29 
depending on water body.” MDE has determined per Thomann and Mueller 
(1987), that it is acceptable to maintain chlorophyll a concentrations below a 
maximum of 100 pg/L, and also to target, with some flexibility depending on 
waterbody characteristics, a 30-day rolling average of approximately 50 pg/L. 
Consistent with the guidelines set forth above, MDE’s interpretation of narra¬ 
tive criteria for chlorophyll a in the Upper and Middle Chester River consists 
of the following goals: 
E Ensure that instantaneous concentrations remain below 100 pg/1 at all 
times and 
2. Minimize exceedances of the 50 pg/1, 30-day rolling average, to a 
frequency that will not result in ecologically undesirable conditions. 
Further development of numerical chlorophyll a criteria for Chesapeake Bay tidal 
waters was advanced with the U.S. EPA 2007b publication Ambient Water Quality 
Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake 
Bay and Its Tidal Tributaries - 2007 Chlorophyll Criteria Addendum. This 2007 
chlorophyll a criteria addendum documented the scientific basis for numerical 
chlorophyll criteria based on: 
1. historical chlorophyll a reference concentrations; 
2. chlorophyll a relationships with dissolved oxygen impairments; 
3. chlorophyll a contributions to water clarity impairments; and 
4. characteristic chlorophyll a conditions associated with specific impairments 
related to harmful algal blooms. 
Recommendations on Chesapeake Bay chlorophyll a criteria were provided and 
structured, tiered sample collection, analysis and assessment procedures were 
recommended. The specific sampling and assessment procedure recommendations 
are directed toward a harmful algal bloom (HAB) based chlorophyll a criterion that 
could be applied to the Chesapeake Bay tidal fresh and oligohaline waters. 
The basic approach used for numerical chlorophyll a criteria assessment procedure 
is documented in Table II-1 in the July 2007 criteria addendum (U.S. EPA 2007a). 
The details of the chlorophyll a criteria attainment assessment are documented 
here in Appendix G. The general application example below is illustrated for the 
James River. 
APPROACH AND PROTOCOL APPLICATION WITH EXAMPLES 
The use of spatially and temporally-intensive DATAFLOW data in conjunction with 
monthly and semi-monthly fixed station data allowed for the generation of daily 
interpolated estimates for each segment. In Virginia, during the 2008 assessment for 
example, more than 500,000 data points were used for the assessment of the three- 
year period. This monitoring approach produced data that generally resulted in from 
1 to 7 individual day-scale interpolation grids in any one month. The day-scale inter¬ 
polation grids were then used to calculate a seasonal average concentration for each 
grid cell. This approach ensures that segments are assessed with as much spatiotem- 
poral variability as possible while minimizing reliance on weak estimates stemming 
from small sample sizes. 
chapter v 
Chlorophyll a Criteria Assessment Procedures 
