Minor changes have been made to the AQUATOX model since Release 2.1 was publicly released. This document describes changes in the model that distinguish Release 2.2 from Release 2.1 and describes any changed equations. It has become increasingly apparent that methods and tools are needed to analyze the combined fate and effects of all stressors on aquatic ecosystems from all sources. Stressors may include nutrients, organic loading, toxic organic compounds, sediments and habitat alteration; sources include point and non point source loadings and atmospheric deposition. Stressors may affect water clarity, algae populations, dissolved oxygen levels, fish and invertebrate communities, levels of contaminants in fish tissue, and many other important environmental conditions. Management approaches that focus on one stressor at a time may miss important interactions that could determine whether overall environmental goals, such as restoration of a more natural aquatic ecosystem, are met. Environmental management programs and activities that could benefit from additional tools for an integrated approach include water quality criteria and standards, Total Maximum Daily Loads (TMDLs), identification of the cause(s) of biological impairment where there are time-variable ecological risk assessment model that simulates the fate and effects of various environmental stressors in aquatic ecosystems. It simulates the fate and transfer of pollutants from loads to the water, sediments, and biotic components, and transfer throughout the food web. Simultaneously it predicts the effects of the stressors on the ecosystem, by simulating the chemical, physical and biological processes that bind the ecosystem together. AQUATOX can predict the fate and ecological effects of nutrients, organic toxicants, and bioaccumulative compounds, as well as the expected ecosystem responses to pollution reductions.