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. The region of ocean near the shore is the neritic zone. The most productive areas are in the neritic zones where upwelling occurs. El Nino is a weakening or reversal of Pacific easterly trade winds. Many populations migrate or decline in response to blocked upwelling which alters food webs.