF AS >0.1, 
c. Then this depth is the lower pycnocline boundary. 
10. If a pycnocline exists, then the upper and lower (if present) boundaries of the 
pycnocline have now been identified. 
LITERATURE CITED 
U.S. Environmental Protection Agency. 2003a. Ambient Water Quality' Criteria for Dissolved 
Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries 
(Regional Criteria Guidance) April 2003. EPA 903-R-03-002. Region III Chesapeake Bay 
Program Office, Annapolis, MD. 
U.S. Environmental Protection Agency. 2003b. Technical Support Documentation for Identi¬ 
fication of Chesapeake Bay Designated Uses and Attainability’. October 2003. EPA 
903-R-03-004. Region III Chesapeake Bay Program Office, Annapolis, MD. 
U.S. Environmental Protection Agency. 2004a. Ambient Water Quality> Criteria for Dissolved 
Oxygen, Water Clarity and Chlorophyll afar the Chesapeake Bay and Its Tidal Tributaries - 
2004 Addendum. EPA 903-R-04-005. Region III Chesapeake Bay Program Office, 
Annapolis, MD. 
U.S. Environmental Protection Agency. 2004b. Technical Support Document for Identifica¬ 
tion of Chesapeake Bay Designated Uses and Attainability - 2004 Addendum. October 2004. 
Region III Chesapeake Bay Program Office. EPA 903-R-04-006. Annapolis, MD. 
U.S. Environmental Protection Agency. 2007a. Ambient Water Quality / Criteria for Dissolved 
Oxygen, Water Clarity and Chlorophyll afar the Chesapeake Bay and Its Tidal Tributaries - 
2007 Addendum. July 2007. EPA 903-R-07-003. Region III Chesapeake Bay Program Office, 
Annapolis, MD. 
U.S. Environmental Protection Agency. 2007b. Ambient Water Quality> Criteria for Dissolved 
Oxygen, Water Clarity ; and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries 
-Chlorophyll a Addendum. October 2007. EPA 903-R-07-005. Region III Chesapeake Bay 
Program Office, Annapolis, MD. 
chapter iii 
Refinements to Procedures for Assessing Chesapeake Bay Dissolved Oxygen Criteria 
