B-2 
linearly interpolated vertically within each column of observed data beginning at 
0.5 meters below the water surface and continuing at one-meter intervals, without 
exceeding the deepest observation in that water column. Data at each depth is then 
interpolated horizontally using inverse distance squared weighting. Data regions 
were specified for each segment to prevent the interpolation algorithm from using 
data points in neighboring tidal tributaries (described in the section below and in 
detail in Appendix D). 
Some designated uses for dissolved oxygen during the summer in the Chesapeake 
Bay and its tidal tributaries and embayments are defined vertically to distinguish 
stable water layers with different criteria levels (U.S. EPA 2003a, 2003b). In areas 
and seasons for which vertical stratified criteria apply, the surface mixed layer (open 
water) is that layer above the pycnocline and, thus, exposed to the atmosphere. The 
transitional middle layer (deep water) is the layer between the upper and lower pycn¬ 
ocline boundaries. The lower layer (deep channel) is the water below the lower 
pycnocline boundary. Given that the pycnocline is dynamic and moves up and down 
with each monitoring cruise, the designated use of each interpolator grid cell must 
also be defined based on the data for each cruise. 
Temperature and salinity are used to calculate density; density, in turn, is used to 
calculate pycnocline boundaries. Density is calculated using the method described in 
Algorithms for Computation of Fundamental Properties of Seawater For each 
column of temperature and salinity data, the upper and lower pycnocline boundaries 
are determined by looking for the shallowest robust vertical change in density of 
0.1 kg/m 3 /m for the upper boundary and the deepest change of 0.2 kg/m Vm for the 
lower boundary. To be considered robust, the density gradient must not reverse direc¬ 
tion at the subsequent measurement and must also demonstrate a change in salinity 
of at least 0.1 psu per meter (not merely a change in temperature). Chapter 7 in U.S. 
EPA 2004, pages 85-87, documents the detailed method for determination of both 
the vertical density profile and the pycnocline. 
The depths to the upper pycnocline boundary (where detected) and the fraction of 
the water column below the lower boundary are interpolated in two dimensions. If 
no lower boundary was detected, then the fraction is set at zero. The depth to the 
upper pycnocline boundary tends to remain stable in the horizontal dimension, 
meaning that spatial definition of that boundary using interpolation generally works 
well. Interpolation of the lower boundary is more complicated because the results 
may conflict with the upper boundary definition or with the actual bathymetry of the 
Chesapeake Bay. Consequently, interpolation of the lower boundary is based on the 
fraction of water column depth. In this way, the constraints of the upper pycnocline 
boundary definition and the actual Bay bottom depth are imposed, eliminating errors 
related to boundary conflicts. 
'Endorsed by UNESCO/SCOR/ ICES/IAPSO Joint Panel on Oceanographic Tables and Standards and 
SCOR Working Group 51. N.P. Fofonoff, and R.C. Millard, Jr., 1983. UNESCO Technical Papers in 
Marine Science. Pans, France. No. 44, p. 53. 
appendix b 
Detailed Chesapeake Bay Water Quality Criteria Assessment Methodology 
