The Curonian Lagoon is a transboundary water shared between Russia and Lithuania and separated from the Baltic Sea by the Curonian Spit. With a surface area of 1,584 km2, it is the largest lagoon in Europe. The lagoon is shallow (mean depth 3.8 m) with a mainly freshwater ecosystem. The main tributary is the Nemunas river that provides the main inflow, both in terms of the amount of water, nutrients and persistent pollution. The Curonian lagoon is a hypereutrophic environment often dominated by cyanobacterial blooms in summer.
Fishing and small-scale fish processing have traditionally provided an important economical basis for many small communities around the lagoon, also tourism, port activities and agriculture play an important and continuously growing role. The water-quality management and nature conservation system in the lagoon are covered by both European directives (in the Lithuanian section) and the Russian Federation Water Code.