In the littoral zone, most producers are rooted plants. In the limnetic zone, phytoplankton predominate. Nutrients fall from the upper layers and support life in the profundal zone. Deep lakes in the temperate zone rely on fall turnover and spring turnover to mix oxygen and nutrients. Young, deep, oligotrophic lakes are clear blue, with few nutrients to support algae.
Nutrients gradually accumulate, and algae tints the water green.
The lake becomes a productive, or eutrophic, lake. In rivers, organisms are adapted to local current conditions. Shoreline ecosystems include estuaries, rocky intertidal zones, sandy beaches, and mangrove swamps. Estuaries occur where a river empties into the sea. The richest marine ecosystems are coral reefs of coastal tropical waters.