Crystals to ice
Scientists have discovered that frozen, wintry lakes can fuel the growth of certain types of algae and zooplankton that thrive during the cold season, blooming under the lake's icy cover.
Here, ice forms on the surface of Lake Pääjärvi, a lake in southern Finland.
Lake Erie, the fourth largest of North America's five Great Lakes, hides concentrated communities of algae under a layer of lake ice, mostly the filamentous diatom Aulacoseira islandica.
Frozen waterfall on the River Jägala in Estonia, in March 2011.
More frozen falls
Another view of the frozen waterfall on the River Jägala in Estonia, in March 2011.
Breaking ice on Estonia's Lake Võrtsjärv, a shallow lake in southern Estonia, in April 2011.
Piles and piles
Along the shore of Estonia's shallow Lake Võrtsjärv in southern Estonia, ice breaks in April 2011.
Cracks and fractures
Another pile of breaking ice on the shores of Estonia's Lake Võrtsjärv, a shallow lake in southern Estonia, in April 2011.
Frozen Lake Võrtsjärv in Estonia, in January 2014.
Võrtsjärv is large but shallow — though it measures 104 square miles (270 square kilometers) the lake is only 20 feet (6 meters) deep.
In January 2014, the large but shallow Lake Võrtsjärv in Estonia is frozen.
Ice on Russia's Lake Baikal, an ancient and enormous lake in Siberia. It measures approximately 400 miles (644 kilometers) in length and over 5,000 feet (1,637 meters) in depth, and is the oldest and deepest lake on Earth.