Hydrobiologia (2006) 553:89–109 Ó Springer 2006 DOI 10.1007/s10750-005-6427-9 Primary Research Paper Is Lake Prespa jeopardizing the ecosystem of ancient Lake Ohrid? A. Matzinger1,*, M. Jordanoski2, E. Veljanoska-Sarafiloska2, M. Sturm1,B.Mu¨ ller1 &A.Wu¨ est1 1Swiss Federal Institute of Environmental Science and Technology (EAWAG), Limnological Research Center, CH-6047 Kastanienbaum, Switzerland 2Hydrobiological Institute, Naum Ohridski 50, MK-6000 Ohrid, Macedonia (*Author for correspondence: E-mail:
[email protected]) Received 16 July 2004; in revised form 30 March 2005; accepted 25 April 2005 Key words: Lake Ohrid, Lake Prespa, karst, phosphorus, eutrophication, water level fluctuation Abstract Lake Prespa and Lake Ohrid, located in south-eastern Europe, are two lakes of extraordinary ecological value. Although the upstream Lake Prespa has no surface outflow, its waters reach the 160 m lower Lake Ohrid through underground hydraulic connections. Substantial conservation efforts concentrate on oligotrophic downstream Lake Ohrid, which is famous for its large number of endemic and relict species. In this paper, we present a system analytical approach to assess the role of the mesotrophic upstream Lake Prespa in the ongoing eutrophication of Lake Ohrid. Almost the entire outflow from Lake Prespa is found to flow into Lake Ohrid through karst channels. However, 65% of the transported phosphorus is retained within the aquifer. Thanks to this natural filter, Lake Prespa does not pose an immediate threat to Lake Ohrid. However, a potential future four-fold increase of the current phosphorus load from Lake Prespa would lead to a 20% increase (+0.9 mg P m)3) in the current phosphorus content of Lake Ohrid, which could jeopardize its fragile ecosystem.