Hypoxic/anoxic conditions typically manifest themselves on diel or seasonal time scales during 
critical times of the annual cycle. For southem/semi tropical regions, the critical period is 
typically from May to October; for northem/temperate regions the period is from June to 
September. Seasonal hypoxia generally develops as a consequence of water column 
stratification, whereas diel cycles generally occur in non-stratified or partially stratified systems. 
Seasonal hypoxia is persistent throughout the critical period whereas diel hypoxia may be cyclic 
(regular frequency and duration; e.g., diel, tidal) or episodic (irregular frequency and duration). 
DO Criteria 
Part of the process of setting nutrient criteria based on DO involves determination of the 
minimum DO requirements of aquatic organisms. NHEERL is in the process of providing the 
scientific basis for setting minimum DO criteria for ecologically and commercially important 
organisms in coastal receiving waters through laboratory exposures. Survival data from 
laboratory exposures, using controlled DO concentrations, will provide risk assessment managers 
with the basic information needed to set minimum DO protection limits for the Nation’s waters. 
General Classification Variables 
The classification scheme for the DO endpoint will be a common effort across all Divisions. Our 
plan is to link and improve existing models of flushing, light limitation, primary production 
controls, and oxygen supply dynamics to sort coastal receiving waters into groups of similar 
overall relative sensitivity. The number of groups will depend on the range and variability of 
estimates. Chapter 6 of Clean Coastal Waters (NRC 2000) lists 12 factors that influence the 
susceptibility of coastal receiving waters to nutrient over-enrichment. We will start with this list 
and focus on those factors that will be most useful in a classification scheme to determine the 
relationship between nutrient loading and DO as an effect of nutrient over-enrichment. The 
relative magnitude of atmospheric oxygen entrainment to the bottom waters and respiratory 
depletion determines whether a portion of the water column experiences episodes of low DO. 
The most important factors that affect entrainment include: density, salinity, and/or temperature 
stratification. In turn, these factors are influenced by climate/weather (temperature, wind), 
geomorphology (tides), and circulation patterns (tides and freshwater input) of the receiving 
waters. 
Once the classification scheme with the key parameters influencing the response of DO to 
nutrients has been developed, we will test the classification using field data and comparable data 
from literature or other institutions and agencies where available. Both empirical and numerical 
simulation models will be used to improve our understanding of how these parameters interact 
and control the DO response to increased nutrient supply across a wide range of coastal receiving 
waters. 
Measurement Endpoints 
As mentioned above, "low DO" is one of the assessment endpoints that is to be related to excess 
nutrients in receiving waters. There are many methods and approaches to determine DO in 
coastal receiving waters. Providing sound basis for a national nutrient criteria based on DO will 
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