Oceans, lakes and streams
To understand present- and past aquatic environments we can use the sediment as an environmental archive. In aquatic environments you will find a range of different organisms. For instance one important group single celled organisms are foraminifera, they exist in all marine environments and have often a shell of calcium carbonate. The calcite shells make the foraminifera excellent environmental records as shell is preserved in the sediment and we can use the shell to perform various geochemical analyses.
Another important group is the diatoms which are siliceous phytoplankton at the base of the food web. Diatoms exist both in fresh and marine environments and can tell us about changes in pH, nutrients, light conditions, temperature etc.