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chapter VII 
Upper and Lower Pycnocline 
Boundary Delineation 
Methodology 
Vertical stratification is foremost among the physical factors affecting dissolved 
oxygen concentrations in some parts of Chesapeake Bay and its tidal tributaries. If 
the density discontinuity is great enough to prevent mixing of the layers and consti¬ 
tutes a vertical barrier to diffusion of dissolved oxygen, then a pycnocline is said to 
exist (Figure VII-1). For the purposes of water quality criteria attainment assessment, 
the Chesapeake Bay and tidal tributary waters are separated into a surface mixed 
layer (e.g., open-water designated use), an inter-pycnocline layer (e.g., deep-water 
designated use) and a lower mixed layer (e.g., deep-channel designated use) (U.S. 
EPA 2003a, 2003b). 
Accurate estimates of the pycnocline are important for assessing criteria attainment. 
The method documented here for assessing upper and lower mixed layer depths 
differs from the standard Chesapeake Bay Water Quality Monitoring Program field 
sampling cruise method (Chesapeake Bay Program 1996) in that this methodology 
uses a measured density gradient based on salinity and temperature rather than 
relying on the field surrogate, conductivity. 
Defining the depth of the upper mixed layer based on the physical barrier of a density 
gradient is discussed in Brainerd and Gregg 1995. Culver and Perry (1999) and 
Larsson et al. (2001) propose particular density gradient thresholds for defining this 
layer. The critical density gradient is dependent on many factors, most importantly 
the strength of the turbulent mixing. Generally, for the Chesapeake Bay the upper 
pycnocline depth, defining the surface mixed layer, is the shallowest occurrence of a 
density gradient of 0.1 kg/m 4 or greater. The lower mixed layer depth is the deepest 
occurrence of a density gradient of 0.2 kg/m 4 , if a lower mixed layer exists below it. 
These limits were based on an extensive review of thousands of density profiles 
throughout the Chesapeake Bay and its tidal tributaries throughout 19-year record of 
the Chesapeake Bay Water Quality Monitoring Program. These density gradient 
thresholds are consistent with the values published for other tidal water bodies and 
with similar studies in the Chesapeake Bay (Fisher 2003). Since pycnocline delin- 
chapter vii 
Upper and Lower Pycnocline Boundary Delineation Methodology 
