A layer of permafrost lies beneath the surface. During the spring and summer, meltwater forms rivers and pools. Lichens are common in the treeless tundra, and animals include caribou, reindeer, lemmings, and snowy owls. Eskers are sandy ridges that break the icy terrain and are remnants of rivers that ran beneath glaciers. Desert plants are well adapted for obtaining and storing water, with rapid life cycles and succulent leaves.
Animals also minimize water loss with tough integuments, and they are active at night.
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.