Seasonal distribution of organic matter and copper under stratified conditions in a karstic, marine, sulfide rich environment (Rogoznica Lake, Croatia)

Closed, isolated small systems, as the representatives of a “unique-environmental feature”, are valuable natural laboratories for studying different biogeochemical processes. The saline Rogoznica Lake (“Dragon Eye”), situated on the Eastern Adriatic coast is such a system (10 276 m2, 15 m deep) typical of many stratified, sulfide rich water bodies. The depth of mixolimnion changes seasonally and it is greatly influenced by meteorological conditions, i.e. temperature and rainfall. Vertical mixing usually occurs during winter when cold, oxygen-rich water from the surface sinks downwards. 9 we monitored seasonal distribution and variation of copper complexing capacity (LT), related apparent stability constants (Kapp), concentration of Cu2+ ions, surfactant activity (SAS), dissolved organic carbon (DOC) and reduced sulfur species (RSS). Our results have shown that LT is increasing with depth up to 8 m depth, while the concentrations of copper ions decrease with the depth due to the higher amount of RSS species in deeper layers. The values of log Kapp are also decreasing with depth as a consequence of a competition of organic ligands and sulfide ions for binding Cu. Below 8 m depth the presence of high amounts of RSS (∼1 mM) influences the electrochemical measuring of copper ion and LT determination, contributing to the copper ion binding.