• Arno River
• CDOM
• Coastal area
• Dissolved organic carbon
• Dissolved organic matter
• Tyrrhenian Sea

Rivers represent the link between the land and the ocean, and their input of large amount of organic matter and nutrients, stimulates biological activity in coastal areas, making them one of the most productive ecosystem on the Earth. Climate change is expected to have a profound impact on rivers and coastal areas. Small rivers can be good systems to study and predict the effects of climate change since, due to their small watershed, they are more likely to have fast response to events like floods and intense precipitation.
The main goal of this study is to report a detailed description of the dissolved organic matter (DOM) cycle along the aquatic continuum, from the inland (spring of the Arno River) to the sea (coastal area of the Tyrrhenian Sea). This study aims to contribute to the estimate of organic carbon fluxes by small rivers to the Mediterranean Sea and to get insights into the role of DOM cycle in the CO2 fluxes in rivers and estuaries.
Dissolved organic carbon (DOC) and the optical properties (absorption and fluorescence) of chromophoric DOM (CDOM) were weekly measured in the Arno River, in order to gain information on DOM annual cycle. Three sampling campaigns were carried out from the spring to the mouth of the river in order to identify the main sources of DOM along the river flow. The mineralization processes, affecting DOM at the estuary, were investigated in different seasons with incubation experiments with natural microbial community. Finally, data collected during an oceanographic cruise, carried out in a large coastal area of Tyrrhenian Sea in April 2013, were used in order to study the impact of river inputs on DOM cycle in coastal areas.
Our results showed that in the Arno River, DOM has a clear annual cycle, driven by discharge in winter, net production between spring and early summer and net removal between the end of summer and autumn. In the river, DOC concentration ranged between 169 µM in winter and 528 µM in summer, in both 2014 and 2015. These data allowed estimating a total flux of 5-20 g DOC year-1 from the Arno river, that represents up to 3% of the total flux of riverine DOC into the Mediterranean Sea. About 20% of the year flux was due to a single flood event.
Along the Arno River, DOC increased moving from the spring to the estuary, showing peaks in correspondence with the input of the most polluted tributaries. CDOM highlighted changes in molecular properties and origin of the molecules along the river and in the tributaries. Fluorescence excitation-emission matrixes (EEMs), elaborated with parallel factorial analysis (PARAFAC), allowed for the identification of anthropogenic organic pollution.
At the estuary of the Arno River, conservative mixing mainly controlled DOM dynamics, however the small deviations from the linear relationship between salinity and DOC suggested that removal or production processes were also present. The removal processes were investigated trough incubation experiments at intermediate salinities, and showed that approximately 8-9% of riverine DOC is labile, since it is removed in 48h in autumn and winter, respectively. The bacterial growth efficiency (BGE), estimated by DOC and heterotrophic prokaryotes abundance variations during the experiments, was 5-10% suggesting that the microbial community had a different efficiency in transforming DOC in biomass in different seasons. These BGE values allowed to roughly estimate that microbial mineralization of DOC can release from 14.4 to 24.5 µM CO2 in 48h at the Arno River mouth.
Rivers flowing into the coastal Tyrrhenian Sea, had an impact only on a restricted area close to the river mouth, and DOC concentration in the river plumes was mainly affected by anthropogenic activities along the watershed. CDOM optical properties were useful to discriminate between terrestrial and autochthonous sources of DOM. The good linear relationship between CDOM and DOC in the Arno estuary and coastal Tyrrhenian Sea area suggests that DOC can be estimated by satellite retrieval of CDOM absorption in coastal areas, with the appropriate calibration of the algorithms.