EPA-823-F-05-007
October 2005
United States	Office of Water
Environmental Protection Agency	43005T
AQUATOX (RELEASE 2.1): MODELING
ENVIRONMENTAL FATE AND ECOLOGICAL EFFECTS
IN AQUATIC ECOSYSTEMS
The U S Environmental Protection Agency is releasing an enhanced version of the freshwater
ecosystem simulation model AQUATOX which predicts the fate of various pollutants, such as
nutrients and organic chemicals, and their effects on the ecosystem, including fish, invertebrates,
and aquatic plants AQUATOX is a valuable tool for ecologists, biologists, water quality
modelers, and anyone who performs ecological risk assessments for aquatic ecosystems Release
2.1 has enhanced scientific capabilities to more effectively simulate the fate of nutrients and
toxic orgamcs and their effects on aquatic ecosystems
Background
One of the biggest challenges to protecting or restoring
our Nation's waters is to adequately understand the
relationships between the chemical and physical
environment and the organisms that live there.
Ecosystems are complex, with seasonal and annual
variations and multiple interactions among species.
Excess nutrients are leading causes of water quality
impairment. To address this problem, states need the
technical resources to establish scientifically sound
nutrient criteria and methods to implement them in
water quality regulatory programs. Ecosystem models
such as AQUATOX, which mechanistically simulate
nutrient dynamics, can be one tool for deriving and
implementing nutrient criteria.
About AQUATOX Release 2.1
AQUATOX is a PC-based ecosystem model that predicts the fate of nutrients and organic
chemicals in water bodies as well as their direct and indirect effects on the resident organisms.
Release 2.1 improves its ability to simulate nutrients and their biological effects by
o More realistic tracking of nutrients, by allowing different ratios of nutrients to organic
matter in different components of the ecosystem,
o Ability to better use monitoring data as input or comparison by enabling input and output
of multiple forms of nutrient data. The linkage to the BASINS watershed modeling
system has been updated accordingly
» Improved simulation of phytoplankton (algae in the water column) and periphyton
(attached algae), particularly in nver systems
o Un-ionized ammonia (which may be toxic) and variable pH may now be simulated

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•	Simulation of toxicity due to organic compounds that do not bioaccumulate has been
improved because it can now be based on external, rather than internal, concentrations
•	The user can now get have Steinhaus community similarity indices calculated and
exported. The Steinhaus index can be used as one measure of how the predicted
composition of the biotic communities varies between simulations.
What can Release 2.1 do?
•	Evaluate which of several stressors is causing biological impairment. For example- Are
nuisance levels of attached algae in streams controlled primarily by nutrients, sediments,
grazing by snails, or flow conditions9
•	Explore how changes m land use or agricultural practices in a watershed might affect
aquatic life, by linking to the watershed models in BASINS.
•	Develop targets for nutrients in lakes and reservoirs with nuisance algal blooms.
•	Estimate time to recovery of fish or invertebrate communities after reducing pollutant
loads.
Who can use AQUATOX?
AQUATOX is a valuable tool for ecologists, biologists, water quality modelers, and anyone
involved in performing ecological risk assessments for aquatic ecosystems.
To get more information
You can get more information about the AQUATOX model and other water quality models by
visiting our internet site at www epa.gov/waterscience/wqm/ For more specific technical
information, call the Standards & Health Protection Division, Office of Science & Technology in
EPA's Office of Water Ask for the AQUATOX manager at 202-566-0400.

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