Seealpsee, Lake in the Alpstein range, Appenzell Innerrhoden, Switzerland

Seealpsee is a lake at 1,143.2 m in the Alpstein range of Appenzell Innerrhoden, Switzerland, covering 13.6 hectares. The Berggasthaus Forelle am Seealpsee, with a large open-air terrace, dominates the western end. A low bank of pale earth and scattered light-colored rocks separates the water from the trees on the far shore.

The lake became an energy source in the 20th century when the Kraftwerk Seealpsee-Wasserauen power plant was built to supply electricity to Appenzell town. This development connected the mountain region to modern infrastructure and changed its role in regional supply.

The name Seealpsee derives from the adjacent Seealp; a 1682 record calls it See Alper See. The Berggasthaus Forelle, with a large open-air terrace, stands at the western end of the lake. In hot summers people swim in the lake after descending from a mountain tour.

From Wasserauen, the terminus of the Appenzeller Bahnen, an asphalted but partly steep path climbs to the lake in about 60 minutes. A longer, gentler unpaved route runs up the southeastern valley side via Hüttentobel. The steep path down from Ebenalp through the Wildkirchli cave past Gasthaus Aescher to the lake is for experienced hikers with hiking boots only; children should be roped.

In July 2009 and 2010 the lake turned red, caused by dense blooms of the dinoflagellate Tovellia sanguinea, which thrives in nutrient-poor cold water above 1000 m. Researchers from ETH Zurich, the Universities of Zurich and Copenhagen, and divers from the University of Konstanz sampled the sediment. The alga is discussed as a bioindicator of very clean water.