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Organic matter and sulfur compounds as an indication of changes in the environment; long-term research (1990-2021) in the northern Adriatic and marine system of Rogoznica lake

Organic matter in the northern Adriatic shows very pronounced changes in quantity and properties. During the research years (1989-2021), periods of high and low concentration of organic carbon can be observed, with evident changes in organic matter reactivity in relation to the content of the surface active dissolved organic carbon. The observed changes on the long-term scale are predominantly due to: (a) annual changes in freshwater intake, mainly the Po River, (b) changes in circulation of water masses in the Adriatic Sea, as a consequence of the Adriatic- Ionian bimodal oscillating system (BiOS) and (c) the occurrence of excessive phytoplankton and gelatinous zooplankton blooms with appearance of mucilage aggregates, which contribute to eutrophication characterized by unusual accumulation of organic carbon. BiOS as a system responsible for advection of either highly saline ultraoligotrophic levantine intermediate waters from the eastern Mediterranean or lower salinity waters from the western Mediterranean richer in nutrients, reflected in the amount and properties of organic matter with different phase lags (from -3 to -4 years), has an increasing impact on the northern Adriatic. Also, on the middle Adriatic coast, marine lake Rogoznica is an ideal system for monitoring changes in the environment. Long- term data (1992- 2021) indicate that the lake shows changes observed through water column warming, accumulation of toxic sulfides and ammonia, and organic carbon in the bottom anoxic layer whose volume has increased several times since the early 1990s. Observed changes affect microbiological and biochemical processes in the lake. Long term results from this study might be significant contribution to general understanding of the response of organic carbon to a changing (warming) ocean. This work is supported by the MARRES project, IP- 2018-01-1717.

organic carbon ; sulfur compounds ; BiOS ; Northern Adriatic ; Rogoznica Lake

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