Determining the Ecological Status of the Dinaric Karst Natural Lakes in Croatia: Eight Natural Lakes, Seven Lake Types, and One Very Significant Equation

The delicate balance of aquatic environments and communities living in them is closely linked to water quality, making its assessment a crucial factor in environmental management. Despite the progress driven by the European Water Framework Directive, challenges remain, especially in understanding and addressing the specificities of rare freshwater ecosystems, such as the Dinaric karst lakes in Croatia. We aimed to find a solution to two very uncommon issues in freshwater ecological classification and monitoring of lake ecosystems: (1) specific karst lake habitats, resulting in eight natural lakes being classified as seven specific lake types, and (2) a very narrow pressure gradient, as most lakes are in good or high ecological quality status or a near-natural state. A modified proportional stratified sampling approach was used, in which between 2 and 7 sampling sites were identified for each of the eight natural lakes based on various factors, including lake surface area and sediment composition. Environmental parameters and anthropogenic pressures were assessed for each lake. This process led to the creation of a stepwise linear regression model to estimate the reference conditions and a multimetric index to determine the final ecological quality ratios (EQRs) of the lakes. Through extensive sampling and analyses, the study contributes valuable insights to the assessment of water quality and proposes a model applicable to freshwater ecosystems with similar typological challenges and habitat specificities worldwide.