Untersee, also known as Lower Lake Constance, is the smaller of Lake Constance’s two main basins. Situated at the northern foot of the Alps, this freshwater body is a distinct aquatic environment. It connects to Upper Lake Constance (Obersee) via the Seerhein, a short stretch of the Rhine River. This unique geographical arrangement contributes to its specific ecological characteristics and scientific interest.
Geographical and Hydrological Profile
Untersee is located where the German state of Baden-Württemberg meets the Swiss Cantons of Thurgau and Schaffhausen. The international border between Germany and Switzerland crosses its waters. With a surface area of 62 to 63 square kilometers, Untersee is considerably smaller than the Obersee.
The lake has an average depth of 13 meters, reaching a maximum depth of 45 meters. The Seerhein, a four-kilometer section of the Rhine, links Obersee and Untersee, flowing through Konstanz before entering the lower basin. Additional inflows include the Radolfzeller Aach, while the High Rhine serves as the main outflow near Stein am Rhein. Untersee’s total water volume is 0.8 cubic kilometers, with a surface elevation of 395.11 meters above sea level. Water temperatures typically range between 18°C and 26°C in August, influenced by the region’s mild microclimate.
Unique Ecosystems and Biodiversity
Untersee supports over 600 plant species along its shores. These include the Lake Constance forget-me-not (Myosotis rehsteineri), adapted to undisturbed limestone gravel beaches. Other notable plants are spring gentian, mealy primroses, and Siberian irises, contributing to diverse vegetation, particularly in extensive reed beds.
The lake is one of Central Europe’s most significant bird regions, serving as a primary resting and wintering ground for migratory species. Tens of thousands of tufted ducks from Siberia and cob ducks from Spain winter here. Nature reserves like the 774-hectare Wollmatinger Ried host over 290 bird species.
Untersee’s waters host 30 to 45 fish species. A distinctive whitefish, “Sandfelchen” (Coregonus lavaretus), thrives in its shallower, more eutrophic conditions. An unusual variant of European perch (Perca fluviatilis) with yellow fins is also found. The lake’s food web has changed due to non-native species like the quagga mussel (Dreissena bugensis), which spread rapidly since 2017. The three-spined stickleback (Gasterosteus aculeatus), once near-shore, now dominates open water, impacting native fish populations like whitefish.
Ecological Significance and Conservation
Untersee is an important wetland habitat in Central Europe, supporting diverse plants and animals. Its diverse topographical and ecological features contribute to rich biodiversity. The lake also serves as a drinking water source for about 4 million people, supplied through an extensive pipeline network to various communities.
The ecosystem faces environmental pressures, including invasive species like the quagga mussel, which can alter the lake’s natural balance. Climate change also affects the lake’s stratification and water level patterns. Historically, agricultural runoff was a concern, though efforts have reduced phosphorus levels.
Conservation measures safeguard Untersee’s unique characteristics. A substantial 96.72% of the Untersee of Lake Constance Key Biodiversity Area is protected. The 774-hectare Wollmatinger Ried–Untersee–Gnadensee nature reserve on the German shore is one such protected area. International cooperation, through the International Commission for Water Protection (IGKB) established in 1959, coordinates water quality monitoring and conservation. The lake also holds Ramsar and EU Natura 2000 designations.