The potential to retain nitrogen in beaver ponds and delta floodplains of the River Nemunas.
Lamsodis, Romanas; Vaikasas, Saulius
The River Nemunas (97,860 km2) flows via the Curonian Lagoon into the Baltic Sea, and has 400 km2 of floodplains in the delta. 82 % of the hydrographical network in the Lithuanian sector (consisting of 47 %) of the basin is regulated and converted into drainage channels. Due to its catchment size and significant pollution from agriculture, the Nemunas River basin is identified as an agricultural hot spot in the Baltic Sea basin. On average, the total annual inorganic nitrogen runoff of the river is 26.96106 kg; it corresponds to the load of 275 kg km-2 yr-1. About 11,000-12,000 beavers (Castor fiber L.) inhabit these drainage channels. It is estimated that beaver impounding activities in drainage channels in Lithuania might reduce the nitrogen load from the River Nemunas between 42-121 kg km-2 yr-1. Reduction is most intensive in spring and summer. Man-controlled inundation (summer type polders) of floodplains in the Nemunas delta may further reduce its runoff nitrogen load by 21 kg km-2 per flood event. Low flow velocities and long retention times allowing for sedimentation and denitrifying processes favour this self-purification. Beaver impoundments in drainage channels as well as inundation of the Nemunas delta floodplains are potentially natural means to assist the rehabilitation of trained river reaches and valleys.