Riverine load of inorganic nutrients and carbon into the Gulf of Trieste: the REDEFINE project preliminary results

The REDEFINE (RivEr DischargEs in the GulF of TrIeste: a coNtribution to the improvEment of the digital twin of the northern Adriatic Sea) project aims to contribute to the implementation of the digital twin of the northern Adriatic Sea (NAd) by studying the contribution of the main rivers of Friuli Venezia Giulia, namely Isonzo, Timavo and Tagliamento, to the Gulf of Trieste (GoT), the northernmost basin of the NAd. The GoT is a shallow basin whose hydrology, biogeochemistry and productivity can be influenced by rivers, the major allochthonous source of freshwater, total alkalinity (TA) and nutrients in the area. The Isonzo River (ISO) is the main freshwater input into the GoT and generally shows significant seasonal variations in discharge, with two main flooding periods related to snowmelt and rainfall. The second freshwater source is the Timavo River (TIM), which flows underground for about 38 km before re-emerging in proximity of its mouth. The Timavo has complex hydrological characteristics related to its karstic nature, as the flow at the mouth is also influenced by underground circulation within the karst aquifer. Moreover, since Timavo has several minor springs scattered along the coastline, the permanent spring located in Aurisina (AUR) was selected for this study. Although the Tagliamento River (TAG) is known to have the highest discharges compared to Isonzo and Timavo, it is rarely considered a relevant tributary to the GoT due to the prevailing cyclonic circulation characterising the NAd. Nevertheless, intense and frequent wind conditions can generate highly variable and even opposite surface layer circulations [5], making the contribution of TAG relevant for the GoT. Here we present the data from the monthly sampling carried out for the REDEFINE project (from October 2024 to June 2025) in different hydrologic conditions (dry and flood periods) together with previous data collected, with the same frequency, since January 2022. During the investigated period, TIM was generally characterized by higher dissolved inorganic carbon (DIC) and TA concentrations and lower isotopic composition of DIC (δ13C-DIC) than ISO and TAG, likely as a consequence of the different catchment basins and the nature of the rivers’ course (hypogeous vs. surface). This difference is particularly marked in AUR, because of the increased mineral dissolution, lower degassing and reduced biological consumption during the hypogean course of Timavo river. The distinct characteristics of the catchment basins of the rivers investigated are also highlighted by different Mg/Ca and Sr/Ca molar ratios. The average annual discharge of TA, DIC and nutrients under different hydrological conditions highlighted that riverine nutrient and inorganic carbon load are highly related to the runoff which strongly varies on the interannual scale. However, the total river transport of nutrients and carbon is in line with previous studies. The REDEFINE results will be coupled with discharge data measured by a local authority (Environmental Protection Agency of the Region Friuli Venezia Giulia) and a water management company (AcegasApsAmga S.p.A., Trieste, Italy) to: (i) provide a baseline for assessing the impact of FVG rivers on the GoT; (ii) support future studies on oligotrophication and acidification; (iii) contribute to the development of the NAd digital twin and the forecasting models on climate change effects on the GoT. Ultimately, the REDEFINE project also aims to assess the investigated rivers as sources of microbial pollution, including antibiotic resistant bacteria, potentially pathogenic bacteria and genetic determinants playing a role in the spread of antimicrobial resistance (AMR). These data will be interpreted in light of the biogeochemical characterisation of the river waters to provide new insights into the ecology of AMR dissemination through the environment, essential for developing effective action plans against AMR threat.