28 
objectionable conditions—or other render tidal waters unsuitable for designated 
uses'' (U.S. EPA 2003a). 
However, the EPA also strongly encouraged states to develop and adopt site-specific 
numerical chlorophyll a criteria for tidal waters where algal-related impairments are 
expected to persist even after the Chesapeake Bay dissolved oxygen and water 
clarity criteria have been attained. 
In Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chloro¬ 
phyll a for the Chesapeake Bay and Its Tidal Tributaries - 2004 Addendum (U.S. 
EPA 2004) guidance was developed on determining where numerical chlorophyll a 
criteria should apply to Chesapeake Bay and tidal tributary waters. A general recom¬ 
mended methodology was developed by the Chesapeake Bay Program partners for 
use by the jurisdictions with tidal waters to determine consistently which local tidal 
waters will likely attain the published Chesapeake Bay dissolved oxygen and water 
clarity criteria yet show the persistence of algal-related water quality impairments. 
Examples of possible salinity-zone-specific, numerical chlorophyll a thresholds 
(pg/L) drawn from a variety of resources and approaches were provided with deri¬ 
vations based in: 
1. historical Chesapeake Bay levels; 
2. ecosystem trophic status; 
3. phytoplankton reference communities; 
4. potentially harmful algal blooms; 
5. water quality impairments; and 
6. user perceptions and State water quality standards (Table IX-1 in U.S. EPA 2004). 
From 2004 through 2006, Delaware, Maryland, Virginia and the District of 
Columbia promulgated narrative chlorophyll a criteria into their water quality stan¬ 
dards. Virginia promulgated numerical segment- and season-specific chlorophyll a 
criteria for the tidal James River. The District of Columbia promulgated numerical 
chlorophyll a criteria for its reach for the tidal Potomac River and its remaining 
waters, having previously adopted numerical criteria for chlorophyll a criteria for the 
protection of the tidal Anacostia River. 
Quantitative interpretation of Maryland’s narrative criterion for chlorophyll a is cited 
in the following excerpt from Maryland Department of the Environment’s (MDE’s) 
“Total Maximum Daily Loads of Nitrogen and Phosphorus for the Upper and Middle 
Chester River Kent and Queen Anne’s Counties, Maryland” (approved by U.S. EPA 
November 2006). The text below also describes MDE’s interpretation of this criterion 
in terms of quantified goals for application in Total Maximum Daily Loads (TMDLs). 
The Chlorophyll a level goals used in this analysis are guidelines set forth by 
Thomann and Mueller (1987) and by the EPA Technical Guidance Manual for 
Developing Total Maximum Daily Loads, Book 2, Part 1 (1997). The 
chlorophyll a narrative criteria ((COMAR 26.08.02.03-3 C (10)) states: 
“Chlorophyll a - Concentrations of chlorophyll a in free-floating microscopic 
aquatic plants (algae) shall not exceed levels that result in ecologically 
undesirable consequences that would render tidal waters unsuitable for desig¬ 
nated uses.” The Thomann and Mueller guidelines above acknowledge 
“‘Undesirable’ levels of phytoplankton [Chlorophyll a] vary considerably 
chapter v • Chlorophyll a Criteria Assessment Procedures 
