16 
For each vertical column of temperature and salinity data throughout the water 
column, the existence of the upper and lower pycnocline boundaries are determined 
by looking for the shallowest robust vertical change in density greater than 0.1 
kg/mVm for the upper boundary and deepest change of greater than 0.2 kg/m Vm for 
the lower boundary. To be considered robust, the density gradient must not reverse 
direction at the next measurement and must be accompanied by a change in salinity 
and temperature. 
Upper and lower pycnocline boundaries, where present, are interpolated in two 
dimensions. The depth to the upper pycnocline boundary tends to be stable across 
horizontal space in the estuary and so spatial definition of that boundary using inter¬ 
polation generally works well. However, interpolation of the lower boundary is more 
complicated because the results can conflict with 1) the upper boundary definition or 
2) with the actual bathymetry of the Bay. As a result, interpolation of the lower 
boundary should be performed based on “fraction of water column depth”. 
In the computations, the lower pycnocline is actually stored as “fraction of water 
column below lower pycnocline,” and calculated by dividing the lower pycnocline 
depth by the total depth and subtracting the product from 1 as follows: 
Example: Lower pycnocline depth = 10 m 
Total depth = 15 m 
% of total depth below lower pycnocline = 1 -(10/15) = -.333 or about 33%. 
When counting violations, the measures are converted back into an actual depth 
before comparing measurements to it. To locate the lower pycnocline, multiply the 
total depth at the given measurement location for that day by (1- %below lower 
pycnocline), in this example it is 15(1-.33) = 10.01. 
This calculation produces essentially the same depth of lower pycnocline. It is 
important to proceed in this approach since total depth measurements may differ 
across sampling dates. By following this procedure for working with the lower pycn¬ 
ocline calculation it avoids the case where you could have a lower pycnocline value 
below the total depth. If no lower boundary is detected then the fraction is zero. 
The standardized method for calculating upper and lower boundaries of the pycno¬ 
cline uses water column measurements of water temperature and salinity. Ambient 
Water Quality Criteria for Dissolved Oxygen , Water Clarity and Chlorophyll a for 
Chesapeake Bay and its Tributaries - 2004 Addendum (U.S. EPA 2004a) provided 
two basic rules for determination of pycnocline depth: 
1. From the water surface downward, the first density slope observation that is 
greater than 0.1 kg/m 3 /m is designated as the upper pycnocline boundary 
provided that: 
a. That observation is not the first observation in the water column and 
b. The next density slope observation is positive. 
2. From the bottom sediment-water interface upward, the first density slope 
observation that is greater than 0.2 kg/mVm is designated as the lower pycno¬ 
cline depth provided that: 
a. An upper pycnocline depth exists; 
chapter iii 
Refinements to Procedures for Assessing Chesapeake Bay Dissolved Oxygen Criteria 
