Water Quality Assessment of a Meromictic Lake Based on Physicochemical Parameters and Strontium Isotopes (87Sr/86Sr) Analysis: A Case Study of Lubińskie Lake (Western Poland)

In 2017, hydrochemical surveys of meromictic Lubińskie Lake (W Poland) and its water inflows were carried out. The lake experienced complete mixing in 2008 due to a series of orkan winds, and since 2015, intensifying worsening of water quality in the lake has been observed. Our aim was to determine the degree of transformation of Lubińskie Lake based on water chemistry and to identify the source of pollution of the lake using strontium isotopes (87Sr/86Sr) as a new chemical tracking tool. The physicochemical analysis confirmed the meromictic character of the lake. The comparison with previous studies (2003 and 2008) showed significant year-to-year differentiation, indicating intensifying eutrophication of the lake’s water, both in the epilimnion and the hypolimnion. Nine spring niches, directly supplying the lake, provide water with very high phosphorus and nitrogen concentrations (up to 10 kg of nitrogen and 0.9 kg of phosphorus daily). The strontium isotopes (87Sr/86Sr) analysis indicated that the lake’s water was supplied mostly by the springs, and recharge from deep aquifers is of secondary importance. Moreover, strontium isotope data and the relationship between Sr and Cl content support the finding that the high load of nutrients is of anthropogenic origin and reaches the lake through springs