98 
Table IX-1 . Example numerical chlorophyll a thresholds (p g liter 1 ) drawn from Ambient Water Quality Criteria 
for Dissolved Oxygen, Water Clarity and Chlorophyll a for Chesapeake Bay and its Tidal Tributaries' 
reflective of an array of historical concentrations, ecosystem trophic status, potential harmful algal 
blooms, water quality impairments, user perceptions and state water quality standards. 
Chlorophyll a Concentration Thresholds (pg liter' 1 ) 
Salinity 
Regime 
Historical 
Chesapeake 
Bay Levels 2J 
Ecosystem 
Trophic Status 
Phytoplankton 
Reference 
Communities 6 
Potentially 
Harmful Algal 
Blooms 7 
Water Quality 
Impairments 8 
User 
Perceptions 
State Water 
Quality 
Standards" 
Tidal Fresh 
Spring: 4 
Summer: 7 
Mainstem 
(annual): 3 
2-15 4 
Spring: 4.3 
Summer: 8.6 
Microcystis 
aeruginosa: 15 
Water Clarity: 
9-16 
Dissolved 
Oxygen: 
4-5 ~ 
Vermont Lakes: 
<15 9 
Minnesota 
Lakes: < 15 1 " 
AL: 16-27 (res.) 
CN: 2-15 (meso.) 
GA: 5-20 (lakes) 
NC: 15(lakes, 
res.) 
Oligohaline 
Spring: 6 
Summer: 8 
Mainstem 
(annual): 3 
Spring: 9.6 
Summer: 6.0 
Microcystis 
aeruginosa: 15 
Water Clarity 
9-16 
Dissolved 
Oxygen: 
7-12 
NC: 40 (tidal) 
Mesohaline 
Spring: 6 
Summer: 8 
Mainstem 
(annual): 4 
Spring: 5.6 
Summer: 7.1 
Prorocentrum 
minimum: 5 
Water Clarity: 
<8 
Dissolved 
Oxygen: 
5-6 
NC: 40 (tidal) 
Polyhaline 
Spring: 4 
Summer: 4 
Mainstem 
(annual): 1 
2-7 5 
Spring: 2.9 
Summer: 4.4 
Prorocentrum 
minimum: 5 
Water Clarity: 
<8 
Dissolved 
Oxygen: 
4-5 
NC: 40 (tidal) 
HW: 2; 5 <10%; 
10 <2% 
Sources: 1. U.S. EPA 2003; 2. Olson 2002; 3. Harding and Perry 1997; 4. Wetzel 2001, Ryding and Rast 1989, Smith et al. 1998, 
Novotny and Olem 1994; 5. Smith. 1998, Molvaer 1997; 6. U.S. EPA 2003; 7. U.S. EPA 2003; 8. U.S. EPA 2003; 9. Smeltzer and 
Heiskary 1990; 10.Heiskary and Walker 1988; 11. U.S. EPA 2003. 
decision rules for defining “persistently higher”. The jurisdictions should finally 
evaluate the degree of non-attainment of the dissolved oxygen and/or water clarity 
criteria within surrounding or “downstream” tidal waters. If these waters are in 
attainment of the dissolved oxygen and water clarity criteria, yet are persistently 
higher than the applicable target chlorophyll a concentrations, then these waters 
should be targeted for adoption of numerical chlorophyll a criteria. 
The jurisdictions should also evaluate results from Chesapeake Bay water quality 
model-simulated water quality conditions with achievement of the assigned 
nitrogen, phosphorus and sediment cap load allocations. The jurisdictions would 
then identify those Chesapeake Bay Program segments where the model simulated 
surface chlorophyll a concentrations are above a range of season and salinity regime- 
based target concentrations. The jurisdictions are encouraged to factor in findings 
from state-generated local TMDL modeling in the smaller tidal tributaries and 
embayments (e.g., Nanticoke River in Delaware, Anacostia River in the District of 
Columbia and several tidal tributaries in Maryland) as an additional source of 
chapter ix 
Determining Where Numerical Chlorophyll a Criteria Should Apply 
