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Chlorophyll a Criteria 
Assessment Procedures 
BACKGROUND 
Phytoplankton are small often microscopic plants floating in the water. These organ¬ 
isms form the base of the Chesapeake Bay’s food web, linking nutrients and sunlight 
energy with higher trophic levels such as fish (e.g. menhaden, bay anchovy) and with 
bottom dwelling oysters, clams and worms via primary producer and detrital path¬ 
ways. The majority of the Bay’s animals feed directly on phytoplankton or on 
organisms that directly consume the phytoplankton. Therefore, the Bay’s carry 
capacity, or its ability to produce and maintain a diversity of species, depends in 
large part on how well phytoplankton meet the needs of the consumers. 
A primary characteristic of algae is the presence of photopigments. Chlorophyll a is 
a primary photosynthetic pigment in algae and cyanobacteria (blue-green algae). 
Since chlorophyll a is a measure of photosynthetic activity, it is thus also a measure 
of the primary food source of aquatic food webs. Chlorophyll a also plays a direct 
role in reducing light penetration in shallow-water habitats, which has a direct 
impact on underwater bay grasses. Excess algae, uneaten by higher trophic level 
consumers (e.g., zooplankton, filter-feeding fish and shellfish), are decomposed by 
bacteria, and in the process, exert a biological oxygen demand upon the system. 
Decomposition of the algal organic matter through bacterial respiration can remove 
oxygen from the water column faster than it can be replaced and lead to hypoxia and 
anoxia, impairing habitat conditions for much of the Bay life. From a water quality 
perspective, chlorophyll a is the best available, most direct measure of the amount 
and quality of phytoplankton with a relationship to impacts on water clarity and 
dissolved oxygen impairments. 
The EPA originally provided the States with recommended narrative chlorophyll a 
criteria applicable to all Chesapeake Bay and tidal tributary waters: 
“Concentrations of chlorophyll a in free floating microscopic aquatic plants (algae), 
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 
chapter v • Chlorophyll a Criteria Assessment Procedures 
