Is anthropogenic nutrient input jeopardizing unique Lake Ohrid? - Mass flux analysis and management consequences [thesis]

Andreas Daniel Matzinger, Alfred Wüest
Lake Ohrid is comparably large (surface area A ~358 km 2 ), deep (maximal depth z max ~289 m) and one of the most voluminous (volume V ~55 km 3 ) lakes in Europe. It is a transboundary lake, with two-thirds in Macedonia and one-third in Albania. Including the underground karst connection to upstream Lake Prespa (A ~254 km 2 , z max ~48 m, V ~3.6 km 3 ), the watershed of Lake Ohrid extends into Greece. The overall karst groundwater inflow -recharged by Lake Prespa and local precipitation
more » ... ecipitation -contributes more than 50 % to the lake's water balance. Because of the comparably small watershed and the dry climate, water exchange is slow with a mean residence time of ~70 years. While a groundwater-fed, large water body in Mediterranean climate is exceptional in itself, the most outstanding quality of Lake Ohrid is its endemism (> 200 species). During the estimated two to five million years of existence, species covering the whole food chain have evolved or persisted in the lake. Given its importance as a global hotspot of biodiversity and being the only ancient, long-lived lake in Europe, Lake Ohrid was declared a UNESCO world heritage site in 1979. However, the unique ecosystem could be jeopardized by human impacts. A particular concern is potential eutrophication of the currently oligotrophic lake from population growth. It is feared that eutrophication could threaten the endemic flora and fauna, which has adapted to nutrient-poor conditions. However, assessment of eutrophication is not straight forward: Because of the long water residence time, changes will be slow. Moreover, expected variations in nutrient conditions are in the range of measurement error because of the low present-day concentrations. As there is a lack in high-quality, long-term data, different measurement schemes must be combined to assess eutrophication. During this work, a basic monitoring program was established, covering the lake, its inflows, as well as settling particles and accumulated sediments. Moreover, available historic information was collected and evaluated. Indeed, slow eutrophication could be detected in sediment cores over the past century. Combining all information in a linear model shows a ~3.5 fold increase in phosphorus (P) concentration from 1.3 mg-P m -3 to currently 4.6 mg-P m -3 . As was expected, the mean P residence time is comparably high at ~5 years, signifying that it takes around 15 years until a new P equilibrium is reached after an increase in input. Major P-input was identified from domestic point sources, which enter the lake diffusively and via tributaries. Agricultural sources seem to be of secondary importance at the moment but may potentially increase in the future if artificial fertilizers become affordable to local farmers. Compared to the P inputs from tributaries and diffusive sources, the groundwater inflows -though being the dominant water source -contribute only ~9 % to the P budget. However there is an eutrophication potential in the connection to upstream, mesotrophic Lake Prespa, which has ~7 times higher P concentrations than Lake Ohrid. The quantification of the Lake Prespa water share in accessible groundwater inflows
doi:10.3929/ethz-a-005165056 fatcat:hjgu2nbheffdthllj6kunjn23m