developed for use at the screening level or tier of risk assessments, to be followed by models 
specific to individual species in hi^er-tiered assessments. 
An important issue regarding population models is the type of individual-level effects they need 
as inputs to provide useful assessments of population-level effects. The toxicity tests and data 
typically available are often limited with regard to the endpoints and life-stages tested, often 
leaving significant gaps in the information needed to assess impacts of toxicity on population 
dynamics. Population model development needs to identify the critical inputs and be coordinated 
with toxicity model research to provide the methodology to make this information available. In 
particular, efforts to improve extrapolation of toxicity data should take such needs into 
consideration. 
Community Model 
Unless a^ssments are simply intended to provide protection to the most sensitive taxonomic 
groups, some synthesis is needed to relate expected effects on populations or individuals to 
consequences to the aquatic community. As mentioned above, current WQC are premised on the 
judgment that occasional low level toxicity to sensitive species constitutes accqjtably low risk to 
those species and to the aquatic communities to which they belong. This might be a sound 
judgment, but the lack of quantification makes it impossible to conclude whether higher 
exposures might also pose minimal risk, or what acceptably low risk means. 
There is thus a need to better quantify the association between toxic effects and effects on aquatic 
communities. This could entail a few lines of efforts. Better meta-analysis is needed of the 
variety of community assessments in experimental or natural ecosystems, linking these to a 
metric that better describes available toxicity data. More efforts are needed to study responses to 
toxic chemicals in complex aquatic ecosystems. Aquatic community models could be developed 
that integrate effects at the individual or populations-level. As part of these efforts, there is also a 
need to identify individual- and population-level endpoints important for supporting better 
community-level assessments, and to coordinate with toxicological research to provide this 
necessary information. 
Bioaccumulative Toxicants 
Toxicity risks associated with PBTs are expected to be advantageously assessed with residue- 
based dose response models. This important concept may be used to describe the minimum 
degree to which chemicals must be persistent and bioaccumulative in order to fall into this class. 
The degree of persistence determines what concentrations in water and sediments are available 
for incorporation of the chemical into benthic and pelagic food chains which lead to exposure of 
vulnerable organisms. The potential for a chemical to bioaccumulate in an organism is 
commonly referenced to concentrations of the chemical which persist in water and sediments. 
Thus, bioaccumulation factors (BAFs) and biota sediment accumulation fectors (BSAFs), in 
accordance with mechanistic food chain models which integrate all routes of exposure, are 
extremely important components of the risk assessment methodology for PBTs. For organic 
chemicals, hydrophobicity, as measured by the octanol-water partition coefficient is the 
primary determinant of bioaccumulation potential with metabolism in the food chain as an 
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