Providing a long-term hydroclimatic perspective for the recent extreme decline of Lake Prespa (SW Balkans) through quantitative reconstruction of lake water-balance changes over the past 5000 years

Ancient lakes throughout the Mediterranean are at risk of disappearing due to a combination of climate change and human impacts. The growing imbalance between water availability and demand is creating unprecedented ecological problems. There is an urgent need for better understanding the patterns of natural lake water variability to improve water resource management and conservation. The incorporation of long-term cycles is particularly important for assessing low frequency – high magnitude trends in lake water levels.

The Ohrid-Prespa Lake system is amongst the oldest permanent lake systems in Europe, with an age of >1 million years, and hosts a globally significant biodiversity. The closed-basin Prespa Lakes are particularly sensitive to climatic variability with long-term water level changes informing on the dynamic balance between [1] runoff and precipitation supplying water to the lakes, and [2] water loss from the lakes by evaporation and underground karst outflow.

The large, ongoing, fall of the Prespa Lakes that started in 1987 threatens the biodiversity and water resources of the interconnected lake system. This decline is caused by climate change, specifically by decreases of 10% in precipitation and 25% in runoff, amplified by water abstraction. There is no precedent for this water level fall in the observational record (1951-present), although geological archives indicate equally low water levels at least twice over the past five millennia. 

Here we present the first quantified estimates of changes in the lake water balance over time that are based on the strong relationship between open water surface area and water loss. This quantification allows direct comparison of lake low- and highstand events across time and assessing magnitudes of regional hydro-climatic changes. This study uses a novel approach that reconstructs absolute lake levels and related open water surface areas for different past periods, using the landform-sediment record.

The hydro-climate of the Prespa catchment shows a drying trend of since the mid Holocene. The recent (2001-2018) lake lowstand is the most significant over the past 700 years in terms of water loss changes. A lake lowstand period of a similar magnitude occurred about 2000 years ago. The most extreme lowstand period over the past 5000 years occurred between 1100-800 years ago during the Medieval Climate Anomaly, when water loss changes were >50% higher compared to the present lowstand. However, the renewed decline in lake level and surface area since 2019 requires close monitoring; if lake level falls a further 2m to 840m.a.s.l. it would become the largest recorded fall over the entire Holocene, with unknown impacts for the wider system.