structural characteristics. Following these tests and the derivation of the necessary information, 
mesocosms would be used in the laboratory and field (enclosures) to determine effects on 
populations and communities of seagrasses. The results of the single species, population, and 
community exposures will need field validation, which box lb addresses in part. Following 
these experiments, mesocosms could be used to determine effects on populations, and 
communities of seagrasses and associated biota, particularly relating the role of seagrass beds as 
habitat to fish and shellfish populations and communities. 
The technical information on stressor/response relationships can be used to generate thresholds 
for sediment effects (box 4). These thresholds can be used in the development of criteria (box 4) 
for suspended and bedded sediments. These thresholds can serve as input to both the 
development of criteria (box 5) and the TMDL process (box 6). These criteria will be 
“integrated” criteria, similar to those discussed in the Toxic Chemical Section (Section 7). They 
will consider the effects of suspended and bedded sediments in a more holistic manner than the 
standard criteria do, taking into account effects on both benthic and water colunm organisms, and 
direct as well as indirect effects on aquatic life use. These criteria also will have to take 
magnitude, duration, and frequency of changes in suspended and bedded sediments into account. 
The availability of sediment criteria and thresholds for sediment effects will allow for a TMDL 
process that is “effects based”. The current TMDL methodologies focus much more on exposure 
and reduction of exposure, than on effects. Acceptable levels of suspended and bedded sediment 
can not presently be based on effects, because the models and stressor-response relationships to 
be derived in boxes 1 and 2 are not currently available. All that the NHEERL effort can provide, 
at this time, is information on effects on certain organisms and classes of organisms. Resource 
managers will have to use that information to make management decisions. At the same time, 
we will have to make sure that the data are provided in such a way that is useful in the context of 
designated uses. The data will have to be presented in as general a form as possible, as opposed 
to just presenting a list of data on individual species for specific magnitudes and durations of 
elevated suspended and bedded sediment concentrations. Priority will be given to those species 
which are tied to designated uses. 
Once TMDLs are produced, their effectiveness will be assessed (box 7) and this will allow 
further refinement of the models developed in boxes 0,1, and 2. 
The final piece of the TMDL process is the exposure component. Loading estimates and models 
are currently being developed by NERL, OW, and NHEERL (under Goal 8) (box 8). These 
loading estimates and models will allow estimation of the changes in suspended and bedded 
sediments inputs which might be needed to reach the targets set on the basis of the effects of 
suspended and bedded sediments effects (boxes 4 and 5). These loading estimates and models 
also provide input to the conceptual model, setting the bounds of bedded sediment and suspended 
solids inputs to aquatic systems. 
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