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The karstic Krka River is characterized by having lower dissolved organic carbon (DOC) concentrations (~30 μM) than coastal seawater (~60 μM). This peculiarity, together with the pristine nature of this area, makes the Krka River estuary a natural laboratory where it is possible to discriminate among the different dissolved organic matter (DOM) sources (riverine, marine and produced in-situ) and to study the main processes of DOM production and removal. The hypothesis behind this work is that in winter, due to the high discharge of the river, most of the DOM has a terrestrial signature, whereas in summer autochthonous DOM compose the main fraction of the DOM pool because of the reduced discharge, the high temperature and primary production. Our data shows that DOM in the river mainly consists of terrestrial molecules, as suggested by the high chromophoric content and low spectral slope (S275–295) values, as well by the predominance of humic-like substances. DOM in the seawater features the concentration and optical properties of the “typical” marine DOM from open sea waters. In summer, low riverine discharge and high temperature promote the intense biological activity, with an increase in DOC concentrations of up to 148 μM, resulting in a non-conservative behavior of DOM in the estuary. The high stratification combined with a decoupling between production and removal processes can explain the observed DOM accumulation. In the bottom layer DOM was released and quickly removed when oxygen was available, whereas in hypoxic waters the production of DOC, chromophoric DOM (CDOM) and fluorescent DOM (FDOM) was linearly related to oxygen consumption. Our work highlights the need of further studies combining chemical and biological information in order to gain new insights into the main processes responsible for DOM dynamics in this system.

Organski ugljik važan je indikator globalnih promjena te sudjeluje u mnogim biogeokemijskim procesima u moru. Ovaj rad dokumentira dinamiku promjena sadržaja i svojstava otopljene organske tvari/ugljika, u odabranim područjima Jadrana koja su pod jakim utjecajem unosa slatke vode kao i različitih vodenih masa: sjeverni Jadran (SJ) na profilu Rovinj – rijeka Po, Boka kotorska i albanski šelf. Ispitivani su sadržaj i svojstva otopljenog organskog ugljika (eng. DOC), partikularnog organskog ugljika (eng. POC) te površinski aktivnih tvari (PAT). Varijabilnost i trendovi u promjeni sadržaja i svojstava organske tvari u SJ u vremenskom razdoblju od 28 godina ukazuju na vrlo dinamičan sustav na prostornoj, a posebno na vremenskoj skali. Rezultati također pokazuju da je područje SJ dominantno oligotrofno, s kraćim, vrlo izraženim eutrofnim/mezotrofnim epizodama. Svojstva i sadržaj organske tvari u sjevernom Jadranu pod jakim su utjecajem promjenjivog unosa slatke vode rijekom Po te pod utjecajem advekcije vodenih masa koje ulaze u Jadransko more iz Mediterana tijekom različitih tipova cirkulacija u sjevernom Jonskom moru uvjetovanih Jadransko-jonskom bimodalnom oscilacijom (BiOS). U vremenskom razdoblju u kojem su istraživana sva tri područja (03/2008- 05/2009), najadsorbabilniji, to jest najreaktivniji organski materijal nađen je u području Boke kotorske (ali bez naznaka eutrofnosti), po svojstvima bliži adsorbabilnijim modelnim spojevima dekstranu T-500 i fulvičnoj kiselini. Slično je pokazano i u jednokratnoj proljetnoj analizi tada oligotrofnog albanskog šelfa. Varijabilnost reaktivnosti otopljene organske tvari u SJ tijekom ovog dugogodišnjeg ispitivanja je bila izraženija nego u ostala dva istraživana sustava. Svojstva organske tvari mijenjala su se od reaktivnijeg, adsorbabilnijeg materijala sličnog fulvičnoj kiselini i dektranu T-500, do slabije adsorbabilnog, odnosno manje reaktivnog organskog materijala predstavljenog modelom visokokomolekularnog polisaharida ksantana. Organic carbon is an important indicator of global change and is a constituent of many biogeochemical processes in the sea. In this thesis organic matter/carbon properties, variability and dynamics in selected areas of the Adriatic (which differ in freshwater influence as well as advected water masses) are shown: transect Rovinj – Po in the northern Adriatic (NA), Boka Kotorska Bay and Albanian shelf. Organic matter in selected areas was monitored regarding its concentration (dissolved organic carbon, DOC; particulate organic carbon, POC) and properties (surface active substances, SAS). The NA was investigated for a period of 28 years. Through investigations of 28-years long time series, it is shown that the NA organic matter has high temporal and spatial variability. Oligotrophic conditions are common in the NA, yet with short but very strong eutrophication episodes. Properties and concentration of organic matter in the NA appears to be under strong local influence of the Po River load, but also under the influence of different water masses entering the Adriatic from the Mediterranean caused by the Adriatic-Ionian Bimodal Oscillating System (BiOS). During the period when all three areas were investigated (03/2008-05/2009), the most adsorbable (hence the most reactive) organic material was found in the Boka Kotorska Bay (with no signs of eutrophication), whose properties are close to more adsorbable model substances such as dextran T-500 and fulvic acid (FA). Similar finding was documented for the single analysis carried out on the oligotrophic Albanian shelf. Reactivity of the dissolved organic matter in the NA during this long-term investigation is shown to be the most variable of all investigated areas. Organic matter properties changed from more reactive/adsorbable material, close to FA and dextrane T-500 in properties, to less adsorbable/reactive organic material close to high molecular polysaccharide xanthan in properties.