**2.3.2 Aquatic ecosystem models**

Aquatic ecosystem models are the successors of water quality models; however there is not a standard definition of a "water quality model" and "a water ecology model" or a very strict border between them. Many well known aquatic ecosystem models or their predecessor water quality models were developed in late 1970s. There are many well written texts related to water quality and aquatic ecosystem modelling, so the reader is referred to those texts. Information related how to obtain them can be reached by simple internet queries. Following paragraphs will give brief information on some well known models that may be useful for aquatic ecosystem modelling especially on estimating the possible impacts of climate change on aquatic ecosystems.

The Water Quality Analysis Simulation Program (WASP) is a water quality model that was developed in early 1980s by United States Environmental Protection Agency. It is a good model for initial studies. The latest version of WASP (Version 7.5) includes an advanced eutrophication module that can simulate the nutrient cycle and primary production up to three phytoplankton groups as well as the detritus cycle. Unfortunately higher trophic levels of the aquatic food web are not covered by the advanced eutrophication module. WASP can be driven by external hydrodynamic simulation models.

CE-QUAL-W2 is a hydrodynamic and water quality model in 2D (longitudinal-vertical). It is applicable to large watershed/water resource systems that contain these types of water bodies such as lakes, rivers and reservoirs. The current model release enhancements have been developed under research contracts between the Corps and Portland State University. The model can simulate basic eutrophication processes such as temperature-nutrient-algae (multi groups)-dissolved oxygen-organic matter and sediment relationships. Additionally, zooplankton (muti groups) can also be simulated.

CE-QUAL-R1 is a one dimensional (vertical) reservoir model developed by Hydrologic Engineering Center (United States Army Corps of Engineers). It can simulate nutrients and phytoplankton (three groups) and zooplankton like CE-QUAL-W2. Additionally, a simplified simulation of fish can be conducted. The model is designed to simulate anaerobic processes and dynamics of reduction processes as well.

AQUATOX is originally developed to assess the fate and effect of chemicals in experimental containers. With the improvements of former versions of AQUATOX, the model has reached the 3rd release, which has the capability of risk assessment combined with the fate and effect of pollutant and toxic chemicals in the aquatic environments. The way AQUATOX characterizes the aquatic system is different from many other models do. Mostly the ecosystem models represent the individuals by the changes in their numbers; hence, they called as population models. However, AQUATOX simulates the ecosystem by changing the concentrations of all components such as chemicals, sediments, and even organisms including the ones on the higher trophic levels of food web. The model is intended to assess dynamic effects of various stressors such as temperature, toxic chemicals, nutrients, sediment; which is applied to aquatic environments from experimental tanks to lake systems.
