Cationic metals and ammonia will be used as examples, where helpful, in the discussion here and 
as candidates for chemical-specific research discussed later. They represent major concerns in 
current aquatic risk assessments and have a rich toxicity literature that will support development 
of dose-response models, and evaluation of the quality of assessments made with limited data. 
However, while the discussion here will in part highlight these chemicals, the issues discussed 
are true for many nonbioaccumulative toxicants. General methods developed for any particular 
chemical should be directly, or by example, applicable to other nonbioaccumulative toxicants. 
Conceptual Model 
As discussed above, research needs for development of risk assessment methods need to relate to 
the conceptual model for the assessment(s). Figure 10 presents a simple conceptual model that 
identifies important elements that need to be considered in conducting and developing methods 
for aquatic risk assessments and criteria development. The horizontal series of boxes on the 
bottom of the figure represent major stages in linking chemical stressors to ecological responses. 
Loadings of toxic chemicals to an aquatic system are distributed throughout the system, resulting 
in exposures to biological receptors. These exposures may adversely affect the survival, growth, 
and reproduction of individual organisms. Such individual-level effects may be expressed at the 
population level as changes in population size, growth rate, and structure. Changes at the 
population level may in turn elicit changes in the community. The "stacked-boxes" used to 
represent these stages are intended to indicate that assessments often will involve multiple 
chemicals and/or biological species. 
The arrows linking the lower series of boxes represent mechanistic relations among these stages, 
and involve the application of various models identified by the series of ovals in the center of the 
figure. The arrows are bi-directional to indicate the fundamental similarities between two modes 
of environmental protection: 1) criteria development and application that determine chemical 
loadings consistent with protecting desired system values (left-to-right) and 2) assessments that 
characterize risks expected from chemical exposures (right-to-left). 
The boxes on the top of Figure 10 identify data needs for the models used in the assessment. All 
of the models will generally require various information on various physical, chemical, and 
biological characteristics of the system. The exposure and toxicity models also will require 
information on the nature of the chemical and its toxicity to various biological receptors. 
This conceptual model helps to identify and evaluate research needed to improve aquatic risk 
assessments and criteria development. Primary attention should be given to the four basic 
models in Figure 10. How are current assessments limited by the inability of exposure models to 
completely and accurately predict and describe the relationship between chemical loadings and 
exposure concentrations? How well do current methods actually describe risks to individual- 
level endpoints under exposure conditions expected in natural systems? What is the significance 
of organismal-level effects to populations and communities? Another research focus would be 
the adequacy of data needed for these models. Can required chemical properties be estimated 
well? Does available toxicity data address the endpoints needed in the assessment? 
83 
