CHAPTER 4 
Revisions to the Chlorophyll a Criteria 
Assessment Methodology 
BACKGROUND 
In the 2003 Ambient Water Quality Criteria for Dissolved Oxygen , Water Clarity and 
Chlorophyll a for Chesapeake Bay and Its Tidal Tributaries, EPA published narrative 
chlorophyll a criteria that states chlorophyll a 
'‘...shall not exceed levels that result in ecologically undesirable consequences- 
such as reduced water clarity, low dissolved oxygen, food supply imbalances, 
proliferation of species deemed potentially harmful to aquatic life or humans or 
aesthetically objectionable conditions - or otherwise render tidal waters as 
unsuitable for designated uses balanced aquatic plant life populations and against 
the overgrowth of nuisance, potentially harmful species” (U.S. EPA 2003). 
From 2004 to 2006, Virginia and the District of Columbia adopted numerical chlorophyll a 
criteria for application in the tidal James River (Virginia) and across the District's jurisdictional 
tidal waters. In the 2007 publication Ambient Water Quality Criteria for Dissolved Oxygen, 
Water Clarity and Chlorophyll a for Chesapeake Bay and its Tidal Tributaries-2007 Addendum , 
EPA published chlorophyll a criteria assessment procedures (U.S. EPA 2007a, p. 62). With the 
establishment of numerical chlorophyll a concentration-based criteria promulgated by the states 
into their water quality standards regulations within Chesapeake Bay tidal waters, it was 
necessary to establish a reference curve for use in the published criteria attainment process (U.S. 
EPA 2003). 
A biologically-based reference curve with which to assess chlorophyll a criteria attainment in 
Chesapeake Bay is not yet available. A dataset has not been identified from which there is 
confidence that a biological reference curve can be derived (U.S. EPA 2007b). The EPA 
Chesapeake Bay Program Office and its partners, in consultation with regional experts in 
phytoplankton and chlorophyll a monitoring and research, have explored the published work of 
Buchanan et al. 2005 and Lacouture et al. 2006 conducted during development of the 
phytoplankton index of biotic integrity (P-IBI). 
In its current form, the published P-IBI work does not provide for a suitable representation of the 
integrated seasonal biological community conditions necessary to inform appropriate seasonal 
reference conditions for Chesapeake Bay chlorophyll a criteria attainment assessments. Benthic 
macroinvertebrates, for example, have life spans that integrate temporally variable environmental 
conditions over space, and the effects of multiple types of environmental stress and habitat 
alteration as used with the B-IBI for Chesapeake Bay (Llanso et al. 2009). However, standing 
crops of phytoplankton communities will respond to nutrient perturbations in 10-14 days 
(Heiskary and Walker 1995). Tracking the P-IBI results indicates any given segment can and 
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