Dissolved organic carbon and surface active substances in the northern Adriatic Sea: Long-term trends, variability and drivers.

The paper presents a unique time series of organic matter content (dissolved organic carbon, DOC, and its surface active substances fraction, SAS) collected in the northern Adriatic along the Po-Rovinj transect between 1998 and 2017. The data were collected on a monthly or bimonthly basis. Seasonal variance of organic matter content does not exceed 30% of its total variance, while the DOC and the SAS trends are significantly negative and positive, respectively, over the whole transect. The organic matter content, however, exhibits pronounced interannual and decadal changes, with periods of high and low carbon content and evident changes in composition of the SAS content. The changes indicate altering episodes between eutrophication and oligotrophication, embedded in the overall oligotrophication trend in the considered period. Both series were correlated with the potential local and regional yearly-averaged drivers in both atmosphere and sea. DOC is most strongly (significant at 99%) correlated with the Po River discharges, at the phase lag of -1 to -2 years. For the SAS, the largest correlations (significant at 99%) are obtained with the Adriatic-Ionian Bimodal Oscillating System index (BiOS index), at the phase lag of -3 to -4 years. Correlations between the organic matter content and the hemispheric or the regional climate indices (North Atlantic Oscillation, East Atlantic/West Russia, East Atlantic, Scandinavian, and Mediterranean Oscillation) are much lower and only sparsely significant at 95% at some phase lags. The same was found for the other local drivers (precipitation and net heat flux). Our analysis highlights the importance of remote processes, like the BiOS, that weren't previously considered to shape the biogeochemical properties of such shallow coastal region impacted by freshwater load. To properly assess such impacts, long-term ecological monitoring and homogenized data series are required.