Dynamics of organic matter in the changing environment of a stratified marine lake over two decades

The marine lake (Rogoznica Lake), which fluctuates between stratified and holomictic conditions, is a unique environ-ment on the eastern Adriatic coast affected by environmental changes. These changes are reflected in the warming of the water column, the apparent deoxygenation of the epilimnion, and the accumulation of organic matter (OM), toxic sulfide, and ammonium in the anoxic hypolimnion. Since the early 1990s, the volume of anoxic water has increased as the chemocline has moved to the surface water layer. A trend toward enrichment of refractory dissolved organic car-bon (DOC) was observed in the anoxic hypolimnion, while a decreasing trend was observed in the oxic epilimnion in the spring DOC. At the same time, the most reactive surface-active fraction of DOC showed the opposite trend. In addition, there is evidence of accumulation of particulate organic carbon (POC) in the water column, followed by an increase in the fraction of POC in total organic carbon (TOC).On a multi-year scale (1996-2020), this work presents a unique time series of the dynamics of OM in the stratified ma-rine system, showing a significant change in its quantity and quality due to climate and environmental variability.DOC-normalized surfactant activity is shown to be a good indicator of environmental change.